The present manuscript relates about the designing and use of an instrumented artificial finger for tactile tasks needs on textile surfaces. The created artificial finger uses a polymeric optical fiber as sensing unit. The manuscript depicts a chronological evolution resulting from the steps in the development of the artificial finger and the reasoning for the overcome of the encountered difficulties. The main difficulty encountered during the development of the artificial finger was the lack of knowledge available for this type of application since this artificial finger is the first created using this type of technology.

The manuscript describes the first handling of the light response of the polymeric optical fiber and the means to obtain a measurement setup to investigate on the artificial finger. Two measurement systems have been investigated. The issues obtained with one system, and the retained improvements for the creation of a new system for the tactile description of textile fabrics are presented along the results obtained with the improved method. The setting of the new system enabled the creation and testing of a fitting method for the understanding of the artificial finger behavior. The fitting method is then tested and discussed.

The artificial finger was then used and recorded during its functioning. The results from the recordings are then discussed and the proposal for improvements in the fitting method is given. Additional testing with the artificial finger was realized for the establishment of sensitivity limits of the artificial finger. Après 3 années (presque 4 me diront certains) de labeur au sein du laboratoire, il reste un élément marquant la fin d'une étape, et il s'agit bien des remerciements. Car il s'agit la de la seule conclusion, celle où l'on peut dire Merci et qui d'un pincement au coeur, correspond finalement à une séparation. Oui, si je souhaite remercier le laboratoire pour son acceuil, c'est parce que je le quitte, pour poursuivre le chemin de la vie, vers une destination encore inconnue. Mais assez de palabres, venons-en aux faits. Il me faut remercier tout d'abord Marie-Ange BUENO, ma directrice de thèse, sans qui ce travail, ce manuscrit, ces remerciements, en bref, rien de tout ça n'existerait. Mais également pour son soutient, sa disponibilité et aussi pour avoir su secouer le poids mort que je suis à certains (longs) moments. Merci également à Michel TOURLONIAS, qui a co-dirigé cette thèse et a apporté beaucoup de temps et a grandement participé aux travaux de conception mécanique qui s ont survenus. Je souhaite également remercier Fabrizio SPANO et René ROSSI, de l'Empa, qui m'ont également beaucoup conseillé et suivi dans mes présentations parfois trop scénarisées et pour lesquelles le suspense était bien trop mis en avant. Je me dois de remercier d'autres personnes qui ont contribué assez directement au projet, je pense en particulier à Brigitte qui a étudié les doigts artificiels bien avant et a réalisé des tests de dureté, Christian qui a usiné des doigts, Philippe pour son aide avec le microscope, Sascha qui a conçu le tribomètre "optique" en l'honneur de son utilisation pour mes doigts artificiels avec oreilles en fibres optique, Daniel et Yann pour les impressions 3D ainsi qu'Agnès et Florence au secrétariat du labo et sûrement bien d'autres encore mais le suicide des mes neuronnes commence à faire son effet. Il ne faut pas omettre de parler des contributeurs indirects, voire très indirects, et je pense au bureau 004, Claire, Donia et Clément, mais aussi au bureau 005 avec Carole, Elise et Wafa, au bureau confection avec Neda, Elham, Raquel et Florianne, au bureau tissage avec Charles et Issam. Pour faire simple, l'équipe des doctorants pendant mon séjour au laboratoire, merci à vous pour la bonne ambiance, et les soirées que nous avons passées sans travailler mais à nous plaindre parce que ça fait quand même du bien. Je souhaite remercier également mes parents, qui m'ont toujours apporté tout leur soutient dans toutes mes entreprises et qui se sont déplacés pour le jour de la soutenance et également ma compagne, Agathe, puisqu'il faut le dire, un thésard en bout de course est particulièrement invivable mais aussi parce que ce chemin vers une destination inconnu, ça fait beaucoup moins peur à deux.

MERCI A TOUS

Il n'existe que deux chose infinies, l'univers et la bêtise humaine... mais pour l'univers, je n'ai pas de certitude absolue.

(Albert Einstein)

Résumé

Le présent manuscrit décrit les étapes de conception et de détermination des paramètres nécessaires pour la fabrication d'un doigt artificiel destiné aux applications pour le toucher de surfaces textiles ainsi que la méthode d'utilisation de ce doigt artificiel sur des bancs de mesure pour obtenir les caractéristiques du frottement de ce doigt artificiel avec des surfaces textiles.

Le doigt artificiel réalisé présente la nouveauté de l'intégration d'une fibre optique polymérique permettant l'acquisition d'information sur le frottement au plus près de la zone de contact. Ce doigt artificiel compte parmi les premiers doigts artificiels instrumentés et est le premier disposant de cette technologie.

Par conséquent, un travail de découverte du système a été mené de prime abord et explique la structure chronologique de ce manuscrit retraçant les étapes du développement de ce doigt artificiel.

Le manuscrit présente en premier lieu la première prise en main du phénomène de réponse optique de la fibre optique polymérique et les moyens mis en oeuvre pour obtenir un banc de mesure adapté au doigt artificiel. Plusieurs 1 Artificial finger design 9

1.1 Artificial finger first approach and preliminary study . . . . . . . . . . . . 9

1.1.1 Identification of important parameters for POF implementation . . 9 2.11 Schematics of an optical fiber with two rays entering the fiber core. One is inside the numerical aperture of the fiber and is reflected inside the fiber, the other is out of the numerical aperture and is refracted out of the fiber.

2.12 Schematics of fiber presenting different modes and of step index of gradient index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.13

The representation of a bent optical fiber with a thin cladding. In this situation the light ray can re-enter the fiber core after having exited the fiber core because of a curve. Extracted from [START_REF] Durana | Dependence of bending losses on cladding thickness in plastic optical fibers[END_REF] . . . . . . . . . . . . . there is always a gap between either the testing probe and the sensor or between the tested surface and the sensors. In order to overcome this problem, an instrumented probe could be used with the possibility to acquire a signal directly from the contact. In fact, this is also the functioning method of fingers, where mechanoreceptors are present inside the finger and acquire the friction signal from the inside. Second is the presence of these external sensors, in the domain of robotics or prostheses, the domain of use of the artificial fingers will be for domestic purposes, where external sensors are not present.

Third, the use of human panels relies on the use of external sensors data, the diversity of the fingers used, and the complex interpretation of each individual in the sensing. For these reasons, the use of an artificial finger representing an "average finger" brings more adaptability and objectivity.

INTRODUCTION

In order to differentiate the instrumented artificial finger of this work from the state of the art, a long term approach on the reproduction of a human finger will be considered.

Another work from the laboratory focuses on the effect of the hydrolipidic film composition on the fingertip during a friction task (2). The humidity of the environment takes an important place in the friction behavior of fingers. With this statement, the designed artificial finger has to remain reliable under moist environment to act as a sensor. The use of optical fibers can bring this property provided the moisture in the environment has no influence on the light transmission of the fiber. Since the instrumented artificial finger will be a tactile sensor and thus deformation will be observed, the measurement is supposed to be unaffected by humidity. Hence, the use of polymeric optical fiber will be investigated.

State of the art

The topic of artificial fingers is a vastly investigated subject. Studies have been made

to show the possibilities of the technology and the means to achieve reproducibility, accuracy and fiability of the measurement. One main objective of artificial fingers is to mimic the behavior of a real finger. In this report, the reviewing of the studies around the subject will follow a simple order, from the basic studies of friction and fingers to the use of instrumented artificial fingers.

The uses of artificial fingers

The uses of artificial fingers relates to many other domains than the tactile description of surfaces. The need to rely on artificial fingers can be found in non tactile needs. As an example, in (3) where the artificial finger created is used in experiments where the influence of mechanical tools, like jackhammer, can damage the internal structure of a finger because of vibration is investigated. Because of the destructive nature of the experiments the use of human panels is not a possibility. For that reason, an artificial finger with structural parameters such as a soft exterior with a hard center representing the bone with joints has been designed. However, the touch has not been investigated with this artificial finger. Artificial fingers may as well be used in mechanical systems where the reproduction of human hand is aimed. An example is given in (4) in which the designing of the hand was entirely aimed at the mechanical reproduction of the hand 0.2 State of the art mechanics. Polymeric fingertips can be seen but the frictional behavior of the hand was not investigated for reproduction of hand behavior but rather ensure grasping. For more precise grasping performance of a robotic hand, it is important to include force sensors in the fingers to have in time control of the hand movement and the effect on the grasped object. The grasping should not degrade the object. An example is given in [START_REF] Astrid | Smart control of a soft robotic hand prosthesis[END_REF] where force sensors are included in the fingertip of the hand and provide feedback on the grasping interaction.

In this thesis, the development of an instrumented artificial finger for touch of textile surfaces was initiated with the goal to avoid the use of human panels. It has been investigated that touch and skin properties largely depend on the individual characteristics (2,[START_REF] Baussan | Experimental simulation and modeling of sock-to-skin friction during running[END_REF][START_REF] Bhushan | Nanomechanical characterization of skin and skin cream[END_REF]8,[START_REF] Condon | Differential sensitivity to surface compliance by tactile afferents in the human finger pad[END_REF]10,11,[START_REF] Hara | The relationship between the Young's modulus of the stratum corneum and age: a pilot study[END_REF][START_REF] Kwiatkowska | Friction and deformation behaviour of human skin[END_REF]. Touch is an important factor for human interaction with its environment. Investigations about touch and ergonomics lead to the study of human perception of tactile tasks and response of human mechanoreceptors [START_REF] Condon | Differential sensitivity to surface compliance by tactile afferents in the human finger pad[END_REF][START_REF] Bueno | COSTaM: Tool Design for a Controlled Tactile Stimulation[END_REF] and the behavior of the finger during friction against different surfaces (11,[START_REF] Asaga | Tactile evaluation based on human tactile perception mechanism[END_REF]16,[START_REF] Fagiani | Contact of a Finger on Rigid Surfaces and Textiles: Friction Coefficient and Induced Vibrations[END_REF]. The development of artificial fingers with the objective to reproduce the friction behavior of a finger is investigated by numerous researchers for various applications. However, compromising between the reproduction of the skin texture, the friction parameters and high resolution instrumentation due to optical systems is a novelty added by this thesis.

As example, Mondal et al. [START_REF] Mondal | Effect of surface modification on frictional properties of polyester fabric[END_REF] used a fingertip made in the form of a half sphere and with polydimethylsiloxane (PDMS). The objective of this work was the recognition of structural parameters of polyester fabrics. The parameters used for recognition were coefficient of friction (COF) and vibrations induced by the friction on the slider.

While COF and vibration are parameters to describe touch, the touch aspect was not thoroughly implemented in this setup because the aim was the recognition of surfaces and not the designing of the touching behavior of the probe. The presented recognition of fabrics corresponds to very similar textile fabrics in terms of structure. A similar setup is presented in [START_REF] Thieulin | Study of the tactile perception of bathroom tissues: Comparison between the sensory evaluation by a handfeel panel and a tribo-acoustic artificial finger[END_REF] for which the probe was of similar shape, but with different testing parameters such as normal load and sliding speed. The authors claimed the parameters to be close to average human discovery parameters. Friction and vibrations are used as discriminatory parameters for the description of the surfaces. In the two above-mentioned works, the coupled use of friction and vibration is used to discriminate textiles. It is stated that the correct reproduction of COF is necessary but insufficient

INTRODUCTION

to describe textile surfaces [START_REF] Dreby | A friction meter for determining the coefficient of kinetic friction of fabrics[END_REF][START_REF] Morrow | The Frictional Properties of Cotton Materials[END_REF]. However the considered shape of probe for the investigation is not adequate for hairy textiles.

Some artificial fingers were realized with the idea that a correct reproduction of the finger friction behavior requires an accurate reproduction of a natural finger shape, in (22,[START_REF] Controzzi | Bioinspired fingertip for anthropomorphic robotic hands[END_REF]24) the case of artificial fingers molded from a natural finger is presented. The artificial finger created then was used to investigate friction differences of several textile structures with an average COF which is, as stated above, necessary but not sufficient.

However, the work of ( 23) reports an accurate reproduction of the multi-layered structure of the finger with bone, pulp and nail with corresponding mechanical properties. This structure was stated as satisfactory in terms of vibration behavior, but lacks in friction level, or COF with stated values around 2.2 with the considered setup. The use of finger shaped artificial fingers is interesting, but the effective investigation on the influence of the shape parameters is lacking, and not sufficient to prove the accuracy of the friction behavior of such artificial fingers since molded fingers from human fingers are considered particular.

In [START_REF] Camillieri | Artificial finger design for investigating the tactile friction of textile surfaces[END_REF], the structural parameters for an accurate reproduction of finger friction against textile fabrics with an artificial finger are investigated. The investigated parameters that show real importance in the reproduction of the finger behavior while sliding against textiles are: rounded shape, stiffness, texture and adhesion. Adhesion, as stated earlier, is the correct reproduction of COF with textiles. The stiffness is the capability of the artificial finger to have enough elastic deformation. Finally, the texture, or fingerprints, is particularly important in the friction against hairy textiles. A description of the accurate reproduction of friction behavior will be held in §1.2 with the use of the produced artificial finger of the present work.

The instrumented artificial fingers

Natural fingers are able to sense their surroundings with internal mechanoreceptors.

Designing artificial fingers as sensor is a logical step in their investigation. The use of artificial fingers for grasping tasks or prosthesis for example requires internal sensing ability to apprehend and give feedback on the current held task. Having a sensing unit in an artificial finger enables the measurement of friction force or effects at the nearest of the contact zone. With the use of tribometers, there is always a bias between the 0.2 State of the art sensors and the contact zone. With a sensing probe used as a slider, it is possible to obtain data at the closest of the contact zone.

Several systems can be used for the description of surfaces with instrumented artificial fingers, as stated in [START_REF] Mohsin | A review of tactile sensing technologies with applications in biomedical engineering[END_REF], several technologies are used for the development of tactile sensors such as capacitive sensors, piezoelectric sensors, piezoresistive sensors, inductive sensors, optoelectric sensors, strain gauges or multi-components. In (26) a bioinspired piezoelectric sensor is presented with a structure based on foot structure of the gecko, the spider and some insects. This system achieved a spatial sensitivity close to that of the human hand. A fingertip for the evaluation of the maturity ratio of tomatoes is described in [START_REF] Termehyousefi | Development of haptic based piezoresistive artificial fingertip: Toward efficient tactile sensing systems for humanoids[END_REF]. The sensing ability of the artificial finger is granted by a PDMS/CNT (carbon nanotubes) nanocomposite bonded to a silver electrode which forms a piezoresistive element. The sensitivity achieved with this artificial finger enabled for the determination of the resilience of the skin of tomatoes with study of the normal force signal obtained.

However, the use of only a normal force recording is not sufficient for the description of surfaces.

Combined use of several sensors is required to obtain information on shear forces. In (28) a piezoresistive fingertip composed of four micromechanical systems (MEMS) was designed and used to discriminate roughness of different surfaces. Reproducible results and accurate discrimination was obtained when used on grooved surfaces because of the reproduced fingerprint texture. An artificial finger with perpendicular polyvinylidene fluoride (PVDF) film is presented in [START_REF] Qin | Enhanced surface roughness discrimination with optimized features from bio-inspired tactile sensor[END_REF]. The artificial finger is composed of a polymethylmetacrylate (PMMA) core which acts as a bone and two layers of PDMS. The two layers of PDMS have different mechanical properties, especially the elastic modulus which results from different ratios of base to curing agent. This artificial finger is able to differentiate surfaces with different roughness standards but has no texture at its surface. In (30, 31), a method to discriminate previously tested and registered surfaces is presented and achieved over 90% of accurate recognition. The testing probe is a half cylinder with a PVDF film placed over it. Normal forces considered for this task was of 1,5N which is considered high for human touch tasks. A system with a different approach to identify textiles is presented in [START_REF] Felix Von Drigalski | Textile identification using fingertip motion and 3D force sensors in an open-source gripper[END_REF] where the textile is inserted in a gripper composed of two circular probes of rubber. This method of identification is highly inspired by human movement where testing the quality of a fabric, but for the system to give reliable results, mechanical parameters need to be transmitted through

INTRODUCTION

the probe to the sensors, thus a high COF between the probes and the textile is to be considered.

A highly mimetic artificial finger has been made and commercialized [START_REF] Fishel | BioTac® Product Manual[END_REF][START_REF] Fishel | Sensing tactile microvibrations with the BioTac-Comparison with human sensitivity[END_REF][START_REF] Chen | Tactile perception of fabrics with an artificial finger compared to human sensing[END_REF][START_REF] Kerr | Material recognition using tactile sensing[END_REF]. This artificial finger was realized with the idea of integrating as many types of measurements as realized by a natural finger in the prototype. The artificial finger is able to sense pressure applied on its fingertip, acoustic vibrations transmitted through the finger and heat exchanges with its environment. The parameters acquired where compared with sensorial experiments with human panels. The drawbacks of this finger are: the heat exchange measurement is difficult to analyze, and require knowing what type of surface is in contact with the finger, the pressure and vibration are transmitted through a conductive fluid. However it is difficult to refill the quantity of fluid and this quantity highly affects the measurements. Last, the outer shell is a silicone based artificial skin. However it has a very high COF with textiles: 1.5 [START_REF] Fishel | BioTac® Product Manual[END_REF] while real fingers are situated around 0.5 [START_REF] Camillieri | Artificial finger design for investigating the tactile friction of textile surfaces[END_REF].

The work realized for this thesis was the design of an artificial finger which reproduced friction behavior between finger and textile fabrics. In addition, the ability to acquire measurement for friction against textile fabrics and the possibility to extract determining features for textile fabric description. The investigation about extracting physically accurate features to characterize textile fabrics has already been held with [START_REF] Dreby | A friction meter for determining the coefficient of kinetic friction of fabrics[END_REF][START_REF] Kawabata | Objective hand measurement of nonwoven fabrics Part I: development of the equations[END_REF]38,[START_REF] Hoffman | Some Relations of Fiber Properties to Fabric Hand[END_REF] and the definition of the "hand" of fabrics. However, the "hand" of fabrics is a tool that has been used to determine the features a textile will have for the manufacturing of garment. Today, the objective of the description of textile surfaces aims for instance at the extension to e-market purposes. A design of tactile stimulator has been realized in the University of Lille with a collaboration of the LPMT / University of Haute Alsace [START_REF] Ben | Design and control of a tactile stimulator for real texture simulation : application to textile fabrics[END_REF] and the correct reproduction of COF behavior while sliding against textiles has been achieved with the stimulator [START_REF] Camillieri | From finger friction and induced vibrations to brain activation: Tactile comparison between real and virtual textile fabrics[END_REF].

In order to broaden the uses of the artificial finger designed in this work, a special attention was accorded to perspiration. [START_REF] Gong | An optical fiber curvature sensor based on photonic crystal fiber modal interferometer[END_REF][START_REF] Ignacio Hernández-Serrano | Robust optical fiber bending sensor to measure frequency of vibration[END_REF]. MZI with optical fibers are also used for electrocardiogram acquisition in [START_REF] Fernandes | Electro-optic acquisition system for ECG wearable sensor applications[END_REF]. This shows the versatility of optical fiber measurements. However, the measurement methods described here rely on either difficult and expensive preparation on fibers for microstructuration of gratings and measurements of wavelength shift needs a spectrometer, and interferometers also needs precise execution for their exploitation. A much simpler and cost effective way to use optical fiber for measurement is the acquisition of transmitted power. As presented in [START_REF] Krehel | Characterization of Flexible Copolymer Optical Fibers for Force Sensing Applications[END_REF], the change in power transmission can be related to the force applied and thus to use the built system as a force sensor. The creation of an artificial finger using an optical fiber to sense surfaces is presented in (51) where the objective was depicted as shear force sensing.

Optical fibers are used for other physical properties measurement as temperature (52) or humidity (53). An interesting fact in the setup realized for the measurement of humidity in (53) is the presence of a short portion of decladded fiber, which is then coated with a material containing titanium dioxide to provide hydrophilicity to the cladding material.

This will make the refractive index of the modified cladding material to change depending on environment moisture thus, the setup will be sensitive to humidity changes. In (54), a different system for humidity sensing with an optical fiber is presented but with the presence of a portion of the fiber with a hydrogel as first environment interface, the

INTRODUCTION

hydrogel is subject to refractive index changes relatively to external humidity. With this observation, the hypothesis of moisture immunity of the optical fiber sensing provided the environment of the fiber does not absorb water has been supposed, then the choice of optical measurement for the system presented in this manuscript has been accepted.

