V Pommier-Budinger

O Cherrier

J Morlier

F Lachaud

DAMAGE LOCALIZATION METHOD FOR THIN COMPOSITE STRUCTURES APPLICATION TO AN AIRCRAFT DOOR

Piezoelectric sensors are widely used for Structure Health Monitoring (SHM) technique due to their high-frequency capability. In particular, electromechanical impedance (EMI) techniques give simple and low cost solutions for detecting damage in composite structures. For example, damage indicators computed from EMI deviations between the undamaged and damaged structure can be compared to a threshold in order to provide information about damage presence.

When it is question of damage localization, the simple analysis of the EMIs fails to furnish enough information. We propose a method based both on EMI damage indicators and on the acoustic attenuation level to localize damage. One of the main advantages of our method, so called data driven method, is that only experimental data are used as inputs for our algorithms. It does not rely on any model.

INTRODUCTION

Structural Health Monitoring (SHM) is an emerging technology, dealing with the development and implementation of techniques and systems where monitoring, inspection and damage detection become an integral part of structures. It further merges with a variety of techniques that will provide information about the condition of a structure in terms of reliability and safety before the damage threatens the integrity of the structure [START_REF] Farrar | An introduction to structural health monitoring[END_REF][START_REF] Doebling | A Summary Review of Vibration-Based Damage Identification Methods: Identification Methods[END_REF]. The major advantage of SHM techniques is their online implementation and their