This manuscript relates the development of an artificial finger designed for the touching of textiles surfaces, where shear forces will have high influence. The technology used for measurement is recording of transmission power of an optical fiber during tactile exploration. The presented development is about artificial finger, however an extension of the work to artificial sensing skin is considered. As explained in (55), materials used for the reproduction of skin properties with artificial materials are often textiles or elastomers. Thus, the addition of a sensing capability of the artificial skin is to be assured by the optical fiber, however, to keep the thinness of the system, the optical fiber will supposedly be inserted in the fabric. The use of artificial skin for systems like robots or humanoid manikins, will require to conform to specific shapes, oftenly with high curvature. For a better curvature acceptance of the system, Polymeric Optical Fiber (POF) is considered.

The work will be described here as follows: first, the preliminary studies for the developing of the artificial finger are presented, the observations that lead to the making of the artificial finger structure and evolution that had to be taken in order to validate the measurements. Second is the making of the prototypes of artificial finger, the results obtained and how the measurement can be interpreted. Third are the experiments in our lab to verify the capability of the artificial finger to describe textiles. Lastly the discussion will be concluded with the main results obtained from this study and the future work to be held to pursue the development of the artificial finger and other perspectives.

Chapter 1

Artificial finger design 1.1 Artificial finger first approach and preliminary study

1.1.

Identification of important parameters for POF implementation

The designing of the artificial finger was based on several studies made over a proof of concept given by Empa, the Swiss Federal Laboratories for Materials Science and Technology [START_REF] Spano | Flexible touch sensors based on nanocomposites embedding polymeric optical fibers for artificial skin applications[END_REF]. The proof of concept did not consider the description of surfaces and the detection of shear forces, but showed the possibility to measure variations of normal forces applied on a polydimethylsiloxane (PDMS) patch with an embedded POF. The first approach retained for these samples consisted in the description of the relation between the variation of normal force applied on the patch and the variation of light transmission. The unit retained for the light transmission is W/m 2 . This will be called irradiance at this point.

Several different patches from the previous work [START_REF] Spano | Flexible touch sensors based on nanocomposites embedding polymeric optical fibers for artificial skin applications[END_REF] were tested : two clear PDMS patches, one with a straight POF embedded, and one with a loop formed outside the patch. The objective was to superpose the POF with itself in a crossing to enhance the light transmission loss with the increasing load. Two patches were enhanced with gold nanoparticles, a low concentrated and a high concentrated each with one straight POF embedded. A measurement bench was assembled in the present study to acquire the force-irradiance measurement (fig. 1.1).

The measurement bench was composed of a halogen light source GLI357, a monochromator SDMC 1-04, a photodiode model 262 UDT and an optometer S370. A monochromator was used because of the presence of gold nanoparticles in two of the patches 1. ARTIFICIAL FINGER DESIGN The samples were tested under different loads with two wavelengths of light: 632 nm and 532 nm. The used lamp has a high output power for a wavelength of 632 nm and the resonance wavelength of gold is 532 nm. The loading was realized with precision masses over an aluminum coin to keep a constant loading surface. The masses were placed over the coin with an increasing load until 5 N and followed immediately by an increment unloading until the reach of the lowest mass. Each mass was set put for a few seconds to observe a stabilization, the loading causes movement in the setup and the two fibers ends are not well maintained. The value was put in a table for exploitation. The cycle was reproduced 5 times to enable a statistic description of the loading and the unloading behavior of the different samples.

A few observations can be made after the realization of the graphics corresponding to the tests (fig. 1.2). First, the irradiance measured varies accordingly to the load applied.

A linear relation is exposed here, which is a very good point for further exploitation.

Second, as known for a halogen light source, output power for red wavelengths, here 632 nm is higher than for other colors. In this case, the test with a light input of 532 nm, results with a ten to thirty times lower output power, which is equivalent to lower sensitivity. However, the proportionality coefficient for the 532 nm wavelength is of -0.0001 nW/cm 2 /cN while the proportionality coefficient for the 632 nm wavelength is

1.1 Artificial finger first approach and preliminary study of -0.0039 nW/cm 2 /cN, which means the gold nanoparticles have no increasing effect in the sensitivity. The better signal to noise ratio will be chosen, i.e. with the highest input power. Finally, the error bars show a high standard deviation in the values for a given load, this represents an error of 0.2 N for the sensing and does not suit the use for touch which is realized with normal forces around 0.5 N.

The different patches have been tested and a comparison is shown (fig. 1.3). The loading and unloading curves are represented here. The elastic behavior of the patch sample for the range of forces used is highlighted by the shape of the curves. Because of the high number of error sources (namely the POF length, the sample placement, centering of the weight) the observed deviation is still high. Moreover, the loads used on the considered surface of application, i.e. 0.5 N over 1.7 cm 2 , goes beyond the range of tactile tasks [START_REF] Camillieri | Artificial finger design for investigating the tactile friction of textile surfaces[END_REF]. As a conclusion, the patch is expected to have an elastic behavior during the touching tests. However, as can be observed on the measurements, there is a need to lower the standard deviation for reproducibility purposes. The preliminary tests showed the possibility to use this type of technology for the measurement of forces through the PDMS patch. To go further in the direction of the POF embedded PDMS patch, it is mandatory to produce new test samples. It is important to extract a few parameters such as elasticity to work on before the production of samples.

The use of gold nanoparticles did not bring higher sensitivity in this situation (fig.

1.2). The use of clear PDMS is chosen. Another parameter is the quantity of PDMS in the sample, or the height of PDMS between the contact zone and the POF. To obtain an idea of the influence of this parameter, a study with the current patches superposed has been done. The results of the experiment are assembled (fig. 1.4).

The results shows a higher absolute value of slope for the situation with the lowest distance between the contact and the POF. This will lead to a higher sensitivity of the measurement, thus, for further development, a short distance between the POF and the contact area will be aimed. The relation observed can be considered linear in the range of measurement, however the evolution of light is always positive and will be asymptotic (the maximum transmitted light cannot exceed the source power minus the linear attenuation of the fiber) which means the linear relation cannot be applicable to a large range of measurement. On the preliminary tests realized, high standard deviations have been highlighted.

Therefore, in order to pursue with the study of the samples, it is important to reduce the standard deviation. However, it is not possible to adjust deviation caused by the samples, since there is little insight on the causes of the deviation from the samples, and

1.1 Artificial finger first approach and preliminary study most of the deviation measured here came from the measurement bench, and thus, an upgrade has to be realized. The observed sources of standard deviation were:

• The connection

• The POF

• The alignment all along the fiber As stated above, the connection is a source of deviation, since the work is done on POF, as many soft materials, it is difficult to obtain a polished section of the POF, which means the output of the POF will cause irregularities in the measurement (fig. 1.5). In addition, since the POF were not glued to the connectors, after some manipulations, the POF slowly gets out of the connector. The POF and alignment factor stated above are related. On the measurement bench, the sample was connected to the monochromator and the photodiode by both sides, and roughly placed in the middle of their distance.

The linearity of the POF at the exterior of the samples (fig. 1.1). was not verified and could not be verified after each placement.

For the tests that occurred until this observation, the patch was put on a hard surface and the load was placed on the PDMS sample, which means the sample was confined between hard materials. In this situation, the POF is subject to micro deformations due to the compression and state of constraint of the PDMS, but the external POF was freely moving and could be subject to macro deformations due to the placement of the sample. In addition, a qualitative test on the variation of irradiance after a loading on a bent sample was conducted, and the observation was that the sample had more sensitivity while being bent.

Proposed solutions for POF implementation

There are parameters on this bench that also need improvement and are easily attainable.

First, the monochromator greatly reduces the overall measurable output power of the light source, the clear PDMS patch with the crossing POF could not be used because signal could not be measured at the end of the POF. The length of the POF was too long for enough power to be measured. This statement indicates that the POF used has a high transmission loss (around 0.5dB/cm (58)). A second issue with the monochromator is the impossibility to observe simultaneously the influence of the load on the irradiance 1. ARTIFICIAL FINGER DESIGN of several wavelengths. All the observations and interpretations led to the design of a new measurement bench to overcome the difficulties of the preliminary bench. The decision to pursue the work in the form of an instrumented artificial finger was made for the reasons stated in the following.

The specifications of the new system were as follows: the artificial finger would be adapted to function as part of a tribometer, the POF would be embedded the same way as in the PDMS patches, but the connectors would be fixed to the main frame of the artificial finger. This causes the POF to be entirely integrated to the PDMS.

This solves the problem of the free POF out of the sample. The artificial finger will be motionless during an experiment and stuck to the tribometer, this solves the problem of the alignment.

The new mesurement bench consists of:

• A new calibrated light source and a spectrometer. The irradiance variation can be known in real time. There is no need for a monochromator since the spectrometer is able to measure each wavelength separately and almost simultaneously.

• The connection between the light source and the artificial finger is assured by a broadband glass optical fiber. Another broadband glass optical fiber assures the connection between the artificial finger and the spectrometer.

• The light source has been changed to a Avalight HAL Mini, a halogen light of 10 W.

• The spectrometer is a Avaspec-ULS2048CL-Evo with a 25 nm slit.

• The tribometer (fig. The observations made here showed the possibility to measure forces with the embedded POF in a PDMS patch. It is needed to verify that the technology will be accurate for the measurement of shear forces. The objective of the creation of the artificial finger has been made and, according to the results in (1), the artificial finger will be designed as a slider for the tribometer. It has been seen (fig. 1.4) that the maximum of sensitivity will be achieved with a low height of PDMS between the POF and the contact area, and, the bending of the POF will also increase the effectiveness of the sensory attribute of the artificial finger. The technology with the POF is foreseen to be extended to an instrumented artificial skin, or sensing surface. The key point of this kind of work is the use of a textile base for flexibility and mechanical properties. However, inserting an optical fiber in the textile base will cause the fiber to have several points of curvature,

Conception of the artificial finger

To reproduce the behavior of a finger for our needs, it is important to describe the finger structure to understand the mechanics in action within a finger during friction for the reproduction of this mechanic, and how the reproduction will be held. For most of tactile tasks, and especially for tactile discovery, the finger to be used is the index finger of the preferred hand. However, the description of a surface with a finger comes through numerous parameters, e.g. heat exchanges, detection of deformation of the finger with mechanoreceptors, or vibrations induced by the friction against the surface to explore. The designed artificial finger will not cover all fields of detection of the finger, but will focus on mechanical sensing. The finger structure is complex, and the cellular structure of the finger will not be detailed here. A natural finger roughly consists of a bone, pulp, skin and nail. The finger pad possesses four kinds of mechanoreceptors [START_REF] Johnson | The roles and functions of cutaneous mechanoreceptors[END_REF][START_REF] Johnson | The roles and functions of cutaneous mechanoreceptors[END_REF][START_REF] Burgess | Cutaneous Mechanoreceptors and Nociceptors[END_REF][START_REF] Iggo | Somatosensory System[END_REF][START_REF] Breugnot | Contribution à la caractérisation mécanique du toucher des surfaces textiles à partir de critères neurosensoriels[END_REF][START_REF] Roudaut | Touch sense: Functional organization and molecular determinants of mechanosensitive receptors[END_REF], the Merkel cells (SA I) located in the superficial epidermis and sensitive to indentation and responsible for fine texture perception, the Ruffini endings (SA II) located in the dermis and more sensitive to skin stretch, the Meissner corpuscles (FA) located in the epidermis and detect global vibration on the skin like slip for grasping and the Pacinian afferent (PC) that detects non localized vibrations to the skin. The addition of all these mechanoreceptors gives the tactile capacity of the hand, and the touching sensitivity of fingers (fig. 1.7). The segmented skin will not be reproduced.

However, the artificial finger will consist of an external PDMS simulating the external part of skin and a core PDMS simulating the pulp.

A previous study (1) aimed at describing the structural needs for an artificial finger to reproduce the friction behavior of a finger against textiles. The investigated parameters : shape, hardness, texture and adhesion properties showed to be the most effective parameters on the reproduction of the friction behavior of the finger. The hardness and friction are responsible for the transmission of internal forces in the finger and how the textile surface reacts to interaction with the finger. In the reaction to the finger friction against the textile, the adhesion properties will act on the deformation of the finger and on the resulting visible contact area. Friction and normal forces act and may cause deformation of the textile surface if the friction force is too high. It has been seen that coefficient of friction (COF) of silicones against textile ( 1.5 [START_REF] Fishel | BioTac® Product Manual[END_REF]) is usually too high to compare to COF of human fingers with textiles ( 0.5 (1)). The designing of this artificial 1. ARTIFICIAL FINGER DESIGN

ARTIFICIAL FINGER DESIGN

Friction behavior of a natural finger highly depends on individual's characteristics such as finger stiffness, hydrolipid film water content or texture (2). The shape of fingerprints can be questioned because of the plurality of possibilities for human fingers.

In the objective of creating an average finger, it is mandatory to select a neutral shape of fingerprints.

As seen in [START_REF] Hoover | The Science of Fingerprints[END_REF][START_REF] Martijn Van Mensvoort | FINGERPRINTS WORLD MAP -Global Distribution of Whorls[END_REF], 95% of human fingerprints belong to three types of fingerprints (fig. 1.9):

• 60% are loops

• 30% are whorls

• 5% are arcs On these types of fingerprints, it is possible to see that the shape of fingerprints can be approximated as parallel ridges perpendicular to the direction of friction. For symmetrical reasons, concentric circles enable for the same shape of finger prints whatever the sliding direction may be. As a consequence, concentric circles are the chosen shape for the artificial finger. Skedung ( 68) has made the same choice. Fingerprints have an average dimension of 500 µm of spatial period and 100 µm in depth (2). This average dimension will be chosen for the production of our artificial finger. The last evoked parameter is the shape. The finger has a roughly circular shape, and this shape was investigated in (1), but no major differences between rounded edges and sharp edges were observed for plane surface exploration. Then, our finger will have a flat ended cylindrical shape, which means sharp edges on the borders. This choice of shape was chosen due to machining difficulties because of the texturing that has been decided to be done on the mold for the artificial finger.

The apparent contact area between an index finger and a textile surface to explore is 1.5 cm 2 for a normal force applied of 0.5 N ( 6). The contact is determined by the nature of the surfaces in contact, thus for the finger, the nature of the surface is the skin, the epidermis part and more precisely the Stratum Corneum (SC). This SC is responsible for the COF of the finger against a reference surface and the shear behavior of skin, but has little impact on the skin normal elasticity [START_REF] Pailler-Mattei | Contribution of stratum corneum in determining biotribological properties of the human skin[END_REF].

A textile fabric will be used as a reference to adjust the COF of the artificial finger in the order of magnitude of the COF of real fingers with the reference fabric. The production of a few patch samples without POF of the PDMS has been realized

to evaluate the influence of the base to curing agent weight ratio on the COF. An average value of 0.5 between natural fingers and a cotton twill woven fabric has been determined in internal project of the laboratory and will be used as COF to reproduce against the same textile sample that will serve as a reference. The results of the production of different compositions of PDMS are shown in (fig. 1.10). On these results, the COF of 0.5 was not obtained with the considered method. A base/curing agent weight ratio of

ARTIFICIAL FINGER DESIGN

The elastic modulus of the artificial finger has been modified to correspond to the needs for the COF reproduction. The obtained elastic modulus for the external PDMS is 12.7 ± 0.2 MPa and has been calculated with several normal loading curves (ex. fig.

1.11) with a cylindrical flat indenter. The relation used for the determination of the modulus was Hertz contact for the used geometry [START_REF] Sneddon | The relation between load and penetration in the axisymmetric boussinesq problem for a punch of arbitrary profile[END_REF][START_REF] Johnson | Contact Mechanics[END_REF]:

F N = 2 • a • E • d 1 -ν 2 (1.1)
In which:

• F N is the normal force measured (N)
• a is the radius of the cylinder (m)

• d is the depth of penetration (m)

• E is the modulus of the indented body

• ν is the Poisson ratio for this body

This type of formula adapted by considering an infinitely rigid flat indenter and an elastic body, which can be assumed to be conform for the present case.

In comparison with a real finger, for which the elastic modulus of SC is comprised between 2.0 to 1000 MPa depending on the measurement method and moisture [START_REF] Hara | The relationship between the Young's modulus of the stratum corneum and age: a pilot study[END_REF], the value obtained for the PDMS used fits the range of values available for SC. However, the SC thickness is around 10µm and the external PDMS is 3 mm of height so the dimensions cannot compare and the influence on the overall finger modulus is different. A real finger has a modulus comprised within 0.03 to 0.2 MPa according to (2,[START_REF] Maeno | Relationship between the Structure of Human Finger Tissue and the Location of Tactile Receptors[END_REF][START_REF] Opris ¸an | Experimental determination of the Young's modulus for the fingers with application in prehension systems for small cylindrical objects[END_REF], and the created artificial finger has a modulus calculated at 2.0 MPa according to Hertz contact mechanic solution for a contact between a spherical indenter and a half space (72):

F N = 4 3 • E * • d 3 2 • R 1 2 (1.2)
With R being the radius of the sphere

The obtained modulus for the whole finger is higher than the modulus of a real finger. However the objective here was the calibration of the COF which was conducted successfully (1.2). The assembly method generated interrogations about the structure of the build while preparing the different parts. First, as stated above, the external PDMS has to be prepared separately in a special mold which will not degrade with a temperature of 200 • C. Next the external PDMS will be added to the rest of the structure, which will need centering and fixing means. Finally, the preparation of all components in PDMS need to be removed of any air bubble remaining, which means a degassing will be executed. The making of the structure will be made with a 3D printer. In foreseeing of the later addition of the POF, and the possibility of acquiring the friction with a camera, transparent borders have been made in polycarbonate to ensure the visibility of the POF (fig. 1.12). As can been seen on the pictures of the prototype (fig. 1.12), a few issues can be noted. First, the centering of the PDMS stud was not assured on this prototype. This needs a correction. Moreover, the easiest way to make the main build of the artificial finger while allowing for transparency is to have a square shape of the body structure. For further exploitation of the artificial finger, it is important to ensure symmetry of the artificial finger. The possibility of having multiple directions of friction, has been considered and the artificial finger is mounted on the tribometer with an octagonal part, thus a circular shape is to be preferred for further investigation on the shape of the artificial finger.

With the remarks done on the shape of the artificial finger, the assembly of the hard stud on the soft core of PDMS went as expected and the method of making the artificial finger can be maintained. In terms of production, it has been seen that the PDMS is not easy to remove from aluminum molds. Polytetrafluoroethylene (PTFE) will be used for the next molds because the low surface tension of PTFE is supposed to facilitate removing of the PDMS from the molds. The next prototype will still be 3D printed, 1.2 Conception of the artificial finger with a circular shape, and the POF will be inserted. As observed in the preliminary study, the POF will have a curvature inside the artificial finger, for testing purposes, the curvature will be arbitrary decided, an angle of 45°has been chosen. A prototype artificial finger was made and testing of the measurement setup was held to validate the method and the making of the artificial finger.

The first results of friction on the tribometer showed promising results with an irradiance signal similar to the force signal (fig. 1.13). With this observation, the development of a final version of the artificial finger can be started. It has been observed that the prototype has still issues about the stability of the measurement. The final artificial finger needs to be stable and reusable and thus, show repeatability and reproducibility.

However, due to the materials used in the making of the prototype, reproducibility cannot be attained, a change of materials has to be held, and the final artificial finger will be made in aluminum. The main source of deviation observed in the prototype was in the stability of the connectors. The main body of the artificial finger was 3D printed with polymer and the connectors are made in stainless steel, thus during connection and disconnection of the optical fibers the connectors can move a little, and this has high influence on the measured signal.

The making of a prototype in aluminum was realized and the centering of the stud, the stability of the connectors and symmetry of the artificial finger were rectified. However, some questions were raised with this new configuration. First, the stability of the connector was not ensured as expected with the aluminum frame, this is due to the machining of the pieces and the tolerance of the mechanism, as a solution, a resin for metals will be used. Next, it has been seen that the positioning of the POF inside the connector is difficult to achieve. Since it is not possible to hold the POF during the making of the artificial finger, a guide will be inserted in the connector to keep the fiber centered. This guide is not made to be removed from the artificial finger. Lastly, unlike for the printed polymer, the used PDMS does not adhere to aluminum, after some time, the core of the artificial finger unstuck from the main structure and a movement of the PDMS can be seen during friction. This behavior correlates with the tilt of the finger during friction, however this causes the understanding of the functioning of the artificial finger more complex. A prototype has been made with a chemical surfacing of the aluminum to make the PDMS adhere on the surface. The prototype with surfacing and the prototype without surfacing have been compared. The measurement results will be explained in §2.1. However, as shown in fig. 1.13, the tangential force measurement enable the detection of the sliding direction of the slider on the textile. As a reminder, the tribometer uses an alternative linear translating table as a movement base for the determination of friction forces, this means the slider used will slide on the textile with two directions. The results presented (fig. 1.13) show the capacity of the artificial finger to discriminate both sliding directions. In the case of the stuck PDMS, the transcription of the sliding direction by the artificial finger comes with a much lower amplitude and artifacts, this makes the interpretation of the sliding signal difficult to realize with an automated program. For further production, the PDMS will not be stuck in the main frame. Once the parameters for the fabrication of the artificial finger have been decided, the making can be achieved.

Fabrication protocol

Fabrication protocol

As stated in the conception of the artificial finger, the structure of the prototype is composed of different parts with specific properties. The fabrication of the artificial finger requires several steps to ensure reproducibility of the fabrication of the artificial finger. The first step in the fabrication of the artificial finger is the manufacturing of the main aluminum body of the artificial finger. This part was not investigated and will not be detailed here. Once the structure is complete, the making can take place in a few steps. First is the preparation of the artificial finger and the POF. The connectors for the POF are placed in the main body and glued with the resin. The length required of POF is measured and inserted in the connectors with the guides (fig. 1

.14a).

The second part is the making of the PDMS parts. It is important to separate the different steps for making the external part of the PDMS and the core since the PDMS recipe is different and thermal treatment is also different. The external part is molded within a PTFE mold, the texture and contact area are set for the finger by the mold.

Because of thermal retreat of the polymer and the differences of thermal expansion of PTFE and PDMS a special care has to be taken while making the PDMS mix, with a base to curing agent weight ratio of 1. In the validated protocol, the polymer mix has to be degassed. This operation is realized within a vacuum bell in the laboratory. Once the polymer mix has regained the initial volume in the vacuum chamber, it is possible to remove it to fill the mold. The temperature setting for the external part is of 200 • C for 3h. However, going straight to this temperature will cause the formed PDMS to curve inside the mold. A solution is to put in a first oven at 100 • C for 1h then remove the PDMS from the mold and put it in form in the oven at 200 • C. Once the PDMS is treated at 200 • C, it becomes fragile, and easy to break, after the first passage at 100 • C, the PDMS is still soft enough to bend without breaking, which is easier to remove from the mold. However, the mold is composed of two parts, the bottom of the mold, with the texture, and a ring, which serves at setting the height of the external part. The part which needs to be removed is the bottom, with the texture and not the ring, this is important for the next part of the making.

The next operation is the making of the core polymer mix with a base to curing agent ratio of 20. This polymer mix will also be degassed within the vacuum bell. The main body of the artificial finger needs to be capped and filled upside down with the 1. ARTIFICIAL FINGER DESIGN polymer mix (fig. 1.14b). Then the ring with the previously made PDMS is used to close the bottom of the artificial finger (fig. 1.14c). The whole is turned so that the cap can be removed. Some polymer mix can be added to complete the artificial finger and a degassing will be executed. Then the artificial finger is closed on top with the part used to mount it on the tribometer (fig. 1.14d) and placed in an oven at 70 • C for 1h. This step is due to the structure not being completely hermetic and polymer fluid can flow out of the finger before the curing. This part is used to form a cured envelop of PDMS for the completion of the core curing. Then the artificial finger is removed from the oven and left at ambient temperature for 48h for the completion of the curing of the polymer. Friction investigation of the instrumented artificial finger

Fabrication protocol

Validation of the friction properties of the artificial finger

The artificial finger has been manufactured and needs to be tested in order to validate the correct reproduction of the friction behavior as depicted in reference (1). The artificial finger was mounted on the tribometer and used against several textile fabrics.

Description of the textile surfaces

The textile fabrics considered in this section are based on the ones used in (1) to compare the results obtained here with the ones provided for real human fingers. A plain woven fabric, a twill woven fabric and a velvet has been used to compare the friction properties of the artificial finger. The plain woven fabric is characterized by a symmetric behavior on all directions of sliding. The twill woven fabric is a fabric with grooves of one of the surfaces, the grooves are 45°to the direction of threads. However in this study, the sliding will be realized perpendicularly to the grooves. The velvet is a hairy textile which presents highly anisotropic behavior with the sliding direction. Two directions can be highlighted, the direction along pile and the direction against pile. This is a situation where there is a high difference in mechanical behavior between the two sliding directions of the reciprocating linear movement given by the tribometer.Thus,

FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER

during a sliding movement against the pile main direction, the pile lift the artificial finger, while in the opposed direction, the pile lie down for the artificial finger (75).

First friction experiments with the artificial finger

The experiments were realized with the tribometer and with the light measurement setup. The tribometer was set in a dead weight position, which means the normal load applied on the artificial finger during sliding is constant and corresponds to the weight of the mass put on top of the artificial finger. With this setup, the tangential force measured varies accordingly to the variations of the COF of the surface. If a surface has a high variation of altitude at one point, this will affect this proximity between the tangential force and COF. To verify the variations in altitude of the artificial finger, a laser measurement is located at the end of the arm maintaining the artificial finger, as explained in §1.1.

Friction experiment measurements

The results described here were obtained with the use of different artificial fingers build before the final protocol described in 1.3, the artificial finger used will be stated for each measurement investigated. Several artificial fingers were made during the work, the denomination for each finger is as follow :

• Slider 45p is the only artificial finger with a 3D printed body. The texturing of the stud was not in accordance with fingerprint dimensions: the depth of the ridges was of 0,5 mm and the periodicity of the ridges was 1 mm. There is no fiber guide in the connectors. The fiber angle was of 45°.

• Slider 45al is the artificial finger for testing the improvements of aluminum body frame. The texturing of the stud was realized accordingly to 1.3. There is no fiber guide in the connectors. The fiber angle was of 45°.

• Slider va (for adhere) is a second aluminum finger. A primer lining of the internal face of the aluminum body was realized in order to make the PDMS of the core adhere to the aluminum body. There is no fiber guide in the connectors. The fiber angle was of 45°.

Validation of the friction properties of the artificial finger

• Slider 45fg is another aluminum finger. The aluminum body was not treated with the primer. The fiber angle is of 45°, and the fiber guide is present in the connectors.

The connectors were also fixed to the body with a resin for metals. This is the design validated in 1.3.

• Slider 30 was realized accordingly to 1.3 and with a fiber angle of 30°.

• Slider 30 n2 is made with the same parameters as slider 30.

• Slider 30 arr is made with the same parameters as slider 30 but the stud was realized with rounded edges, maintaining the apparent contact area of 1 cm 2 .

All individual measurements realized with each artificial finger will not be displayed in this manuscript for readability. Most of the measurements realized with artificial fingers until slider 45fg were realized for comparison and amelioration of the design.

The measurements from slider 30 will partially be explained in this part, the most of the analyzing of the friction tests is discussed in later chapters as well as results from slider 30 n2 and slider 30 arr. As slider 45p was a proof of concept, it was not used for much testing on surfaces. Figure 1.13 was obtained with this artificial finger. The other uses of the artificial finger were for the improvement of the building, thus this was the purpose of the prototype slider. With this design, the importance of the correct attachment of the connectors to the build was observed. Indeed, with this slider the connectors could move and rotate in the build, which caused the light measurements to be highly different even on a same surface. The observation of a global variation of light power transmitted following the variations of tangential force motivated for the build of a more robust finger.

The comparison between both artificial fingers is made with figure 2 In figure 2.1, the graphic is obtained after compilation of the recording realized with the spectrometer, unlike figure 2.2 where the irradiance acquired is given by the Pulse acquisition setup and the evolution of signal is represented relative to displacement. It is noticeable that the sampling of both systems is not compatible, i.e. the Pulse system records with higher frequency than the spectrometer sends to the Pulse system. However, there is still a delay between the correct spectrometer acquisition and the setting of the analogical value for the Pulse recorder. The most relevant data sheet is to be extracted from the spectrometer. Another observation to be done is the comparison in friction behavior against the same fabric with real human fingers (1) (fig. 2.3). With real humans, it is difficult to impose a fixed position and/or normal load to the finger for a friction measurement.

Moreover, the stiffness of the medial-lateral joint of the finger produces the tilt observable on the graphic shape. After several tests on the plain woven fabric, the two sliders (45p and 45al) were compared and a difference in behavior was observed, the PDMS adhere to the printed body, but not to the aluminum body, which means a tilt is observable on the artificial finger stud depending on the friction direction (fig. 2.4).

The presence of the tilt on this particular slider was a source of questioning for the However, the tilt is not controlled and may be source of deviation in measurements, as observed on slider 45p, the tilt is non mandatory for the detection of the sliding direction, but due to the manufacturing process of slider 45p, the symmetry of all parts of the artificial finger is not assured. In addition, the stud texture has been improved for aluminum sliders. As a consequence the making and testing of an artificial finger with a primer lining of the internal face of the aluminum was realized, resulting in an artificial finger for which the PDMS core adhere to the aluminum body. In this situation, the PDMS of the artificial finger has low moving amplitude available. With low deformations, the functioning principle of the artificial finger may prove to be easier 2.1 Validation of the friction properties of the artificial finger to understand. A measurement realized with this configuration has however a major drawback (fig. 2.5), the difference between sliding directions is not very clear and the change in direction is not sufficiently distinguishable for an automatic computing without knowing of the sliding conditions. Moreover the variations obtained from a measurement on this fabric can't be used for a description of the surface, since none of the variations observed here have an explanation from the surface characteristics. This artificial finger can be accurate for specific surfaces, but can't be used for a description of an unknown fabric. Because of this observation, the primer lining of the internal face of the aluminum body has been abandoned. of constant normal force applied. The observed shape is similar to that of a real human finger against fabrics (fig. 1.8). The observed COF here is high and around 1. This result validates the artificial finger shape for the reproduction of finger friction behavior against hairy textiles. Apart from the velvet, which has a specific behavior resulting in the flipping of the pile on sliding direction changes, there is little shape specificity for both plain and twill woven fabrics.

Observations and difficulties

For the first results obtained, there is a high similarity between the friction signal obtained with the tribometer and the optical signal measured with the artificial finger.

A first observation that can largely be done is the signal variations given by the artificial finger. It is easy to understand the change in value for each sliding direction for the tangential force, since the direction is opposed, the direction of the friction force is also opposed. The change in signal sign is artificially made with the friction force sensor to show this change in direction. However, the light measurement can not be explained the same way because the signal can only be positive (light is transmitted) but changes in amplitude, meaning the quality of transmission differs for one sliding direction to another. This shows a possibility to detect the sliding direction with the artificial finger.

With this observation, it is important to compare the results obtained from different sliding directions with the initial static position in load. What is observed in this case is that one direction increases the light transmission and the other decreases the light transmission. In fact, the initial value of transmission is roughly centered between each value of transmission.

The observed difficulties were for the prototypes before the use of the resin to fix the connectors, deviation was due to the connectors rolling inside the main frame. A second difficulty was for the time synchronization for each measurement setup. In fact, the measurement realized with the tribometer was synchronized with the Pulse Recorder, while the light measurement was recorded with the proprietary software of the spectrometer on a separate computer. Two main problems come from this separation of the measurement. First is the determination of the start of the friction test. The two measurements setups have to be started separately, so one of the measurement starts before the second one. A trigger signal from the translating plate was used but caused problems for the creation of the measurement file for the spectrometer. The manufacturer was contacted

FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER

but no solution was found. The second problem was the sampling time. The analyzer had an internal clock and synchronized all signals for the tribometer with a specified sampling time. However, the spectrometer software was limited by the computing time of the software, all hardware components were sufficient to attain the sampling time of the analyzer, however, a computing of the software was necessary and was not stable in time for fast acquisition (0.5 kHz) and gave irregular results. The use of the analyzer to get the signal from the spectrometer was investigated, since an analogical output was present on the spectrometer. However, the analogical output was created through the software computing, thus, the analogical output still had the sampling problems from the software. The synchronization of the starting of the experiment was realized manually with an Excel macro. Because of this problem, the analysis of the results could not have a strong computing base, and only insights are given in this part. A new measurement bench will be described later to enable for a better analysis on the friction results.

The difficulties of the setup being discussed, there are still difficulties due to different artificial fingers and the interpretation of the measurement. The raw data from the irradiance coming from slider 45al and slider 45fg are similar in variation but not in value. Power transmitted from slider 45al is twice the power transmitted from slider 45fg (fig. 2.2 & 2.6). Also power transmitted through slider 45al is four times the power transmitted through slider 45p, this increase in transmission quality was attributed to the change in materials and the better alignment of the different parts of the artificial finger, however, the improvement of build should be in favor of slider 45fg over slider 45al, which in not observable through transmitted power value.

Added to that observation, the actual variation of irradiance transmitted depending on the sliding direction is problematic. Indeed, the artificial finger was made in order to have symmetry by the fiber axis and perpendicular to the fiber axis. Which means the transmitted irradiance should be comparable for each sliding direction with this setup. This is not the case here, even though the possibility to detect sliding direction is an interesting capability of the artificial finger. In order to better understand the behavior of the transmission within the artificial finger, a starting step is the test of the artificial finger under one type of force stimulation. The easiest setup to achieve this stimulation is normal force sensing, thus indentation/compression experiments were issued on the artificial finger.

Validation of the friction properties of the artificial finger

Normal loading influence on irradiance in the artificial finger

The testing of the artificial finger under normal load was issued on the machine developed and described in [START_REF] Melki | Etude du mouillage de structures fibreuses multi échelles: robustesse de l'hydrophobicité[END_REF]. The artificial finger is put upside down on a plate and a flat indenter goes down to press against the stud of the artificial finger. The setup records the position of the indenter and the force applied. The artificial finger is wired to the spectrometer and the irradiance is recorded. An Excel macro has been created to allow for the irradiance to be graphed relative to the indentation depth.

The graphics presented in fig. 2.7 show an interesting behavior of the artificial finger.

Until a penetration depth of 0.4 mm, which corresponds to 1.5 N, the irradiance increases with the load, then a plateau occurs for 0.15 mm of penetration then the irradiance decreases with the increasing load. The issue is the value of irradiance is equal for two distinct values of normal force apart of roughly 2 N. The artificial finger is supposed to be used under normal loads according to touch conditions which is around 0.5 N.

The artificial finger has a linear variation of irradiance with increasing normal load from 0 to 0.6 N, this can be sufficient but this is too restrictive for the use of the artificial finger. However, the forces employed here cannot help to understand the behavior of the artificial finger during friction, which is the distinction of each sliding direction. The tilt explained earlier (fig. 2.4) is an hypothesis for an explanation of this behavior. It can be observed that during the tilt, even with a constant normal force, the height of PDMS differs on both sides of the artificial finger because the connectors have a fixed position within the artificial finger which is not the case for the PDMS where a movement has been allowed, and by extension, the POF also can move inside the artificial finger. In order to replicate this difference in height during tilt, the artificial finger was indented on both sides of the stud at the connector position.

The situation depicted in fig. 2.8 shows an asymmetric behavior of the artificial finger for each side. However, for low deformations (under 0.2 mm) the difference in behavior cannot explain the detection of the sliding direction: 2.0 mW/cm 2 for the direction according to fig. With the previously given information about the asymmetry of the artificial finger, it can be helpful to change to artificial finger direction on the tribometer. The reference will be the optical path of light, there is an input and an output on the artificial finger and the first sliding direction will be changed for comparison (fig. 2.9).

The influence of the change described in fig. 2.9 is highlighted by fig. 2.10. In this graphic, the upper and lower values of irradiance due to the direction change are opposed in the time relative curve, but correspond to the same sliding direction in the artificial finger reference. This curve shows two main behaviors of the artificial finger.

First, the detection of the sliding direction is effectively related to the sliding direction of the artificial finger, which confirms the hypothesis of asymmetry in the artificial finger system. Second, the variations observed on each sliding distance shows the same profile, this means that for the same position on the surface, the measurement of the artificial finger is equivalent. This is an important observation since this confirms the objective of having a sensor sensitive to the tested surface.

FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER

A A B B dir.1 dir.2
The artificial finger is shown to be sensitive to the tested surface but the variation of the irradiance measured needs to be explained. The friction measurement on the plain woven fabric shows a smooth profile in terms of tangential force (fig.

Optical fiber description and behavior

The principal component of the instrumented artificial finger is the optical fiber. This POF serves as a sensor for the measurement. It has been formulated that all variations in light transmission are due to the POF deformation. However, it is important to describe the working principle of optical fibers.

FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER

An optical fiber serves as a guide with light transmission. The functioning principle of the transmission relies in the equation from Snell and Descartes :

n 1 n 2 • sin(i 1 ) = sin(i 2 ) (2.1)
In which:

• n 1 is the refractive index of environment 1

• n 2 is the refractive index of environment 2

• i 1 is the angle of the incident light ray to the normal of the interface of environment 1 and 2

• i 2 is the angle of the refracted light ray to the normal of the interface of environment 1 and 2

This formula (2.1) describes the change in light direction when a ray encounters an interface between two distinct environments. The ray is deviated at the interface according to the Snell and Descartes formula. The refracted ray corresponds to the major part of the light energy to be transmitted through the interface, however, there is always a part of energy which is reflected on the surface and stay in environment 1. What is interesting is the limit of the formula. Indeed, when n 1 n 2 is above 1, there is a possibility for the left part of the equation to be above 1 for high value of incident angle. In this situation the right part of the equation sin(i 2 ) cannot exceed 1, thus this translates physically as the absence of refracted light ray, all the power is reflected and stay in environment 1. For an optical fiber, the environment 1 is the core of the fiber and the environment 2 is the cladding of the fiber (fig. 2.11). Optical fibers are classified in several types: the single mode or multi mode fibers and step index or gradient index fibers. Single mode fiber means that the fiber allow for only one optical path to be followed in the fiber.

Step index fibers corresponds to the situation in fig. 2.11 in which the different environments of the fiber have constant refractive index within one environment. The interface between the two environments correspond to a step change in refractive index and is the only change in direction of the light ray. For gradient index fibers, the refractive index inside the core of the fiber is gradually decreasing with the distance to the axis of the fiber, resulting in multiple changes in the light ray direction (fig. 2.12). A parameter for 2.2 Optical fiber description and behavior the characterization of optical fibers is the numerical aperture, this dimensionless value correspond to the sine of the admission angle of the fiber times the refractive index of the environment of the fiber, in this case, it is air. The admission angle is the highest angle with the fiber axis for which the light ray will be transmitted through the fiber.

In other words, this is the maximal angle θ with

n 1 n 2 • sin( π 2 -θ) ≥ 1. The maximum angle θ max is obtainable with : n • sin(θ max ) = n core 2 -n clad 2 . i 1 i 2 NA Figure 2
.11: Schematics of an optical fiber with two rays entering the fiber core. One is inside the numerical aperture of the fiber and is reflected inside the fiber, the other is out of the numerical aperture and is refracted out of the fiber.

For the description of the fiber, there is also the need to describe the cladding, which is the external environment of the fiber core. The cladding parameters will take effect as stated above for the numerical aperture of the fiber (higher numerical aperture results in higher transmitted power in this case) and, will have effects in the loss quantity during bending [START_REF] Durana | Dependence of bending losses on cladding thickness in plastic optical fibers[END_REF]. With a thin cladding, there is a possibility for a light ray to re-enter the fiber core after having left the core because of refraction (fig. 2.13.) in the case of this artificial finger, the fiber core does not have a cladding, the external environment of the fiber core is the PDMS of the artificial finger, this can be considered as an infinite cladding because of the large dimensions of the PDMS core. As shown in [START_REF] Durana | Dependence of bending losses on cladding thickness in plastic optical fibers[END_REF], with a cladding thickness superior to 4mm, the transmission curve converges with the infinite thickness curve. In other words, light which exits the fiber core will not be redirected into the core because of reflections in the cladding.

The results showed in [START_REF] Durana | Dependence of bending losses on cladding thickness in plastic optical fibers[END_REF] describe the behavior of an optical fiber during a bending situation. It is easily understandable and intuitive that the higher the bend on an optical

FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER

Figure 2.12: Schematics of fiber presenting different modes and of step index of gradient index fiber there is, the higher are the losses due to the bending, which is explained in [START_REF] Durana | Dependence of bending losses on cladding thickness in plastic optical fibers[END_REF]. This was also the idea retained for the functioning of the artificial finger, during friction, the forces applied on the finger induce deformations on the artificial finger and by extension to the fiber. With more deformations the observed losses should be higher. This is not what is described in fig. 2.7, the normal force applied increases with deformation as depicted by the penetration depth and the stress strain curve, but the transmission does not evolve accordingly to the increasing deformation. Moreover, there are no insight on reasons for the deformations to be non-symmetric, thus a decrease in transmission power, or irradiance, should be observed for each sliding direction, which is not currently the case. These two information showed the need to try to better understand the functioning of the optical fiber. A measurement device was realized for the testing of the optical fiber in the same restraining conditions as in the artificial finger in terms of fiber angle, but there will be no PDMS as external environment of the optical fibers. The measurements will be realized on bare fibers.

2.3 Experiments realized on a bare fiber locked in a specifically made frame by a distance of 5 mm. The indentation will be realized manually with translating plates.

The penetration depth will be set as with previously mentioned indentation tests. The maximum normal force registered was associated with a penetration depth of 0.8 mm, the same distance will be applied with the translating plates. The 0.8 mm are applied after the contact is observed. The translating plates are equipped with micro-metric screws to enable the measurement of the penetration depth. A picture of the POF in each position is made and the value of irradiance transmitted is recorded for interpretation.

All pictures were acquired with the same light parameters and the frames were made as to be able to reuse the same POF sample in different positions so that all differences in output power will not be due to differences in POF quality. A simple observation on the pictures of the POF in each frame (fig. 2.15) is enough to foresee a difference in transmitted light within the fiber, as the POF in the 60°frame seems brighter, this means a higher amount of power exits the fiber at the curvature level resulting in lower transmitted power. This accords to the optical fiber behavior in bending depicted in [START_REF] Durana | Dependence of bending losses on cladding thickness in plastic optical fibers[END_REF].

As expected, there is a decrease in transmitted irradiance with the higher curvature 2.3 Experiments realized on a bare fiber locked in a specifically made frame imposed on the POF (fig. 2.16). The curvature has been measured by using a graphical tool on the pictures taken with the camera placed on top of the pressing setup. The measurement protocol was realized with the bottom part of the POF approached with a circle realized with the graphical tool. The radius of the circle was then measured and the actual value of the curvature radius was obtained by using the known fiber length which is 20 mm as in the artificial finger (fig. 2.17).

Each situation has been realized 5 times for a statistical analysis. The pressing realized at the center of the fiber was realized in order to simulate the deformation of the fiber while being used for normal force sensing. The maximal penetration depth of 0.8 mm used corresponds to the application of a normal load of 4.5 N as in the indentation tests realized. The application of a deformation with an offset of 5 mm relative to the centered mark was used to simulate the fiber deformation while a friction occurs on the artificial finger, i.e. which is subject to the application of tangential forces.

The first observation on the deformation of the POF shows a different behavior for the two types of stimulation. When a centered load is applied on the POF, the curvature of the POF is split in different parts : there are two high curvature zones link by aligned parts of POF. The curvature radius value of the two zones of curvature is higher than for the fiber without applied force. However, the length of POF subject to the curvature is lowered (fig. 2

.18).

As opposition to this situation, the pressing of the fiber at an offset position generates a part of linear fiber and only one part of curved POF (fig. 2. [START_REF] Thieulin | Study of the tactile perception of bathroom tissues: Comparison between the sensory evaluation by a handfeel panel and a tribo-acoustic artificial finger[END_REF]). The difference in deformation for the POF in the two considered types of simulation can indicate a difference of behavior in the artificial finger use for the type of measurement to be made. A difference in behavior is expected to be seen for centered and offset positions of pressing.

However, there is no specifically different deformation behavior for each offset situation, as a consequence, the proximity of the measurements cannot explain a difference of behavior as observed for the friction of the artificial finger. There is the possibility that the symmetry has been respected for the pressing tests but there are still some uncontrolled positioning in the making of an artificial finger.

A comparison of the measurement has been realized for each frame, the objective was to determine the influence of the initial curvature on the irradiance transmitted by the system for a given deformation. 2.3 Experiments realized on a bare fiber locked in a specifically made frame As expected, there is two behaviors of the POF with these experiments, the central load and the offset loads. Also, the two offset positions behave relatively symmetrically.

The difference in power transmission is of 10% apart but the deviations measured can cover this difference in mean value. Thus the two situations concerning the simulation of the friction motion of the POF are equivalent. This is in disagreement with the results obtained with the artificial fingers, and the detection of the sliding direction. The tests realized here show the symmetry that should be observed with sliding direction.

FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER

The conclusion to be made here is the either inaccuracy of the experiments to show the behavior of the POF during the friction, or that unexplored parameters regulate the behavior of the artificial finger. To discuss the validity of the experiments, it can be said that the main difference between the tests and the actual composition of the artificial finger is the presence of the PDMS core in the artificial finger that is responsible on the motion of the POF in the artificial finger. Next is the actual method used for the simulation of the friction situations. The POF was pressed with an indenter perpendicularly to the direction that should be the forces of friction forces, and lastly, the distance of deformation is not verified. The penetration depth used is 0.8 mm and corresponds to the penetration depth obtained for the indentation tests realized on the artificial finger for normal load sensing, the deformation of the POF inside the artificial finger during friction will not be of 0.8 mm deep in the direction of the friction forces.

Moreover, the force to be used for friction tests is only around 0.5 N, and will not cause deformations this high. Nonetheless, the observed behavior should depict the situation for high forces applied on the artificial finger, and should describe the amplitude of variations that can be observed with the artificial finger.

The graphic (fig. 2.20) enables the distinction of the two types of stimulation and highlights the possibility to adjust the sensitivity of the artificial finger to a specific situation with the initial curvature of the POF. Indeed, the curve shows a better sensitivity to be attainable with the 30°fiber angle artificial finger for shear forces sensing, while a higher fiber angle, 60°, seems more suited for normal load sensing. Based on this observation, a new artificial finger with a fiber angle of 30°is made to verify the compatibility of this result with the actual making of an artificial finger.

Friction tests with a 30°fiber angle designed artificial finger

The tests realized on the bare fiber experiments showed the influence of the initial POF curvature on the sensitivity to be attained with the artificial finger. The results obtained lead to the making of a slider with a 30°fiber angle. This artificial finger is called slider 30.

2.4

Friction tests with a 30°fiber angle designed artificial finger

Comparison of the results form 30°fiber angle artificial finger with 45°fiber angle artificial finger

The results obtained with the bare fiber experiments needs to be verified, thus, a test showing the differences of a friction test realized with the same parameters in terms of speed, normal load and surface type has been held with both types of artificial fingers (fig. 2.21). On this graphic two points can be observed. First, the result expected after the bare fiber tests is verified, the artificial finger with a fiber angle of 30°has higher amplitude of deviation for the same friction as an artificial finger with a fiber angle of 45°. Moreover, with slider 30 initial value being higher (around 35 mW/cm 2 for slider 30 against 25 mW/cm 2 for slider 45fg), the same percentage of variation translates as a higher amount of irradiance variation. Next is the asymmetry of the artificial finger. The artificial finger has been validated with a higher amplitude of variations and

FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER

will be tested on various surfaces in attempt to show a specific behavior depending on surface properties. The same fabrics as used with slider 45fg will be tested with slider 30 to validate the shape of curves previously obtained (fig. 2.22). Slider 30 follows the same manufacturing method as slider 45fg but with a fiber angle of 30°.

As can be observed on the measurement realized on the fabrics with slider 30, the previously observed conclusions are maintained. The artificial finger is sensitive to the sliding direction, a correlation between the tangential force and the irradiance measurement can be assumed given the shape of the curve, however, the change in sign for the tangential force brings difficulty in the correlation establishment. For the case of velvet, the peaks relative to pile flip are present. However, the variation of the tangential force and the irradiance are not exactly stackable. The graphic from cotton twill woven fabric in particular has high peaks on the irradiance measurement. Some peaks are reported on the tangential force measurement but not all the variations. The question of the source of these variations is relevant, since these variations are not present on the other fabrics.

There is however a slight variation along a course on the plain woven fabric which is not present in the tangential force measurement. In order to verify if the source of these variations comes from the contact with the fabric, which would mean the artificial finger is actually able to sense contact related deformations that the remote force sensors are not able to detect, the testing of a neutral surface has been made. Thus, a piece of blank paper was used to verify the behavior of the artificial finger on a smooth surface (fig.

2.23).

This friction test is reassuring in the sense that the smooth surface cause a smooth measurement. The tangential force is horizontal for each course. However, there is a slow progression in the irradiance measurement. This cannot be explained as the tangential force shows a steady value for each direction. By the shape of the irradiance measurement signal, the possibility of a viscous phase coming from the PDMS has been investigated.

To verify the time relative artificial finger behavior, the same testing has been realized with different sliding speeds (fig. 2.24). The results from these measurements show that for each sliding speed, the time needed to attain the stabilized value of irradiance differs for each measurement. In fact, the progression corresponds for each measurement to a sliding distance of 20 mm. Because the irradiance progression is displacement related, the artificial finger is considered purely elastic in the domain of touching and has no viscous behavior in the presented study. The behavior of the artificial finger on textile fabrics shows variations that could be associated with variations of friction during sliding, thus paper with blank and printed zones will be used.

First, it is important to measure the difference of COF between blank and printed zones. For that purpose, a piece of paper was printed on half the sliding distance allowed by the tribometer and the other half sliding distance is blank. Friction with the artificial finger was realized on this paper and the result is shown fig. 2.25. The signal recorded shows a slight increase in tangential force on sliding on the printed paper. The irradiance seems to be following that tendency but it is hard to perceive since the global variation of irradiance is not smooth as seen on the blank paper (fig. 2

.23). A change in the

surface parameters was realized in order to try to increase this behavior. The paper was printed with black squares of the artificial finger stud size. On this test, the influence of the friction properties of the surface on the irradiance transmission can clearly be seen (fig. 2.26).

On this graphic, the correlation between friction and irradiance is better highlighted.

With this observation, the functioning of the artificial finger can be simplified as having higher variations of irradiance transmission with higher forces applied on the artificial finger. The problem of the study of the behavior of the artificial finger remains in the quantification of the variations of irradiance transmission. As were seen on the normal force experiments on the artificial finger, the increase of force does not necessarily mean a decrease of irradiance, as can be concluded from the functioning of optical fibers and the tests on the bare fiber. The artificial finger is showing a complex behavior, that cannot be studied with simplifications of the different parts of the system. The artificial finger needs to be studied as a whole.

A testing of the capability of the artificial finger was then decided, and several pieces of paper were realized in order to determinate the detecting capability of the artificial finger. Pieces of paper were printed with a line with different width to show the size threshold of detection for the artificial finger (fig. 2.27).

The line of 1 mm was not detected by either a change of friction (tribometer signal) or a change in irradiance. The lines of 2.5 and 5 mm were detected by both systems.

The artificial finger actually detects changes in friction occurring on a distance lower 2.4 Friction tests with a 30°fiber angle designed artificial finger Figure 2.26: Relative irradiance and tangential force relative to time for slider 30 against a printed piece of paper with alternative black and blank squares. The tangential force associated with black squares is higher than blank squares.

than that of the contact area, which shows capability of the artificial finger to sense surfaces. An interesting fact is the presence of the peak due to the line on the top part of the curve for the irradiance measurement corresponding to only the second sliding direction of the motion of the artificial finger on the paper. The graphics also show that the peak is lower for the bottom part of the curve for the tangential force measurement.

It is possible that the artificial finger detects a step due to the presence of the ink on the paper, and not the friction due to the change of COF on the printed paper. To verify this situation, the measurement was realized on the paper with the 2.5 mm line and some adhesive tape was used to cover the line and paper. The COF is between the adhesive tape and the PDMS for all the measurement, with the printed line that could be a step (fig. 2.28). On this measurement is observed a punctual increase of the signal for both tangential force and irradiance at the passage of the artificial finger on the printed line. The result obtained here shows the difficulty to interpret the origin of the observed situation, the artificial finger was able to sense a change in the tested surface and the tribometer also showed a change in tangential force. However, the normal load 2.4 Friction tests with a 30°fiber angle designed artificial finger is constant. With the calculation realized, the COF will be stated as increasing on the passage over the line but the use of rubber band should prevent any change in COF. As a conclusion, the sliding on the line cause a vertical displacement of the artificial finger and the line is an obstacle in the motion, this causes the artificial finger to have a higher tilt of the passage on the line, resulting in a deformation that can be measured by the irradiance and with tangential force measurement.

Setting of a new measurement bench

The tribometer used had a linear translating plate with a translating distance of 50 mm, to increase the steady state of the friction, the increase of this distance is mandatory.

The force measurement relied on a tangential force gauge sensor placed on the arm on which the artificial finger is stuck and the normal force sensor was placed on the other side of the arm, which means the force sensor were far from the contact zone of the artificial finger and the textile. Moreover, the tangential force sensor was an element that forbid the rotation of the artificial finger, in this situation it was only possible to measure the friction with the axis of the POF having an angle of 0°with the sliding direction. In order to improve the situation, a new tribometer was realized to enable the

FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER

sensing of surface with an oriented artificial finger and for longer distances. The angle of the POF axis with the sliding direction will be called the POF angle. Figure 3.1 depicts a POF angle of 90°and figure 3.2 depicts a 0°POF angle. The force sensors were also replaced and a tri-axis force sensor was placed below the tested surface, enabling for a nearer sensing of the friction.

Because of some difficulties to obtain accurately sampled measurements with the spectrometer, the use of a photodiode was preferred. A photodiode 918D-SL-OD3R manufactured by Newport has been used. The response time of the photodiode has been selected to correspond to the sampling of the Pulse acquisition system used with the tribometer. The measurement realized with the new tribometer is entirely acquired with the Pulse acquisition system, which allows for an accurate sampling of each signal considered, i.e. normal force, tangential force, irradiance and translating plate position.

The setup as realized here enables the averaging of friction tests realized with same conditions for statistical purposes.

FRICTION RESULTS OBTAINED WITH SOME TEXTILE SURFACES

Artificial

New tribometer used and results from friction tests

The developed tribometer was necessary to access statistical analysis of the friction measurements and proper linking of the results provided by the artificial finger and the tribometer. With the use of a single acquisition interface, is was possible to access this type of interpretation on the results. The friction tests presented in Chapter 2 were used to improve the characteristics of the sensing protocol and the repeatability of the manufacturing of the artificial finger. In this chapter, the signals obtained have the same sampling and are synchronized and thus can be correlated and provide a better understanding of the measurement of the artificial finger.

Characteristics of the new tribometer

In the previous chapter, the tribometer used had a linear translating plate with a translating distance of 50 mm. To increase the steady state of the friction, the increase of this distance is mandatory. Moreover, the force measurement relied on a tangential force gauge sensor placed on the arm on which the artificial finger is fixed and the normal 3.1 New tribometer used and results from friction tests force sensor was placed on the other side of the arm, which means the force sensor was far from the contact zone of the artificial finger and the textile. In another way, the tangential force sensor was an element that forbid the rotation of the artificial finger, in this situation it was only possible to measure the friction with the axis of the POF having an angle of 0°with the sliding direction.

In order to improve the situation, a new tribometer was realized :

• The maximum sliding distance is of 100 mm.

• To enable the sensing of surface with an oriented artificial finger and for longer distances. The angle of the POF axis with the sliding direction will be called the POF angle. Figure 3.1 depicts a POF angle of 90°and figure 3.2 depicts a 0°POF

angle. The force sensors were also replaced and a tri-axis force sensor was placed below the tested surface, enabling for a nearer sensing of the friction.

• As explained in the previous chapter, with the spectrometer used in the previous chapter, it is not possible to accurately sampled measurement, therefore the use of a photodiode was adopted. A photodiode 918D-SLOD3R manufactured by Newport has been used. The response time of the photodiode has been selected to correspond to the sampling of the Pulse acquisition system used with the tribometer.

The signals obtained from this new tribometer is entirely acquired with the Pulse acquisition system, which allows for an accurate sampling of each signal considered, i.e.

normal force, tangential force, irradiance and translating plate position. The setup as realized here enables the averaging of friction tests realized with same conditions for statistical purposes. is the tri-axis force sensor and c) is the translating table. On this picture, the POF angle is 90°relative to the sliding direction.

FRICTION RESULTS OBTAINED WITH SOME TEXTILE SURFACES

Method for testing the textile surfaces

The artificial finger has a set altitude and the changes of normal force are measured with a sensor. The access of the COF is made with the ratio between the values of tangential force and normal force (COF = F T F N ). This method has been proven to show better reproducibility with hairy textiles and changes of COF over surfaces during the tests for [START_REF] Camillieri | Artificial finger design for investigating the tactile friction of textile surfaces[END_REF]. The position of the sensors is also different. As explained above, the sensing of forces will be made by a tri-axis force sensor placed under the support for testing material. In order to prevent any influence of the support of testing material on the profile of the testing material, the support will be a plate made of glass. The glass plate will be mounted on a specific 3D printed holder realized for the tri-axis force sensor.

Moreover, instead of the spectrometer, a photodiode has been adopted. The use of a photodiode brings the advantage of fast response, here lower than 2 µs which enable an acquisition up to 500 kHz, and directly wired on the acquisition setup. However, the main disadvantage of the photodiode is the loss of the information of the wavelength of the acquired signal. The photodiode transforms all light received into electrical current which is proportional to the light power received, this is linked to the irradiance since the power received is transformed only for the light acquired on the photoreceptor, which has a finite dimension of 1 cm 2 . The irradiance measured is an integration of the transmitted irradiance by the artificial finger times the responsivity function of the photodiode.

Because of the responsivity function (fig. 3.3) of the photodiode, a verification of the sensitivity of the system has to be realized. Indeed, because of this responsivity function, the measured signal can be sensitive to the wavelength of the transmitted light. However, the artificial finger measurement with this setup should not be sensitive to the color of the tested fabric.The evaluation of this sensitivity has been done with a measurement realized on a sheet of paper printed with different colored stripes (fig. 3

.4).

As can be seen on the results provided, the error bars present on the graphic correspond to 95% of confidence interval. It can be concluded that there is no statistical difference between each measurement realized on the different colors. The main issue was for the red color, which was the color which could reflect the more power from the source light which is from a hallogen bulb, and the sensitivity of the photodiode to red wavelengths.

Since this issue has been verified, this allows for further study on textile fabrics with this measurement setup.

FRICTION RESULTS OBTAINED WITH SOME TEXTILE SURFACES

0.00E+00

Acquisition results

It has been said that the obtained signal from the photodiode is an integration value of the received irradiance, thus, the irradiance term will be kept for comprehension but the value displayed will be the measurement of the acquisition setup in Volts. The sampling for the acquisition has been set on 200 Hz.

The measurement will provide four signals: the normal force measurement, the tangential force measurement, the irradiance measurement and the position of the translating plate. The force measurements and irradiance measurements will be traced relatively to the position of the translating plate. The corresponding COF to a friction test will also be traced relatively to translation displacement. Thus, four graphics are generated from the measurements. A measurement is started with a trigger signal emitted by the translating plate at the beginning of the motion. The acquisition time is set to acquire at least 3 cycles of the reciprocating movement, this is because of the velvet for which the first cycle has a combing effect on the fabric pile and the second cycle is considered for stabilization.

In this chapter, the artificial finger is slider 30 (fig. 3.1).

Testing of textile surfaces

With this new measurement setup for testing the artificial finger, it is important to make the friction tests again on the fabrics with the new setup and with consideration of the slider angle.

Friction on the Pekin fabric

The fabric used for determination of the influence of surface parameters has changed, the cotton twill fabric was interesting but a fabric with more accessible description of grooves has been selected, a Pekin fabric is used for the detection of grooves on the surface (fig. 3.5). A Pekin fabric is a woven fabric for which the weaving pattern is composed by two or more weaving patterns.

The grooves on the surface of the Pekin fabric have a spatial resolution of 7 grooves/cm, this is in the same order of magnitude as the cotton twill fabric. This is a reason for which the Pekin fabric has been chosen to replace the cotton twill fabric. The graphics of fig. 3.6 show the results of a friction with the new tribometer for the Pekin fabric.

FRICTION RESULTS OBTAINED WITH SOME TEXTILE SURFACES

Testing of textile surfaces

This test has been realized with an orientation of the slider as the POF angle with the slider direction is 0°. The presented graphic is an averaging of 3 different measurements.

The fabric was not removed and replaced between measurements to keep the relative position of the grooves with the translating plate encoder.

One of the observations that can be made, is that contrary to the cotton twill fabric, the signal obtained here show rapid variations within one sliding direction. This is due to the measurement of the normal force which requires the artificial finger to be fixed at a given altitude. In the previous chapter, the measurement presented for the cotton twill fabric was made under constant normal load with a dead weight. The signal shows the quantity of variations per length unit corresponds to the size of the grooves. A Fast Fourier Transform (FFT) has been applied on the signal and shows a frequency of 7Hz corresponding to the detection of the grooves with the speed of 10 mm/s (fig.

3.8a). With this observation, the artificial finger is able to detect the grooves on the

Pekin fabric. There is an observable variation in height of the signal, this follows the normal force signal which translates a default in the horizontality of the fabric support.

However, one of the advantages of the setup with a fixed height of the artificial finger, is the accurate measurement of the COF for the friction test. The graphic representing the variations of COF depicts the same variations for the grooves but no real change in mean COF value, which is concordant to reality since the fabric is consistent for the whole length of the friction test. This error in the experiment however brings an important information, this COF remains constant in mean value for the whole measurement, but there is a decrease in irradiance signal corresponding to the bottom side of the support, and associated with a lower normal force. This observation is important for the building of a calculation method which will be explained later. It is interesting to compare the friction results obtained in fig. 3.6 with the ones obtained in fig. 3.8 where the POF angle has been changed to 90°.

The force measurements in fig. 3.8 are very similar to the ones in fig. 3.6. This is a very reassuring observation given the conservation of experiment parameters, POF angle excluded. The COF has the same value near 0.5 and the fast variations induced by the grooves texture are still present but with a lower amplitude for the normal force, 0.04 to 0.08 mN for the POF angle of 90°versus 0.08 to 0.12 mN for the POF angle of 0°. Concerning the irradiance measurement, the differences in signal show that the orientation of the artificial finger influences the measurement. This is expected since the

FRICTION RESULTS OBTAINED WITH SOME TEXTILE SURFACES

POF has an orientation along its axis and the artificial finger actually have two planes of symmetry, along the axis of the POF and along a diameter perpendicular to the POF axis.

On the irradiance measurement, the results are interesting and provide some information on the artificial finger behavior. First, the difference in mean value for the two sliding directions is higher for the POF angle of 90°with 4mV against 1.5 mV for the POF angle of 0°, which can be associated to a better detection of the sliding direction.

Second is the lower visible amplitude of the grooves, however a FFT calculation on the signal (fig. 3.8b) reveals this detection. The interpretation to be made at this point for the behavior of the artificial finger is the dependence of the orientation of the artificial finger to obtain a specific information about the friction:

• A POF angle of 90°gives a better detection of the sliding direction.

• A POF angle of 0°gives a better detection of the texture.

• Really importantly, the force measurement realized with the tribometer are similar, which shows the friction is equivalent.

This can be approached to a natural finger behavior for which a friction along the proximal-distal axis gives a different measurement than along the antero-posterior axis because of joint stiffness (unpublished results from LPMT). In other words, the artificial finger is able to give different results depending on the orientation of the artificial finger with conservation of the friction parameters. With this observation, it is possible to say that the artificial finger is very sensitive to the orientation of the friction. The artificial finger then measures a composition of friction and parameters from the contact or orientation of friction. Testing other surfaces is important to observe another influence of the POF angle on the friction measurement.

FRICTION RESULTS OBTAINED WITH SOME TEXTILE SURFACES

FRICTION RESULTS OBTAINED WITH SOME TEXTILE SURFACES

Friction on the velvet fabric

The velvet is a fabric which presents hairiness on its surface. The side picture in fig.

3.9 shows the orientation of the hairiness and illustrates the along pile and against pile directions used to qualify the motion during experiments. As seen for the friction measurements on the Pekin fabric, the results obtained from the two slider directions for the velvet are very similar. Qualitatively, there is nothing specific to observe on the force measurements for the comparison needs of the POF

Testing of textile surfaces

angles of 0°and 90°. However, the irradiance measurement, that can be similar in both cases at first, actually presents a difference. Indeed, for the POF angle of 0°, when the slider moves against pile direction the irradiance is higher than along pile direction while the evolution is reversed for the POF angle of 90°. This shows the influence of the orientation of the motion for the measurement. In the case of the velvet where the sliding is mixed with high changes in normal force the influence of the direction is increased in comparison with the Pekin fabric for which the normal force value are similar for both sliding directions.

The observation realized so far show a high similarity between the irradiance and the force measurement. The new system enables the computation of a fitting between each measurement realized.

FRICTION RESULTS OBTAINED WITH SOME TEXTILE SURFACES

FRICTION RESULTS OBTAINED WITH SOME TEXTILE SURFACES

Relation between force measurement and irradiance by means of a fitting method

The correlation between the irradiance and the tangential and normal forces has been observed and can be explained by the fact where the artificial finger is deformed during sliding because of a combination on friction and normal forces. Therefore, a relation between irradiance and these forces can be expected. A fitting method to express the irradiance in terms of variations of normal and tangential forces is then presented.

The fitting method used relies on the least squares analysis. The difference of the square values of the obtained fitting and the irradiance is calculated and a calculation is realized to obtain the minimal value of the sum of the square differences between the two functions.

The building of the function was realized with consideration of the irradiance measurement. First, the irradiance measurement cannot be negative and is not likely to approach zero because the level of deformation of the fiber to observe no irradiance cannot be attained within the spatial restrictions of the artificial finger. As observed on the measurements with the spectrometer and with the new setup, the irradiance oscillates by a few percents during an experiment. Moreover, without any force applied on the artificial finger, the irradiance has a set value which means the fitting needs to have a constant. The next consideration is the implementation of the forces in the calculation.

The hypothesis made was the non sensitivity of the artificial finger to all external factors except the deformations. For this reason, it has been considered that the artificial finger responds to a resulting force of the tangential and normal forces. However, as explained with the measurement with the Pekin fabric, the signal obtained with the artificial finger changed with the altitude of the fabric support while having a constant COF. This means the artificial finger is not equally sensitive to normal and tangential forces, therefore a weighting factor has been considered between the two forces. The last point to be considered in the building of the fitting function is the detection of the sliding direction. The measured forces for each sliding direction is the same in absolute value, which means a correction has to be added to the calculation for the signal to accord to the differences of amplitude observed for each sliding direction with a constant COF as observed on the Pekin fabric for example.

Relation between force measurement and irradiance by means of a fitting method

The obtained function is written as follow:

Irr(F T , F N ) = c + d • F T 2 + k 2 • F N 2 (3.1)
With:

• F T is the tangential force (N)

• F N is the normal force (N)
• c is a constant to consider the initial state of irradiance which is not 0 (V)

• d is a unit coefficient converting the force in N into the irradiance in V

• k is the weighting coefficient (dimensionless)

The application of this fitting was realized with Microsoft Excel™ Solver® plugin.

The fitting method used is the non linear GRG with centered derivative. The indicator considered for the fitting quality is the coefficient of determination R 2 . The calculation has been realized under several conditions with an averaging of measurements for the velvet fabric (fig. 3.10).

The calculation output was characterized with a coefficient R 2 = 0.99. The high value of R 2 shows, as expected, the irradiance measurement is actually correlated to the forces applied on the artificial finger.

The application of the fitting method was realized for the Pekin fabric. The fitting produced a coefficient R 2 = 0.05. This was to be expected since the calculation cannot take into account the sign of the tangential force which corresponds to a different deformation of the POF in the artificial finger. This was corrected with the definition of a coefficient c i for each sliding direction. The determination of the c i coefficient is made for each sliding direction separately. Indeed, such a coefficient is required since the absolute or square values of each forces are equivalent for both directions in the case of the Pekin fabric. This would not allow for a difference in amplitude as observed in the measurements.

The obtained value of R 2 after the correction is 0.95. Again the correlation between the forces and the irradiance is observed.

It is now important to take a closer look at the coefficients obtained to understand

FRICTION RESULTS OBTAINED WITH SOME TEXTILE SURFACES

their physical signification. The coefficients c i calculated for both cases (Pekin fabric and velvet fabric) were near 0.086. After investigation on the artificial finger behavior, it has been observed that 0.086 is the irradiance value obtained with the artificial finger loaded with the initial load of 0.5 N before the sliding. However, the calculation imposed a change of this value for each sliding direction, which this cannot only be the irradiance with a load of 0.5 N since this value is constant. However, the use of this value for the computing of the fitting can be argued and has been kept. The c i coefficient is maintained for calculation according to the sliding direction. The writing of the equation has been changed as follow:

Irr(F T , F N ) = Irr 0 • (1 + c i ) + d • F T 2 + k 2 • F N 2 (3.2)
With a closer look at this equation, there is a concerning point. The term Irr 0 translates the irradiance transmitted with the initial load, i.e. 0.5 N in our case. However the term F N takes into account the initial load of 0.5 N. In other words, the term under the square root should translate the variations of force around the initial situation, which is true for F T starting from 0 but the normal force is supposed to oscillate around 0.5 N, thus a new term F N 0 is introduced and amputated to F N .

The final considered fitting function is then:

Irr(F T , F N ) = Irr 0 • (1 + c i ) + d • F 2 T + k 2 • (F N -F N 0 ) 2 (3.3)
With:

• F T is the tangential force

• F N is the normal force
• Irr 0 is the initial value of irradiance with the initial load F N 0 applied

• F N 0 is the considered initial load for the testing, in this work 0.5 N

• c i is a constant associated to the sliding direction

• d is the unit coefficient converting the force in N into the irradiance in V

• k is the weighting coefficient 3.3 Relation between force measurement and irradiance by means of a fitting method

Coefficients obtained k c 1 c 2 d R 2
Velvet fabric (POF angle 90°) 10.86 -0.008 -0.002 0.0003 0.87 Velvet fabric (POF angle 0°) 12.69 -0.060 -0.029 0.0005 0.97 Pekin fabric (POF angle 90°) 5.20 -0.002 0.001 0.0009 0.96 Pekin fabric (POF angle 0°) 5.78 -0.039 -0.024 0.0011 0.92 Table 3.2: Recollection of the obtained coefficients for the tested fabrics .

There is still the need to describe one of the coefficients of the fitting. The weighting coefficient, k, is one of the potential coefficient to be used for the description of surfaces with the artificial finger. Indeed, this coefficient translates a preferential influence of F T or F N in the calculation of the irradiance. Thus, two cases should depict different cases.

The case where k is inferior to 1, meaning the tangential force is more represented in the irradiance signal or k superior to 1 for which the normal force is the more represented in the irradiance. Another situation has been observed during the application of the fitting to the measurements of different friction tests. When k is above 5, there is a possibility for the computation to find a better set of coefficient by increasing the value of k to several hundreds. In terms of coefficient of determination, the quality of the approximation increases by value of 1 • 10 -4 each time the calculation is launched. For values of k between 0 and 5, the calculation converges to the same result with multiple trials.

The fitting method established gives good results in terms of coefficient of determination (Table 3.2). This means the hypothesis of the artificial finger measuring a weighted resulting force is confirmed. However, the conditions for which the defined coefficients can characterize types of surfaces are not known. A higher quantity of surfaces and situations are required to propose a physical signification for all coefficients. The coefficient k is a key parameter in the defined formula, the knowledge of this coefficient signification is determinating in the elaboration of surface classes. There is also difficulty in the coefficient c i for which there is no information at how can be calculated this coefficient.

To investigate further on this fitting method, the use of different structures of surfaces will be realized, and the possibility to have this fitting method to suit a high number of cases of friction behavior will be tested. SURFACES As a primary experiment for the observation of the effect of COF, a piece of paper will be printed on half the distance of sliding in order to present a sufficient length of steady state.

The graphics representing the friction measurements realized on printed paper with a POF angle of 0°(fig. 4.1) and 90°(fig. 4.2) show several points. There is a change in COF on the printed part of the paper, which is observed by a step increase in tangential force and normal force. The origin of this is the thickness of the paper. In fact, the profile of the paper surface has been measured (fig. 4.3) and this measurement shows the paper is thicker on the printed zone. This difference of thickness influences the normal force measurement, since the altitude of the artificial finger is constant and the support has been adjusted to present a good parallelism with the translating table.

The irradiance measurement follows the shape of the tangential force. This is problematic in terms of interpretation. Indeed, the irradiance measurement is not centered on 0 as for the tangential force, for which an increase of absolute value corresponds to a practical increase in force. In the case of the irradiance, on the printed part of the 4. ARTIFICIAL FINGER BEHAVIOR paper, for the upper side of the curve, the increase in COF and forces is associated to an increase in transmission. However, for the bottom side of the curve, which corresponds to the other sliding direction, an increase in COF and forces (in absolute value) is associated with a decrease of transmission.

First, the paper has a different thickness where the printing has been done. This influences the normal force measurement, since the altitude of the artificial finger is constant, a change in normal force comes from a change in thickness of the tested sample or from a default in the support. The support has been selected to present no defaults then the roughness of the paper sample has been measured. Concerning the measurement, there is a change in COF on the printed part of the paper, which is observed by a step increase in tangential force and normal force. The irradiance measurement follows the shape of the tangential force. This is problematic in terms of interpretation, indeed, the irradiance measurement is not centered on 0 as for the tangential force, for which an increase of absolute value corresponds to a practical increase in force. In the case of the irradiance, on the printed part of the paper, for the upper side of the curve, the increase in COF and forces is associated to an increase in transmission. However, for the bottom side of the curve, which corresponds to the other sliding direction, an increase in COF and forces (in absolute value) is associated with a decrease of transmission. For the fitting function, this is translated by a symmetric behavior on each side of the friction, which is represented by a negative d coefficient (3.3) for the bottom part of the curve. This allows for a high correlation value but the situation where coefficient d needs to be negative has not been established here.

A few hypotheses has been formulated at this point on this behavior. First, this is a specific behavior of this particular artificial finger, thus another artificial finger, slider 30 n2, with the same characteristics as slider 30 (table 3.1) will be made with the exact same parameters for comparison needs. Second, there is an effect of "sliding memory" to be experimented. The idea behind this second hypothesis is that on this particular shape of surface, the artificial finger always ascends the ink step on the same direction and descends on the other direction, the case of ascending and descending on the same sliding direction has not been verified.

The application of this phenomenon to the fitting function implies the consideration of the coefficient d according to the sliding direction. However changing the sign of the coefficient d also implies that the artificial finger behavior related to forces is opposed.

Thus, a simple verification has been made with the artificial finger placed on the tribometer and the measurement of irradiance has been realized with single increase of normal force and increase of tangential force without sliding.

The testing of the artificial finger relatively to normal force and tangential force will be held for a first approach on the explanation of the asymmetric behavior. • For normal loading and unloading: the slider is on the tribometer and it is load and unload for several cycles. The normal force is measured by the sensor used for 4. ARTIFICIAL FINGER BEHAVIOR the tribological measurement.

• For tangential loading to a direction and the other, the test has been realized on the sample support of the tribometer with the glue used for sampling placement.

The translating table then realized a motion of 1 mm around the initial place of the artificial finger. with the indenter (chapter 2), the artificial finger was upside down, on the tribometer the artificial finger is normally oriented. The positioning on the tribometer corresponds to the positioning while doing friction experiments, thus it is considered as the most reliable. Other curves realized show the same behavior with a maximal normal force of 6.0 N, even if the domain of application of the artificial finger is around 0.5 N of normal force.

The tangential force graphic presents also a linear curve. However, this linear curve translates a non linear behavior. Indeed, the negative tangential force is not to be considered as negative but increasing to the other direction. This can partially explain the behavior observed for several points. First, is the sliding direction detection, on the graphic (fig. 4.4d), a tangential force of 0.5 N is associated to a response of 104 mV on one direction and 98 mV on the other direction. Next is the sign of the d coefficient, on one direction, an increase in tangential force is associated with an increase of irradiance, on the other direction, an increase of tangential force is associated with a decrease of irradiance. This observation is true for both POF angles of 0°and 90°(fig. 4.4c and fig.

4.4d).

However, the application of a negative value for d coefficient on all measurements does not automatically imply an increase in the coefficient of determination value. This is particularly the case for the Pekin fabric where the detection of the grooves is correctly realized on the fitting function without this negative d coefficient, which would imply an inversion of the measurement with the sliding direction.

Behavior of the artificial finger relative to friction

The coefficients established for the fitting function are the main issue on the description of textile surfaces. It is important to determine which coefficient depends of the tested surface and which coefficient depends on the artificial finger. Therefore the results obtained from another artificial finger with the same characteristics, i.e. slider 30 n2 (Table 3.1), were compared.

One of the coefficients of the fitting function is obviously associated to the artificial finger, which is Irr 0 (3.3), which corresponds to the initial value of the irradiance under the initial normal load. The testing of Irr 0 was accompanied with the realization of the graphics for the irradiance evolution with the normal and tangential forces as in fig. 4.4.

The graphics in fig. 4.5 for slider 30 n2 show the same behavior as for slider 30. The normal force response is linear and the tangential force has the same symmetry. The friction measurement has been realized with both sliders, slider 30 and 30 n2 on a piece of paper with printed parts. Both endings of the sliding distance are printed on these pieces of paper, this was to verify the influence of the "sliding memory" effect evoked before.

ARTIFICIAL FINGER BEHAVIOR

ARTIFICIAL FINGER BEHAVIOR

Case of POF angle of 0°T

he observed situation with the asymmetric response of the artificial finger will be investigated with both sliders, slider 30 and slider 30 n2 in the case of the POF angle of 0°. It can be observed on figures 4.6, 4.7 and 4.8 that both artificial fingers share the same behavior which is :

• For one direction, with an increase in measured forces (normal and tangential) associated to an increase of COF, the irradiance measured increases (top part of the irradiance curve).

• For the other direction, with an increase in measured forces (normal and tangential) associated to an increase of COF, the irradiance measured decreases (bottom part of the irradiance curve). This result was also observed with slider 30 n2 on a different setting of paper, with a centered printing. This result is in agreement with the curves presented in 4.4 and 4.5

where the evolution of irradiance with tangential force depends on the direction of the tangential force.

• In one direction the increase of tangential force causes an increase of irradiance, as it can be observed in top part of Figures • In one direction, an increase of the measured forces (normal and tangential) associated to an increase of COF, corresponds to an increase of the irradiance measured (top part of the irradiance curve).

• For the other direction, an increase of the forces (normal and tangential), associated to an increase of COF, induced a decrease of the irradiance (bottom part of the irradiance curve).

The same conclusion about the effect of the response to tangential force can be made in this situation as for the POF angle of 0°.

However, for slider 30 n2 (4.10 and 4.11), for both directions, an increase of forces (normal and tangential), associated to an increase of friction forces, induces a decrease of the irradiance. Therefore for this slider with POF angle of 90°, a more complex phenomenon occurs with the artificial finger. There is one direction for which an increase in tangential force is associated with a decrease in irradiance. However, for the other direction, this situation has not been observed with normal or tangential force influence only. It is supposed in this case that the behavior of the artificial finger results from a combination of normal and tangential forces. to a support made with glass and the artificial finger has a flat stud, which means that for the case of sliding on the paper, the contact between the sample and the artificial finger is a plane on plane contact. This type of contact is known to be source of errors because it is impossible to have a whole plane being in contact with another plane and the difficulty to know the location of the points of contact.

In addition, it can be seen on the graphic of the measurement of slider 30 n2 with a POF angle of 0°(fig. 4.7) that the distinction of the blank and printed parts of the paper is difficult to observe on the irradiance measurement while the difference in COF is easily distinguishable for these parts. It can also be observed on slider 30 with a POF It is also considered that the "sliding memory" effect can have an influence in this measurement. The sliding is made so that the artificial finger starts its motion from an extremity of the surface, which means a printed part of the paper in this case. It is seen that the printed part has a higher COF than the blank part of paper. This means that the artificial finger is subject to the highest deformation of the sliding with the first motion. It is supposed that the drop in COF on the blank paper does not bring enough relaxation to the artificial finger for the measurement to accurately detect this moment.

4.2 Behavior of the artificial finger relative to sliding direction

Sliding direction detection

It has been observed with the friction realized so far that the sliding direction can be obtained with the artificial finger. On each measurement presented, one part of the curve is associated with one motion direction. A specific sensitivity to this sliding direction has been observed with the textile experiments (fig. 4.12). The Pekin fabric situation clearly shows a higher sensitivity to the surface features (the grooves) with a POF angle of 0°than for a POF angle of 90°. However, with a POF angle of 90°, the difference between the two sliding directions is higher than with a POF angle of 0°, the difference in means values of the obtained irradiance between each direction is 1.0 mV for a POF angle of 0°while this difference is of 3.7 mV for the POF angle of 90°.

With the Velvet fabric, the change of POF angle shows a change in behavior relative to the sliding direction (fig. 4.12). For the sliding direction against the fabric hairiness (against pile label, in light blue), the normal force acting against the artificial finger is significantly higher because of the hairiness position. It can be observed that for a POF angle of 0°, irradiance against pile direction is higher than along pile direction. In contrary, for the POF angle of 90°, irradiance is higher for along pile direction.

This observation can be explained by the deformation of the POF with a POF angle of 90°. In fact, during sliding, the fiber is deformed and if initially the fiber axis is 4. ARTIFICIAL FINGER BEHAVIOR not perfectly included in a plane perpendicular to the surface, the deformation has a preferred direction, which can change in function of the initial direction. This different sensitivity to the tangential force can cause the artificial finger to show different behavior with a POF angle of 0°and a POF angle of 90°.

As a conclusion, it can be said that a better detection of the sliding direction has been observed with the artificial finger in a 90°POF angle position while 0°showed a better sensitivity to the surface features. • Why there is a difference of irradiance evolution in the case of the POF angle at 90°between two sliders with the same characteristics (slider 30 and slider 30 n2), while they have the same behavior at 0°?

• What is the origin of the shifted detection of a surface change for the slider 30 n2 for a POF angle of 90°?

The movement of the slider at the contact with the paper for the different cases has been analyzed with the software. Two different aspects can be analyzed:

• The displacement of the slider at the front and at the rear of the contact relatively to the whole sliding displacement

• The horizontal deformation of the slider stud due to friction. • The realized comparison is

x A i -x B i
By comparing these two lengths the state of compression of the stud can be assessed.

In reality, the difference x A i -x B i corresponds to the value of the artificial finger stud decrease in size. Therefore, the difference in positioning has two physical translations:

• If the difference x A i -x B i is positive, the stud is under compression due to the friction forces (reduction of size is positive) tested with slider 30 n2 artificial finger. The same results as above were observed, comprising the difference in compression level between edge detection and global detection.

• If the difference x A i -x B i is
In the case of edge detection, the difference in points positions is higher than for global detection, which can be observed on the differences between situation with POF angle of 0°and 90°in figures 4.20, 4.23 and 4.26. The observation of the measurements obtained with the rounded artificial finger can confirm the difficulty to obtain a good contact between the surface and the artificial finger with a flat ended stud.

Behavior of the artificial finger with rounded border

There has been difficulties with the setting of the fitting function however. For situations as represented in fig. 4.6 where there seems to be a horizontality issue with the support, it is not possible to have a negative value for coefficient d for the bottom part of the curve, the representation uses a positive value for both direction and the determination coefficient is still high. However, it can be seen that there is an issue with the fitting on the steps for this curve. The supposition for this example is that the signal transmitted by the artificial finger can be decomposed in two components. One of the 4. ARTIFICIAL FINGER BEHAVIOR components is the position of the core PDMS and POF in the artificial finger that can be approximated by the tilt of PDMS, and the second component is the deformations induced by the surface on the artificial finger. This shows the limitation of the current fitting method and more parameters need to be added in order to include causes on the artificial finger that are not yet taken into account by the present fitting method.

In order to get information about the deformation of the artificial finger, the friction tests realized on the paper and the textile surfaces have been recorded with a camera.

The investigation of the films of friction will produce information about the visible deformation of the artificial finger.

Analysis of the performed measurements with the camera recording

Conclusions on the measurements campaign

Many measurements were realized in order to describe and understand the behavior of an artificial finger provided with the present technology and mechanical features such as shape, elasticity modulus and structure. A fitting method was proposed and tested under various conditions, namely different surfaces, different artificial fingers and different orientations (POF angle). The objective of the realized tests was to verify the reliability of the fitting method and to understand the responding behavior of the artificial finger. The different surfaces tested, -namely the Pekin fabric and a stable COF but grooves on the surface, the velvet with pile that changes friction depending on the sliding direction, and the printed sheet of paper with a brutal change in COF during the friction -led to the determination of specific parameters in the fitting such as the sliding direction relative computing of coefficients or the negative sign of the scaling coefficient for specific uses.

The high values of coefficient of determination obtained for the measurement realized show the correlation between the force measurement and the optical measurement. In order to obtain a standalone use of the artificial finger, it is mandatory to understand the influence and meaning of the parameters in the formula and provide a link between the forces in action on the artificial finger and the optical measurement acquired. It is also possible to reverse the interpretation of the artificial finger, and use the measurement to provide new parameters for textile identification. This leads to improvements to be held on the POF instrumented artificial finger technology.

Firstly, the requirements for the use of the negative coefficient are unclear. It has been observed (fig. 4.6) that for a situation where the use of negative coefficient should be needed (the artificial finger presents an asymmetric behavior on sliding directions), the negative coefficient is not adapted to describe the measurement the way the fitting formula is written, because of an external factor which is the horizontality default.

Secondly, the surfaces used had for objective to test and verify the artificial finger behavior under specific circumstances, it is now important to investigate to what extent the artificial finger measurement is able to discriminate different textile surfaces.

Thirdly, the sensitivity of the artificial finger under a positioning setting which is the POF angle has been investigated and the following has been observed : Finally, the integration of vibration measurements is to be taken into account in order to describe tactile discovery of surfaces. It is important to preliminary determine the sensing capability of the artificial finger.

In this way, some additional tests were undertaken with the artificial finger to observe the behavior of the artificial finger under different situations.

On another hand, the purpose of the artificial finger is the reproduction of the human finger friction behavior against textile surfaces. The comparison with the results provided in (1) showed a correct reproduction of the COF of the artificial finger against textile surfaces. The addition of the camera recording of the stud during the friction tests and the compression observed with the difference in displacement of the stud, the behavior of the artificial finger can be approached to the observation made by Oddo et al. (28) where the deformation of the finger was shown progressive with time during friction. On these observations, the artificial finger behavior reproduces the human finger friction and thus the measurement acquired by the optical measurement can bring additional information of the friction, which makes this instrumented artificial finger a promising tactile sensor.

Chapter 5

Opening to further investigation of the artificial finger

In this section, the measurements realized were issued to test the capability of the artificial finger to acquire specific information. Due to a low number of tests realized here, the observations mentioned are indicative only.

• The friction behavior of the artificial finger against the Pekin fabric has been analyzed as regarding the finger deformation during sliding.

• A set of 3D printed surfaces with regularly spaced circular pads have been tested with the artificial finger in terms of friction and vibrations.

• The other family of surfaces is constituted from the same velvet cropped with different heights of pile.

Camera recording of the friction against Pekin fabric

The use of the camera recording to obtain the displacement of the artificial finger stud during a friction test was realized with a sliding of slider 30 n2 (artificial finger with a fiber angfle of 30°, flat ended stud, stuck connectors and fiber guides, table 3.1 with Pekin fabric. As observed with the friction measurements and with the artificial finger measurements, the artificial finger shows response to the grooves of the textile surface. It is interesting to observe the motion of the artificial finger during this test. For recording purposes, the forces measurement and the camera recording are not made with the same

OPENING TO FURTHER INVESTIGATION OF THE ARTIFICIAL FINGER

test. Indeed, for a friction test, the artificial finger slides on the center of the fabric.

However, for the camera recording, the artificial finger was placed to a side of the textile to prevent bending of the fabric which would bother the image record. It can be seen on the measurements of the artificial finger on the Pekin fabric that the measurement with a POF angle of 90°(fig. 5.2) shows the same high frequency variations, this frequency corresponds to the grooves on the Pekin fabric surface. Thus, the artificial finger performs a displacement associated to the presence of grooves. Also, the groove detection is better realized with the edge detection observed for POF angle of 90°(stated in previous chapter, not referenced for reviewing). The compression level for both measurements (fig. 5.3) shows no specific variation that occurs during the measurement, which is coherent since COF is roughly constant during friction against the Pekin fabric. There is no difference in the compression level for each sliding direction, this can be related to the stable value of COF on the two sliding directions.

OPENING TO FURTHER INVESTIGATION OF THE ARTIFICIAL FINGER

Testing on 3D printed surfaces

The investigated 3D printed surfaces are constituted of regularly spaced circular pads with different diameters and the same height. The corresponding excited frequency of these surfaces for the presented friction tests speed of 10 mm/s are presented in table 5.1.

All measurements are not displayed in this manuscript, because lower value of frequencies are similar to the presented surfaces. Surfaces S1, S2 and S3 are presented, as their dimensions are the lowest and bring the better information about the artificial finger capability.

The testing of the surfaces with the artificial finger was realized. One surface, called S0 has been realized smooth and was used to show the reaction of the artificial finger on unpadded surface. The artificial finger used was slider 30 n2 with a POF angle of 90°, since this situation with the edge detection was supposed to present the best results as observed on the Pekin fabric grooves.

Testing on 3D printed surfaces

The measurements realized on the different surfaces show that the artificial finger is actually able to detect the pads with S2 (fig. 5.7) and S3 (fig. 5.8) surfaces. The pads are detected with the normal force and tangential force and the irradiance signal. On S1 surface, it is harder to see variations that could be due to the pads on the surfaces.

One easy way to verify if the variations observed on the measurements is actually due to the pad is the FFT applied on the irradiance signal. On figure 5.9, the FFT calculations for the presented surfaces is graphed. The peaks of frequency corresponding to the pads for S1, S2 and S3 are visible (table 5.1). The associated frequencies slightly differ from the calculated frequencies. This difference in value is due to the average realized on the spatial resolution of the pattern. These results show the capability of the artificial finger slider 30 n2 to detect the surface and transmit the frequency of detection through the irradiance signal. The edge detection has been observed, however the question about this type of detection for the rounded artificial finger remains, thus the measurement with the surfaces has been realized with slider 30 arr (artificial finger with a fiber angle of 30°, stuck connectors with fiber guides ans specific rounded edges molded on the stud, see table 3.1). with the printed paper, the rounded artificial finger is able to accurately detect the vibrations issued by the surface properties of the samples. The question about the limit of detection can be issued. This test has been realized with S1 and S2 surfaces with different sliding velocities. S1 has been tested with a speed of 15 mm/s and S2 has been tested with a speed of 50 and 100 mm/s. The S1 surface was not used for more testing at high speeds because the frequency is not sufficiently highlighted with the 10 mm/s.

OPENING TO FURTHER INVESTIGATION OF THE ARTIFICIAL FINGER

Thus the surface S2 is used for the high speeds. The S1 surface was analyzed with a However, that frequency is masked on the graph because of the amplification of other frequencies due to the high speed performed (fig. 5.14c. This can show a limit in the frequency transmission of the artificial finger, a limit in the speed usage of the artificial finger or a combination of speed and default amplitude to detect. Human fingerpad

OPENING TO FURTHER INVESTIGATION OF THE ARTIFICIAL FINGER

detects vibrations at a frequency of 1000 Hz with an optimum at 300 Hz in terms of amplitude, which is higher that the observed capability of this sensor. The verification of this insufficiency can be held as perspective of this work.

On a side note, the camera recording of the friction test was realized with slider 30 Further investigation can be held in this direction where the vertical displacement of the stud is used instead of the normal force, which would ensure better link between the deformation of the stud and the irradiance measured.

OPENING TO FURTHER INVESTIGATION OF THE ARTIFICIAL FINGER

Testing on the cropped velvet fabrics

The velvet used in this part has been cropped to different heights with a specifically realized mower for cropping textile fabrics. The fabrics are named I, II, III and V which roughly correspond to the hair height of the fabrics in mm. for the friction against velvet fabrics. It can be seen that the length of the hairiness also have an influence on the COF value. In fact, the longer the hairiness is, the higher the COF is. Moreover, it can be observed that the length of the flip loop due to the change in sliding direction is higher for longer hairiness. A correlation between this loop length and the hair length of the textile fabric can be subject for further investigation of the description of textiles with the artificial finger. The artificial finger would be able to inform on the length of hair on a fabric since the irradiance measurement follows the loop observed in force measurements.

To conclude this side part of the investigation of the artificial finger, one goal for the tests realized was the capability of the rounded artificial finger. The vibration transmission limit of the artificial finger was tested, the capability to detect surface features with the rounded edges has also been verified and the linking between the displacements, vertical of horizontal, of the artificial finger in the fitting formula has been proposed as perspective. A first tool to describe the fabric length of hairiness in terms of friction is also presented with the testing of the different cropped velvet fabrics.

Chapter 6

Conclusion and perspectives

Conclusion

In this manuscript is described the work held during three years for the subject of designing a POF instrumented artificial finger for the tactile description of textile fabrics.

In this work, the result of the designing of the artificial finger consist in the establishment of a manufacturing protocol for the making of an instrumented artificial finger with the capability to sense features of surfaces while having a friction behavior near to that of a human finger, and particularly in terms of COF and friction behavior. The designed artificial finger comprises a polymeric optical fiber which acts as sensor, a PDMS core and stud specifically designed for having a COF against a given textile fabric of 0.5 and a texturing to reproduce fingerprints which is mandatory for reproduction of a finger behavior on the hairiness of textiles. The friction on textile surfaces has been compared

to the results from real human finger, and the artificial finger has been validated in the reproduction of the friction behavior. The analysis of the measuring results from the artificial finger is the core for a usage of the artificial finger as sensor. For a robust usage of the instrumented artificial finger as sensor, the repeatability and reproducibility of the instrumented artificial finger has been tested. As a result, the addition of fiber guides and connector resin for better placement of the polymeric optical fiber in the artificial finger was adopted. With a validated artificial finger, the testing of textile surfaces was realized in order to determine the sensing behavior of the artificial finger. A fitting method which links the measured forces with the optical measurement realized with the artificial finger has been created, tested, improved and discussed. The fitting method

CONCLUSION AND PERSPECTIVES

has been tested and the finger behavior has been studied from its specific characteristics, the experimental conditions and the tested surface features.

The observation of the artificial finger with a camera and the recording of the motion of the artificial finger during a friction test lead to the observation of a compression of the artificial finger during specific stimulations due to the surface features, such as a change in COF of the tested surface. This compression is not observable with the force measurements realized with the tribometer and shows a specific behavior of the artificial finger. The optical measurement of the artificial finger is dependent on the deformations of the artificial finger, thus, investigations on this compression influence on the optical measurement can lead to understanding of the links between the artificial finger measurement and the surface features.

Moreover, an important point in tactile characterization is the measurement of the induced vibrations. it has been showed the artificial finger is sensitive to vibration up to 60 Hz which is interesting but not sufficient because the optimal sensitivity of real fingers to vibration is around 300 Hz and the maximal frequency detected is around 1000 Hz.

Perspectives

As perspectives to this work, the improvement of the manufacturing process can be held in terms of mass production and automation of the process. The current protocol is sufficient for a single production of an artificial finger and brings some deviation in the resulting artificial finger. These deviations were observed in the measurements realized with the different artificial fingers realized, as example in the edge detection of the COF change for the printed paper. Nethertheless, a finger characterization or calibration will be mandatory before use, as for all trade sensors.

A further investigation on the fitting method is also required to obtain a reliable measurement of the surface features with the artificial finger. A way of improvement of the fitting method can be proposed in the splitting of an influence of the macro geometry of the surface to be tested, and the micro-structure that the artificial finger will be subject to describe and will result in identifying parameters of the surface. The improvement of the fitting formula leads to a high amount of work, with the observation of the reliability of the formula in various conditions of work. The use of the formula 6.2 Perspectives coefficients as describers for textiles is a way for the objective for the description of textile surfaces with the artificial finger. In this direction, a lot of tests are required for the observation of a reliable use of the coefficients as descriptors and provide the description settings of different classes of textile fabrics. For this objective, the testing of numerous types of textiles with different specific features is to be considered for continuing the development of the artificial finger sensor.

1 0. 1 1 0. 2 2 0. 2 . 1 2 0. 2 . 2

 1112221222 évolutions du banc de mesure sont décrites dans ce manuscrit. Les difficultés rencontrées ont permis la réalisation d'un banc de mesure permettant une analyse logicielle des résultats obtenus. Cette analyse a donné naissance à une formule d'approximation présentée et discutée. La mise en place de cette formule a apporté une meilleure compréhension du comportement du doigt artificiel. Une analyse vidéo des essais est également présentée et permet une discussion supplémentaire sur les résultats obtenus par le banc d'essai, le doigt artificiel et la formule d'approximation. Des pistes d'améliorations ont été proposées suite aux remarques réalisées par cette analyse combinée. Des essais additionnels sont également présentés pour montrer les limites de sensibilité du doigt artificiel instrumenté. Introduction Context of the study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . State of the art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The uses of artificial fingers . . . . . . . . . . . . . . . . . . . . . . The instrumented artificial fingers . . . . . . . . . . . . . . . . . . 4

CONTENTS 5 1 . 1

 511 Opening to further investigation of the artificial finger 5.1 Camera recording of the friction against Pekin fabric . . . . . . . . . . . . 5.2 Testing on 3D printed surfaces . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Testing on the cropped velvet fabrics . . . . . . . . . . . . . . . . . . . . . 6 Conclusion and perspectives 6.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preliminary measurement bench with a monochromator on the left, the PDMS sample at the center and the photodiode on the right . . . . . . . 1.2 Irradiance at 632nm and 532nm relative to normal load. . . . . . . . . . . 1.3 Irradiance at 632nm during loading and unloading for the different patches clear PDMS, PDMS with a low and a high concentration of gold nanoparticles. The samples have two main differences, the concentration in gold nanoparticles and the POF length. Regression coefficients in table 1.1. . .1.4 Irradiance relative to the distance of the POF to the surface of the PDMS patch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Binocular microscope picture of the SMA905 connector with a POF. 16X magnification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Picture of the tribometer used after the preliminary study . . . . . . . . . 1.7 Description of functionality of mechanoreceptors. Extracted from (6) . . . 1.8 Evolution of the coefficient of friction (COF) during a friction cycle obtained between a velvet fabric and a real or textured and non-textured artificial finger. Extracted from (1) . . . . . . . . . . . . . . . . . . . . . . 1.9 Picture of the 3 main kinds of fingerprints (66) . . . . . . . . . . . . . . . 1.10 COF for different base/curing agent ratio and for different thermal cycles 1.11 Example of a indentation test realized on a PDMS sample . . . . . . . . . 1.12 Pictures of the prototype of the method of assembly of the artificial finger. 1.13 Example the friction force from the tribometer (top) and the irradiance signals obtained from the artificial finger (bottom) during several sliding cycles against a plain fabric. The curves are displayed relative to time, and the large variations in value are due to the alternative linear movement. vii LIST OF FIGURES 1.14 Schematics representing different stages of the making of the artificial finger. 2.1 Repeatability of the irradiance measurement from slider 45p and slider 45al with the plain woven fabric . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Measurement realized during a friction test with the plain woven fabric from slider 45al . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Evolution of real human finger friction against the plain woven fabric. Extracted from (1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Schematics of the tilt movement occurring on the slider depending on the friction direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Relative irradiance acquisition of slider va (PDMS is stuck in the core) during a measurement with the a plain woven fabric. The relative irradiance was calculated based on the value of irradiance at the start of the test corresponding to the artificial finger loaded with the normal load of 0.5 N before any movement. . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Measurements realized with slider 45fg during friction on different fabrics with same normal load of 0.5 N applied . . . . . . . . . . . . . . . . . . . 2.7 Irradiance and normal force vs penetration depth measured during indentation of the slider 45fg artificial finger. 10 different tests were issued with the same conditions and show the same behavior. . . . . . . . . . . . . . . 2.8 Irradiance vs penetration depth during indentation of each side of the artificial finger slider 45fg at the connector position . . . . . . . . . . . . . 2.9 Schematics of the flipping of the artificial finger for the following measurements. Top view. The arrows represent the optical path of light and the rectangle is the tribometer translating plate. The translating plate will conserve the motion and the connection on the artificial finger will also be conserved, the connectors are physically labeled A and B. Connector A will remain the input connector and connector B will remain the output connector. Direction is shown relative to the support motion. . . . . . . . 2.10 Irradiance vs time for two friction tests on the plain woven fabric with an artificial finger able to flip. Dark curve corresponds to the left situation of 2.9. Light curve corresponds to the right situation of 2.9. . . . . . . . . viii LIST OF FIGURES

2. 14

 14 Picture of the setup used for pressing against the fiber with the frame used to set the POF in the same position as in the finger. On the right is the pressing part. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.15 Pictures of the lit POF in different fiber angle frames. Picture are realized with a constant setting of the camera. . . . . . . . . . . . . . . . . . . . . 2.16 Graphic representing the variation of the transmitted power in terms of irradiance relative to the aligned connector position representing 100% and with the curvature of the fiber expressed in terms of radius of curvature obtained with measurement on the pictures. . . . . . . . . . . . . . . . . . 2.17 Image obtained after editing the picture made by the camera for the measurement of the curvature radius. The example given is for the 45°f rame. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.18 Image obtained after editing the picture made by the camera for the measurement of the curvature radius of the pressed POF at the center of the initial curvature. The decomposition of the considered parts of the deformation is shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.19 Pictures of the POF with pressure locations at the two considered offset positions for the 45°frame. . . . . . . . . . . . . . . . . . . . . . . . . . . 2.20 Graphic of the evolution of the transmitted power with the different positions of press and the initial curvature value . . . . . . . . . . . . . . . . 2.21 Relative irradiance relative to time during friction with slider 30 and slider 45fg against the plain woven fabric . . . . . . . . . . . . . . . . . . . . . . 2.22 Irradiance (a) and tangential force (b) relative to time for different kinds of textile fabrics with slider 30. . . . . . . . . . . . . . . . . . . . . . . . . ix LIST OF FIGURES 2.23 Graphic of the evolution of relative irradiance during a friction of slider 30 against a blank piece of paper. a is the irradiance measurement and b is the tangential force. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.24 Relative irradiance obtained with slider 30 on a blank sheet of paper for different sliding velocities. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.25 Relative irradiance and tangential force relative to time for slider 30 against a printed piece of paper. Half is blank and half is black. The vertical bars separate the blank and printed zones. The first occurrence is blank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.26 Relative irradiance and tangential force relative to time for slider 30 against a printed piece of paper with alternative black and blank squares. The tangential force associated with black squares is higher than blank squares. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.27 Relative irradiance and tangential force relative to time for slider 30 against three different widths of black stripes printed on a blank sheet of paper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.28 Graphic of the evolution of relative irradiance during a friction of slider 30 against a piece of paper with a line of 2.5 mm and a adhesive tape taped above the paper. a is the irradiance measurement and b is the tangential force. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Picture of the new tribometer used for the investigation of textile fabrics with the artificial finger. a) is the artificial finger support arm, a special piece is hidden here that sets the angle of the axis of the POF within the artificial finger and the sliding direction, b) is the tri-axis force sensor and c) is the translating table. On this picture, the POF angle is 90°relative to the sliding direction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Picture of the new tribometer used for the investigation of textile fabrics with the artificial finger. On this picture, the POF angle of the slider is 0°relative to the sliding direction. . . . . . . . . . . . . . . . . . . . . . . 3.3 Graphical representation of the responsivity function of the acquired 918D-SL-OD3R photodiode. Extracted from the calibration certificate enclosed with the photodiode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x LIST OF FIGURES 3.4 Graphical representation of the irradiance acquired with the photodiode with an application of a normal force of 0.5 N of the artificial finger (slider 30) above different colored parts of a piece of paper . . . . . . . . . . . . . 3.5 Picture of different magnification of the Pekin fabric . . . . . . . . . . . . 3.6 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 0°on Pekin fabric relative to the displacement. . . . . . . 3.7 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 90°on Pekin fabric relative to the displacement. . . . . . 3.8 FFT calculation realized on the irradiance measurement from the Pekin fabric. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Picture with different magnifications of the velvet fabric . . . . . . . . . . 3.10 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 0°on velvet fabric relative to the displacement. . . . . . . 3.11 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 90°on velvet fabric relative to the displacement. . . . . . 3.12 Irradiance for Pekin and velvet fabrics with the corresponding fitting curves for POF angles of 0°and 90°. . . . . . . . . . . . . . . . . . . . . . 4.3 Profile of the printed paper measured with a profilometer. The left part is the printed paper and the right part is blank. A step of 10 µm is measured. 4.1 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 0°on the printed paper relative to the displacement. The left part from 0 to -45 mm is printed and the right part 0 to 45 mm is blank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi LIST OF FIGURES 4.2 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 90°on the printed paper relative to the displacement. The left part from 0 to -45 mm is printed and the right part 0 to 45 mm is blank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Graphics for the description of the artificial finger under normal and tangential variations separately. . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Graphics for the description of the artificial finger slider 30 n2 under normal and tangential variations separately. . . . . . . . . . . . . . . . . . 4.6 Graphics for the description of a friction test of the artificial finger slider 30 on the paper with both ending black printed zones and a POF angle of 0°. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 Graphics for the description of a friction test of the artificial finger slider 30 n2 on the paper with both ending black printed zones and a POF angle of 0°. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8 Graphics for the description of a friction test of the artificial finger slider 30 n2 on the printed paper with the black printed zone at the center and a POF angle of 0°. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9 Graphics for the description of a friction test of the artificial finger slider 30 on the paper with both ending black printed zones and with a POF angle of 90°. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10 Graphics for the description of a friction test of the artificial finger slider 30 n2 on the paper with both ending black printed zones and with a POF angle of 90°. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.11 Graphics for the description of a friction test of the artificial finger slider 30 n2 on the printed paper with the black printed zone at the center and a POF angle of 90°. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.12 Graphics of the irradiance measurement for Pekin and velvet fabrics with the fitting curve obtained by the developed calculation . . . . . . . . . . . 4.13 Irradiance cycle highlighting the description of the shifted detection of the surface feature change from the POF angle of 90°orthe POF angle of 0°w ith slider 30 n2 on the same piece of paper. . . . . . . . . . . . . . . . . xii LIST OF FIGURES 4.14 Graphical representation of the profile of the artificial finger obtained with a multifocal microscope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.15 Picture of the screen of the tracker software settings for the detection of the stud displacement. Setting with slider 30 n2 artificial finger. The dimension considered as reference is 13.87 mm. The tracked part corresponds to the point in the red circle. The red circle is the detection zone considered and is compared with the reference labeled "gabarit". . . . . . 4.16 Schematics of the tracked points on the artificial finger. On direction 1, A1 and B1 have a positive acquired position and on direction 2, A1 and B1 have a negative acquired position. . . . . . . . . . . . . . . . . . . . . . 4.17 Schematics of the evolution of the recording during the tracking of the artificial finger points. An image during movement is used as support to explain the values obtained. . . . . . . . . . . . . . . . . . . . . . . . . . . 4.18 Displacement of points A and B of the slider 30 stud during a friction test against a sheet of paper with both sides black printed. The POF angle is of 0°. Only the first cycle is represented. . . . . . . . . . . . . . . . . . . . 4.19 Displacement of points A and B of the slider 30 stud during a friction test against a sheet of paper with both sides black printed. The POF angle is of 90°. Only the first cycle is represented. . . . . . . . . . . . . . . . . . . 4.20 Graphical representations of x A i -x B i for slider 30 with POF angles of 0°or 90°during friction on a paper with both sides black printed. Only the first cycle is represented. . . . . . . . . . . . . . . . . . . . . . . . . . . 4.21 Displacement of points A and B of the slider 30 n2 stud during a friction test against a sheet of paper with both sides black printed. The POF angle is of 0°. Only the first cycle is represented. . . . . . . . . . . . . . . 4.22 Displacement of points A and B of the slider 30 n2 stud during a friction test against a sheet of paper with both sides black printed. The POF angle is of 90°. Only the first cycle is represented. . . . . . . . . . . . . . . 4.23 Graphical representations of x A i -x B i for slider 30 n2 with POF angles of 0°or 90°during friction on a paper with both sides black printed. Only the first cycle is represented. . . . . . . . . . . . . . . . . . . . . . . . . . . xiii LIST OF FIGURES 4.24 Displacement of points A and B of the slider 30 n2 stud during a friction test against a sheet of paper with center part black printed. The POF angle is of 0°. Only the first cycle is represented. . . . . . . . . . . . . . . 122 4.25 Displacement of points A and B of the slider 30 n2 stud during a friction test against a sheet of paper with center part black printed. The POF angle is of 90°. Only the first cycle is represented. . . . . . . . . . . . . . . 122 4.26 Graphical representations of x A i -x B i for slider 30 n2 with POF angles of 0°or 90°during friction on a paper with both sides black printed. Only the first cycle is represented. . . . . . . . . . . . . . . . . . . . . . . . . . . 123 4.27 Picture of the rounded artificial finger on the tribometer. . . . . . . . . . 125 4.28 Graphics for the description of a friction test of the artificial finger slider 30 arr on the printed paper with the mark at the center and a POF angle of 0°. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 4.29 Graphics for the description of a friction test of the artificial finger slider 30 arr on the printed paper with the mark at the center and a POF angle of 90°. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 4.30 Picture of the screen of the tracker software settings for the detection of the stud displacement. Setting with slider 30 arr artificial finger. The dimension considered as reference is 13.87 mm. The tracked part corresponds to the point in the red circle. The red circle is the detection zone considered and is compared with the reference labeled "gabarit". . . . . . 129 4.31 Graphics representing the result of the analysis of the slider30 arr stud motion and the position difference calculation graphic. The friction test was realized against a sheet of paper with center part black printed. The POF angle was of 0°. Only the first cycle is represented. . . . . . . . . . . 131 5.1 Slider30 n2 stud motion. The friction test was realized against the Pekin fabric. The POF angle was of 0°. Only the first cycle is represented. . . . 136 5.2 Slider30 n2 stud motion. The friction test was realized against the Pekin fabric. The POF angle was of 90°. Only the first cycle is represented. . . . 137 5.3 Graphical representations of ε l -ε r for slider 30 n2 with POF angles of 0°o r 90°during friction on the Pekin fabric. Only the first cycle is represented.137 5.4 Example pictures of the 3D printed surfaces tested with the artificial finger.139 xiv LIST OF FIGURES 5.5 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 n2 on the S0 surface with a POF angle of 90°. . . . . . . . . . . . . . . . . . 5.6 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 n2 on the S1 surface with a POF angle of 90°. . . . . . . . . . . . . . . . . . 5.7 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 n2 on the S2 surface with a POF angle of 90°. . . . . . . . . . . . . . . . . . 5.8 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 n2 on the S3 surface with a POF angle of 90°. . . . . . . . . . . . . . . . . . 5.9 FFT of the irradiance signal obtained with slider 30 n2 for the 3D printed surfaces S1, S2 and S3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the S1 surface with a POF angle of 90°. . . . . . . . . . . . . . . . . . 5.11 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the S2 surface with a POF angle of 90°. . . . . . . . . . . . . . . . . . 5.12 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the S3 surface with a POF angle of 90°. . . . . . . . . . . . . . . . . . 5.13 FFT of the irradiance signal obtained with slider 30 arr on the 3D printed surfaces S1, S2 and S3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.14 FFT of the irradiance signal obtained with slider 30 arr on the 3D printed surfaces with different sliding speeds. . . . . . . . . . . . . . . . . . . . . . 5.15 Graphics for the description of a friction test of the artificial finger slider 30 arr on the S4 surface associated with a camera recording of the friction.153 5.16 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the velvet I fabric with a POF angle of 0°. . . . . . . . . . . . . . . . . xv LIST OF FIGURES 5.17 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the velvet II fabric with a POF angle of 0°. . . . . . . . . . . . . . . . . 5.18 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the velvet III fabric with a POF angle of 0°. . . . . . . . . . . . . . . . 5.19 Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the velvet V fabric with a POF angle of 0°. . . . . . . . . . . . . . . . . xvi List of Tables 1.1 Coefficient of proportionality depending on the layers of PDMS . . . . . . 1.2 Coefficient of proportionality depending on the distance of the POF to the surface of the PDMS patch . . . . . . . . . . . . . . . . . . . . . . . . 1.3

3. 1

 1 Recollection of the main features of the different artificial fingers realized 3.2 Recollection of the obtained coefficients for the tested fabrics . . . . . . . 4.1 Roughness parameters measured on the paper on blank and printed parts 5.1 Geometrical features of the 3D printed surfaces in terms of plot diameter, pattern dimension and provoked excitation frequency in the case of a sliding velocity of 10 mm/s. The plots have a height of around 80 µm . . Introduction 0.1 Context of the study This work describes the design and validation of the first artificial finger instrumented with a polymeric optical fiber. The objective is the reproduction of the textile-finger friction during touch and the acquisition of an internal signal obtained at the nearest of the contact between the finger and the textile surface. The use of artificial fingers can reduce the design and development time of new surfaces, being real or simulated with a tactile stimulator, by avoiding the use of human panels. The development of this artificial finger is based on a previous work in the laboratory by Camillieri et al. (1) in which the parameters for an artificial finger to reproduce the friction behavior of an average finger on textiles have been investigated. The making of an instrumented artificial finger comes from several needs. First is the difficulty to obtain accurate measurements of friction in the contact zone because of the use of external sensors, i.e. which are far from the contact zone. In other words,

Figure 1 . 1 :

 11 Figure 1.1: Preliminary measurement bench with a monochromator on the left, the PDMS sample at the center and the photodiode on the right

Figure 1 . 4 :

 14 Figure 1.4: Irradiance relative to the distance of the POF to the surface of the PDMS patch

Figure 1 . 5 :

 15 Figure 1.5: Binocular microscope picture of the SMA905 connector with a POF. 16X magnification

Figure 1 . 7 :

 17 Figure 1.7: Description of functionality of mechanoreceptors. Extracted from (6)

(a)

  Real nger (b) Smooth arti cial nger (c) Textured arti cial nger

Figure 1 . 8 :

 18 Figure 1.8: Evolution of the coefficient of friction (COF) during a friction cycle obtained between a velvet fabric and a real or textured and non-textured artificial finger. Extracted from (1)

Figure 1 . 9 :

 19 Figure1.9: Picture of the 3 main kinds of fingerprints[START_REF] Hoover | The Science of Fingerprints[END_REF]

  The adjusted COF wil be tested on other fabrics after the first adjustment. COF of elastomers, such as silicones, with textile fabrics are too high for the accurate reproduction, thus the reproduction of the COF of a finger requires a special care with the making of the contact area of the artificial finger. The first difficulty is in the choice of parameter to enable a modulation of the observed COF. The PDMS used for the development of our artificial finger is Sylgard®184 by The Dow Chemical Company. The choice has been made on the correction of the elastic modulus, which is the easiest parameter to change in our formula of the PDMS. The method to change the elastic modulus are with a change in the curing time, and a change in the recipe in terms of ratio polymer base with curing agent. The table 1.5 from (70) indicates the evolution of the elastic modulus with the curing time and temperature. This served as a base to produce our recipe. Experiments were done to show the influence of the polymer base and curing agent ratio. 3 different compositions have been selected for our investigation: a weight ratio of 5:1, 10:1 and 20:1. The weight ratio 10:1 is the recommended weight ratio for the making of the PDMS.

1: 1 Figure 1 .

 11 Figure 1.10: COF for different base/curing agent ratio and for different thermal cycles

1. 2 Figure 1 . 11 :

 2111 Figure 1.11: Example of a indentation test realized on a PDMS sample

1 .

 1 ARTIFICIAL FINGER DESIGN (a) Picture of the process of degassing the polymer preparation (b) Sided picture of the printed prototype with the future emplacement for the SMA connector for the POF (c) Sided picture of the printed prototype with the visible transparency

Figure 1 . 12 :

 112 Figure 1.12: Pictures of the prototype of the method of assembly of the artificial finger.

Figure 1 . 13 :

 113 Figure 1.13: Example the friction force from the tribometer (top) and the irradiance signals obtained from the artificial finger (bottom) during several sliding cycles against a plain fabric. The curves are displayed relative to time, and the large variations in value are due to the alternative linear movement.

  (a) The frame of the arti cial nger is empty with the POF inserted in the connectors. (b) The arti cial nger is presented upside down and the PDMS is poured by hand enough to cover the inside part of the connectors. (c) The arti cial nger is presented upside down and capped with the bottom part (on top in the schematics) comprising the stud within the spacer ring (a) and the mold for the texture (b).

  (d) Arti cial nger with all components for resting until removal of the molding parts.

Figure 1 . 14 :

 114 Figure 1.14: Schematics representing different stages of the making of the artificial finger.

  .1 in which two tests are represented on both graphics corresponding to slider 45p and slider 45al. The graphic represents the evolution of the irradiance transmitted relative to time. The choice of an aluminum frame has improved the repeatability of the measurement as expected (fig. 2.1). However, it can be seen that power value of irradiance for each artificial finger is very different, for comparison of value between sliders it may be necessary to use relative irradiance. With slider45al a friction test was executed with the plain woven fabric. It can be observed from figure 2.2 that there is a correlation between the 2. FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER Irradiance transmitted by slider45p during two di erent tests with the plain woven fabric Irradiance transmitted by slider45al during two di erent tests with the plain woven fabric

Figure 2 . 1 :

 21 Figure 2.1: Repeatability of the irradiance measurement from slider 45p and slider 45al with the plain woven fabric

2. 1

 1 Validation of the friction properties of the artificial finger -

Figure 2 . 2 :

 22 Figure 2.2: Measurement realized during a friction test with the plain woven fabric from slider 45al

Figure 2 . 4 :

 24 Figure 2.4: Schematics of the tilt movement occurring on the slider depending on the friction direction

Figure 2 . 5 :

 25 Figure2.5: Relative irradiance acquisition of slider va (PDMS is stuck in the core) during a measurement with the a plain woven fabric. The relative irradiance was calculated based on the value of irradiance at the start of the test corresponding to the artificial finger loaded with the normal load of 0.5 N before any movement.

Figure 2 . 6 :

 26 Figure 2.6: Measurements realized with slider 45fg during friction on different fabrics with same normal load of 0.5 N applied

  2.6 against 0.5 mW/cm 2 due to the difference in behavior for each side according to fig.2.8. The asymmetric behavior of both sides can be related to manufacturing sources of deviation in the artificial finger or a misinterpretation of the influence of the deformation of the fiber on the irradiance. The manufacturing deviations are present since differences can be seen on each versions of the artificial finger, but the2. FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER Normal force vs penetration depth

Figure 2 . 7 :

 27 Figure 2.7: Irradiance and normal force vs penetration depth measured during indentation of the slider 45fg artificial finger. 10 different tests were issued with the same conditions and show the same behavior.

2. 1

 1 Validation of the friction properties of the artificial finger /cm²) Penetration depth (mm) Loading left side Unloading left side Loading right side Unloading right side

Figure 2 . 8 :

 28 Figure 2.8: Irradiance vs penetration depth during indentation of each side of the artificial finger slider 45fg at the connector position

Figure 2 . 9 :

 29 Figure2.9: Schematics of the flipping of the artificial finger for the following measurements. Top view. The arrows represent the optical path of light and the rectangle is the tribometer translating plate. The translating plate will conserve the motion and the connection on the artificial finger will also be conserved, the connectors are physically labeled A and B. Connector A will remain the input connector and connector B will remain the output connector. Direction is shown relative to the support motion.

Figure 2 .

 2 Figure 2.10: Irradiance vs time for two friction tests on the plain woven fabric with an artificial finger able to flip. Dark curve corresponds to the left situation of 2.9. Light curve corresponds to the right situation of 2.9.

Figure 2 . 13 :POFFigure 2 . 14 :

 213214 Figure 2.13:The representation of a bent optical fiber with a thin cladding. In this situation the light ray can re-enter the fiber core after having exited the fiber core because of a curve. Extracted from[START_REF] Durana | Dependence of bending losses on cladding thickness in plastic optical fibers[END_REF]

2 .

 2 FRICTION INVESTIGATION OF THE INSTRUMENTED ARTIFICIAL FINGER (a) 30° ber angle (b) 45° ber angle (c) 60° ber angle

Figure 2 . 15 :

 215 Figure 2.15: Pictures of the lit POF in different fiber angle frames. Picture are realized with a constant setting of the camera.

Figure 2 .

 2 Figure2.16: Graphic representing the variation of the transmitted power in terms of irradiance relative to the aligned connector position representing 100% and with the curvature of the fiber expressed in terms of radius of curvature obtained with measurement on the pictures.

Figure 2 . 18 :

 218 Figure 2.18: Image obtained after editing the picture made by the camera for the measurement of the curvature radius of the pressed POF at the center of the initial curvature. The decomposition of the considered parts of the deformation is shown.

Figure 2 . 19 :

 219 Figure 2.19: Pictures of the POF with pressure locations at the two considered offset positions for the 45°frame.

Figure 2 . 20 :

 220 Figure 2.20: Graphic of the evolution of the transmitted power with the different positions of press and the initial curvature value

Figure 2 . 21 :

 221 Figure 2.21: Relative irradiance relative to time during friction with slider 30 and slider 45fg against the plain woven fabric

2. 4

 4 Friction tests with a 30°fiber angle designed artificial finger

Figure 2 .Figure 2 . 23 :

 2223 Figure 2.22: Irradiance (a) and tangential force (b) relative to time for different kinds of textile fabrics with slider 30.

2. 4 Figure 2 . 24 :

 4224 Figure 2.24: Relative irradiance obtained with slider 30 on a blank sheet of paper for different sliding velocities.

Figure 2 . 25 :

 225 Figure 2.25: Relative irradiance and tangential force relative to time for slider 30 against a printed piece of paper. Half is blank and half is black. The vertical bars separate the blank and printed zones. The first occurrence is blank.

  Black stripe of 5 mm in width.

Figure 2 . 27 :

 227 Figure 2.27: Relative irradiance and tangential force relative to time for slider 30 against three different widths of black stripes printed on a blank sheet of paper.

Figure 2 . 28 :

 228 Figure2.28: Graphic of the evolution of relative irradiance during a friction of slider 30 against a piece of paper with a line of 2.5 mm and a adhesive tape taped above the paper. a is the irradiance measurement and b is the tangential force.

Figure 3 . 1 :

 31 Figure 3.1: Picture of the new tribometer used for the investigation of textile fabrics with the artificial finger. a) is the artificial finger support arm, a special piece is hidden here that sets the angle of the axis of the POF within the artificial finger and the sliding direction, b)is the tri-axis force sensor and c) is the translating table. On this picture, the POF angle is 90°relative to the sliding direction.

Figure 3 . 2 :

 32 Figure 3.2: Picture of the new tribometer used for the investigation of textile fabrics with the artificial finger. On this picture, the POF angle of the slider is 0°relative to the sliding direction.

Figure 3 . 3 :Figure 3 . 4 :

 3334 Figure 3.3: Graphical representation of the responsivity function of the acquired 918D-SL-OD3R photodiode. Extracted from the calibration certificate enclosed with the photodiode.

( a )

 a Picture of the Pekin fabric without magni cation (b) Picture of the Pekin fabric with 16x magni cation. The weaving pattern can be observed on this picture.

Figure 3 . 5 :

 35 Figure 3.5: Picture of different magnification of the Pekin fabric

Figure 3 . 6 :

 36 Figure 3.6: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 0°on Pekin fabric relative to the displacement.

Figure 3 . 7 :

 37 Figure 3.7: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 90°on Pekin fabric relative to the displacement.

Figure 3 . 8 :

 38 Figure 3.8: FFT calculation realized on the irradiance measurement from the Pekin fabric.

( a )

 a Picture of the velvet fabric without magni cation. The stripes are aesthetics made from two di erent colors. (b) Picture of the velvet fabric in the thickness direction. The hairiness orientation can be seen on the picture. No load was applied for the picture.

Figure 3 . 9 :

 39 Figure 3.9: Picture with different magnifications of the velvet fabric

Figure 3 . 10 :

 310 Figure 3.10: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 0°on velvet fabric relative to the displacement.

Figure 3 . 11 :

 311 Figure 3.11: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 90°on velvet fabric relative to the displacement.

Figure 3 . 12 : 1

 3121 Figure 3.12: Irradiance for Pekin and velvet fabrics with the corresponding fitting curves for POF angles of 0°and 90°.

Figure 4 . 3 :

 43 Figure 4.3: Profile of the printed paper measured with a profilometer. The left part is the printed paper and the right part is blank. A step of 10 µm is measured.

4. 1 . 1

 11 fig. 4.4 show the sensitivity of the artificial finger under single direction stimulation:

  This experiment was realized to observe the capability of the artificial finger to measure single component forces. In the case of the normal force, the results obtained can be compared to the experiments realized with the large indenter in §2.1 where the result is reproduced in fig. 4.4a). It can be seen form fig. 4.4b and fig. 4.4a a difference of behavior. Nevertheless, that can be explained by the position of the artificial finger:

  Irradiance with its tting curve, R 2 = 0.96

Figure 4 . 1 :

 41 Figure 4.1: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 0°on the printed paper relative to the displacement. The left part from 0 to -45 mm is printed and the right part 0 to 45 mm is blank.

Figure 4 . 2 :

 42 Figure 4.2: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 with a POF angle of 90°on the printed paper relative to the displacement. The left part from 0 to -45 mm is printed and the right part 0 to 45 mm is blank.

Figure 4 . 4 :

 44 Figure 4.4: Graphics for the description of the artificial finger under normal and tangential variations separately.

Figure 4 . 5 :

 45 Figure 4.5: Graphics for the description of the artificial finger slider 30 n2 under normal and tangential variations separately.

Figure 4 . 6 :Figure 4 . 7 :

 4647 Figure 4.6: Graphics for the description of a friction test of the artificial finger slider 30 on the paper with both ending black printed zones and a POF angle of 0°.

Figure 4 . 8 :

 48 Figure 4.8: Graphics for the description of a friction test of the artificial finger slider 30 n2 on the printed paper with the black printed zone at the center and a POF angle of 0°.

  Irradiance with its tting curve. R 2 = 0.97

Figure 4 . 9 :Figure 4 . 10 :

 49410 Figure 4.9: Graphics for the description of a friction test of the artificial finger slider 30 on the paper with both ending black printed zones and with a POF angle of 90°.

  Normal force measurement of the friction of slider 30 n2 on the printed paper Tangential force measurement of the friction of slider 30 n2 on the printed paper COF calculated for the testing of friction of slider 30 n2 on the printed paper Irradiance measurement of the friction of slider 30 n2 on the printed paper with its tting and a POF angle of 90°. R 2 = 0.86

Figure 4 . 11 :

 411 Figure 4.11: Graphics for the description of a friction test of the artificial finger slider 30 n2 on the printed paper with the black printed zone at the center and a POF angle of 90°.

  Pekin fabric with a POF angle of 0°. Pekin fabric with a POF angle of 90°. Velvet fabric with a POF angle of 0°. Velvet fabric with a POF angle of 90°.

Figure 4 . 12 :

 412 Figure 4.12: Graphics of the irradiance measurement for Pekin and velvet fabrics with the fitting curve obtained by the developed calculation

Figure 4 . 13 :

 413 Figure 4.13: Irradiance cycle highlighting the description of the shifted detection of the surface feature change from the POF angle of 90°orthe POF angle of 0°with slider 30 n2 on the same piece of paper.

Figure 4 . 14 :

 414 Figure 4.14: Graphical representation of the profile of the artificial finger obtained with a multifocal microscope.

4. 2 . 2 . 2

 222 Monitoring of the stud deformationThe artificial finger was recorded with a camera during the friction tests realized on the different surfaces. The obtained films were analyzed with the Tracker® software.The software enables the tracking of a specific point on a film and produces a table in which the instantaneous positions of the point are recorded. The software requires a scaling which has been realized with the diameter of the artificial finger stud measured with 13.87 mm. The tracked point on the artificial finger is the edge with the surface for slider 30 n2 with the same characteristics as slider 30 (fig. 4.16) (3.1). The pink lines correspond to the axis used for reference. The increase in coordinate is from left to right and from bottom to top on the image. The tracked points, labeled A and B as shown in figure 4.16 are the extreme left and extreme right points of the stud. The recording was used to observe the actual horizontal displacement of the artificial finger stud during a friction test. The objective was a comparison of the observed position with the irradiance measurement realized with the POF. In fact these measurements with the slider30 and slider30n2 (3.1) showed a difference in behavior with the POF angles of 0°and 90°(fig. 4.9, fig. 4.6, fig. 4.10 and fig. 4.7). In fact there is to different questions: 4.2 Behavior of the artificial finger relative to sliding direction

Figure 4 . 15 :

 415 Figure 4.15: Picture of the screen of the tracker software settings for the detection of the stud displacement. Setting with slider 30 n2 artificial finger. The dimension considered as reference is 13.87 mm. The tracked part corresponds to the point in the red circle. The red circle is the detection zone considered and is compared with the reference labeled "gabarit".

On figures 4 .Figure 4 . 16 :

 4416 Figure 4.16: Schematics of the tracked points on the artificial finger. On direction 1, A1 and B1 have a positive acquired position and on direction 2, A1 and B1 have a negative acquired position.

Figure 4 . 17 :

 417 Figure 4.17: Schematics of the evolution of the recording during the tracking of the artificial finger points. An image during movement is used as support to explain the values obtained.

Figure 4 . 18 :

 418 Figure 4.18: Displacement of points A and B of the slider 30 stud during a friction test against a sheet of paper with both sides black printed. The POF angle is of 0°. Only the first cycle is represented.

Figure 4 . 21 :

 421 Figure 4.21: Displacement of points A and B of the slider 30 n2 stud during a friction test against a sheet of paper with both sides black printed. The POF angle is of 0°. Only the first cycle is represented.

Figure 4 . 27 :

 427 Figure 4.27: Picture of the rounded artificial finger on the tribometer.

4. 3

 3 Behavior of the artificial finger with rounded border COF calculated from normal and tangential forces during friction of slider 30 arr on the printed paper Irradiance with its tting curve. R 2 = 0.77

Figure 4 . 28 :

 428 Figure 4.28: Graphics for the description of a friction test of the artificial finger slider 30 arr on the printed paper with the mark at the center and a POF angle of 0°.

Figure 4 . 29 :

 429 Figure 4.29: Graphics for the description of a friction test of the artificial finger slider 30 arr on the printed paper with the mark at the center and a POF angle of 90°.

Figure 4 .

 4 Figure 4.30: Picture of the screen of the tracker software settings for the detection of the stud displacement. Setting with slider 30 arr artificial finger. The dimension considered as reference is 13.87 mm. The tracked part corresponds to the point in the red circle. The red circle is the detection zone considered and is compared with the reference labeled "gabarit".

4. 3

 3 Behavior of the artificial finger with rounded border • The artificial finger is better sensitive to the sliding direction with a POF angle of 90°•The artificial finger is better sensitive to surface features causing normal force variations with a POF angle of 0°N ext, the production of the artificial finger with rounded edges showed the importance of the contact zone of the artificial finger with the textile surface. The optical measurement with this artificial finger has to be improved compared to the flat ended artificial fingers.

  Stud motion view from the left point. ε l Stud motion view from the right point. εr

Figure 5 . 1 :

 51 Figure 5.1: Slider30 n2 stud motion. The friction test was realized against the Pekin fabric. The POF angle was of 0°. Only the first cycle is represented.

5. 1 Figure 5 . 2 :igure 5 . 3 :

 15253 Figure 5.2: Slider30 n2 stud motion. The friction test was realized against the Pekin fabric. The POF angle was of 90°. Only the first cycle is represented.

Figure 5 . 4 :

 54 Figure 5.4: Example pictures of the 3D printed surfaces tested with the artificial finger.

Figure 5 . 5 :Figure 5 . 6 :

 5556 Figure 5.5: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 n2 on the S0 surface with a POF angle of 90°.

Figure 5 . 7 :

 57 Figure 5.7: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 n2 on the S2 surface with a POF angle of 90°.

Figure 5 . 8 :

 58 Figure 5.8: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 n2 on the S3 surface with a POF angle of 90°.

Figure 5 . 9 :

 59 Figure 5.9: FFT of the irradiance signal obtained with slider 30 n2 for the 3D printed surfaces S1, S2 and S3.

Figure 5 . 10 :

 510 Figure 5.10: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the S1 surface with a POF angle of 90°.

Figure 5 . 11 :

 511 Figure 5.11: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the S2 surface with a POF angle of 90°.

5. 2

 2 Testing on 3D printed surfaces

Figure 5 . 12 :

 512 Figure 5.12: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the S3 surface with a POF angle of 90°.

  arr and surface S4 (fig.5.15). The objective here was the understanding of the source of the detection of the features of the surface with the artificial finger. With the specific shape of the rounded artificial finger, it has been possible to acquire a reliable vertical displacement of the stud because an angle is realized between the rounded part of the stud and the upper part of the stud. Thus in this graphic is displayed the vertical displacement of the stud during the friction against surface S4. The correlation between the vertical displacement and the irradiance can be observed. It is known that the vertical displacement of the stud is linked to the normal force measured with the tribometer.

5. 2 Figure 5 . 13 :

 2513 Figure 5.13: FFT of the irradiance signal obtained with slider 30 arr on the 3D printed surfaces S1, S2 and S3.

Figure 5 . 14 :

 514 Figure 5.14: FFT of the irradiance signal obtained with slider 30 arr on the 3D printed surfaces with different sliding speeds.

Figure 5 . 15 :

 515 Figure 5.15: Graphics for the description of a friction test of the artificial finger slider 30 arr on the S4 surface associated with a camera recording of the friction.

69 Figure 5 . 16 :Figure 5 . 17 :

 69516517 Figure 5.16: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the velvet I fabric with a POF angle of 0°.

Figure 5 . 18 :

 518 Figure 5.18: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the velvet III fabric with a POF angle of 0°.

5. 3 Figure 5 . 19 :

 3519 Figure 5.19: Evolution of the tribological criteria (normal force, friction force and COF) and the irradiance obtained from the artificial finger slider 30 arr on the velvet V fabric with a POF angle of 0°.

  Description of the textile surfaces . . . . . . . . . . . . . . . . . . . 35 2.1.2 First friction experiments with the artificial finger . . . . . . . . . 36 2.1.2.1 Friction experiment measurements . . . . . . . . . . . . . 36 2.1.2.2 Observations and difficulties . . . . . . . . . . . . . . . . 43 2.1.3 Normal loading influence on irradiance in the artificial finger . . . 45 2.2 Optical fiber description and behavior . . . . . . . . . . . . . . . . . . . . 49 2.3 Experiments realized on a bare fiber locked in a specifically made frame . 53

1.1.2 Proposed solutions for POF implementation . . . . . . . . . . . . . 15 1.2 Conception of the artificial finger . . . . . . . . . . . . . . . . . . . . . . . 19 1.3 Fabrication protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2 Friction investigation of the instrumented artificial finger 35 2.1 Validation of the friction properties of the artificial finger . . . . . . . . . 35 2.1.1

  Table of the Young moduli for several kinds of used POF in literature.

*POF provided by the Empa . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Table of the COF of the artificial finger and a human panel against test textiles. Results with * reported in (1) for the same fabric. . . . . . . . . 1.5 Table of the elastic modulus variation of PDMS Sylgard® 184 depending of the curing time and temperature. Results from (70). . . . . . . . . . . .

Table 1 .

 1

	1. ARTIFICIAL FINGER DESIGN

Table 1 . 4 :

 14 Table of the COF of the artificial finger and a human panel against test textiles. Results with * reported in (1) for the same fabric.

	1.2 Conception of the artificial finger

Table 1 . 5 :

 15 Table of the elastic modulus variation of PDMS Sylgard® 184 depending of the curing time and temperature. Results from (70).

	1. ARTIFICIAL FINGER DESIGN
	Temperature (cure time) Young's Modulus E (MPa) Hardness (Shore A)
	25 °C (48h)	1.32 ± 0.07	43.8 ± 0.76
	100 °C (48min)	2.05 ± 0.12	48.3 ± 0.65
	125 °C (33min)	2.46 ± 0.16	49.8 ± 0.43
	150 °C (23min)	2.59 ± 0.08	52.8 ± 0.73
	200 °C (18min)	2.97 ± 0.04	54.0 ± 0.08

  2.6) butthe observed irradiance is not smooth. The variations observed can be due to close interactions of the surface that are not recorded by the remote force sensor. The question is now the functioning of the POF behavior.

					2.2 Optical fiber description and behavior
		1.06
		Dir. 1	Dir. 2	Dir. 1	Dir. 2
	Relative irradiance	0.96 0.98 1 1.02 1.04
		0.94
	-5	0	5		10	15	20	25	30
					Time (s)

Table 3 .

 3

		Fiber angle Fiber	Stuck con-	Stud shape Body mate-
	finger name	guides	nectors		rial
	Slider 45p	45°No	No	Cylindrical	3D printer
				flat ended	polymer
	Slider 45al	45°No	No	Cylindrical	Aluminum
				flat ended
	Slider va	45°No	No	Cylindrical	Aluminum
				flat ended	treated for
					PDMS core
					adhesion
	Slider 45fg	45°Yes	Yes	Cylindrical	Aluminum
				flat ended
	Slider 30	30°Yes	Yes	Cylindrical	Aluminum
				flat ended
	Slider 30 n2 30°Yes	Yes	Cylindrical	Aluminum
				flat ended
	Slider arr	30°Yes	Yes	Cylindrical	Aluminum
				rounded

1: Recollection of the main features of the different artificial fingers realized .

Table 4 . 1 :

 41 4.1 Behavior of the artificial finger relative to friction Roughness parameters measured on the paper on blank and printed parts

	Part of paper	Ra	Rq
	Blank	2.59 ± 0.05 µm 3.23 ± 0.05 µm
	Printed	2.01 ± 0.10 µm 2.6 ± 0.12 µm

  Behavior of the artificial finger relative to friction • In the other direction an increase in tangential force causes a decrease in irradiance, as it can be observed in the bottom part of Figures 4.6d, 4.7d and 4.13b.

	4.1 In another way, in both directions, the increase in normal force should cause an
	increase in irradiance from the evolution of the irradiance in Figures 4.6 to 4.8. The
	observed sensitivity of the artificial finger to normal force does not follow its previously
	observed sensitivity. The artificial finger sensitivity to tangential force is however re-
	versed depending of the direction as seen in Figures 4.6 to 4.8. It can be concluded that
	the tangential force response is predominant in these cases.
	4.1.3 Case of POF angle of 90°T
	he observed evolution with the asymmetric response of the artificial finger will be
	investigated with both slider 30 and slider 30 n2 in the case of the POF angle of 90°.
	It can be observed from Figures 4.9, 4.10 and 4.11 that both artificial fingers present
	a different behavior to forces depending on the sliding direction. In fact for slider 30
	(fig. 4.9):
	4.6d, 4.7d and 4.13b.

Table 5 . 1 :

 51 Geometrical features of the 3D printed surfaces in terms of plot diameter, pattern dimension and provoked excitation frequency in the case of a sliding velocity of 10 mm/s. The plots have a height of around 80 µm

	.

Figure 2.17: Image obtained after editing the picture made by the camera for the measurement of the curvature radius. The example given is for the 45°frame.

ARTIFICIAL FINGER BEHAVIOR

Remerciements

LIST OF TABLES

Chapter 3

Friction results obtained with some textile surfaces The artificial finger slider 30 shows the asymmetric behavior in terms of irradiance measurement for the two directions with both POF angles of 0°and 90°(fig. 4.6 and 4.9).

The analysis of the recording of the artificial finger slider 30 on figures 4.18 and 4.19 shows the same shape of curve than the irradiance and COF measurements. One observation can be made, when a point is the forward point, the amplitude of the variation due to change in COF is higher than in backward position for the same point. Second observation is the difference in amplitude of deformation due to the friction on backward and forward positions.

Because of this observation, a calculation of the deformation state of the stud is realized. Because of the definition of the system, the comparison of the values corresponds to the comparison of the displacements from A point and B point. In other words : • For the tests presenting variations of COF due to a change in the friction properties of the surface, the compression levels of the stud are of similar value for each COF zone, and this is better observed on the low COF zones (blank).

• For situations where the change in COF is detected by an edge of the artificial finger, the difference of the position of the points shows that the situation with a shifted detection is associated with a compression of the stud implying an uplifting of the backward part of the artificial finger stud. While for global detection there is little difference in the measured positioning of the outer points of the artificial finger stud implying a whole displacement of the stud during the friction.

In this chapter, the compression of the slider has been observed along with the amplitude of displacement of the artificial finger. However, the observation of these phenomenons in regards to the tribological measurements and the irradiance measurements could not be used to explain the asymmetric behavior of the artificial finger in irradiance between the two sliding directions and evolution of irradiance with friction forces.

Behavior of the artificial finger with rounded border

To verify the hypothesis of the contact of the artificial finger being a source of deviation in the irradiance measurement, the realization of an artificial finger with rounded edges to verify has been made and will be tested on the printed paper with centered mark first.

As a reminder, slider 30 arr is realized with a POF angle of 30 degrees, possesses fiber guides in the connectors and has texturing on the stud and additionally had rounded Given the observations made with the slider 30 arr artificial finger, the use of the rounded edges can be questioned. Indeed, the optical measurements obtained with the slider 30 arr artificial finger show no sliding direction detection with a POF angle of 0°and the changes in COF cannot be extracted from the graphics obtained with both POF angles. The correlation between the light measurement and the forces measured can however be high since a R 2 coefficient of 0.86 was obtained for the situation with a POF angle of 90°. It is necessary to produce another artificial finger with rounded edges to compare its behavior with the current one to conclude on the influence of the contact shape in the measurement. The investigation on the artificial finger behavior considering the different observations made however ends at this point due to the termination of the thesis period.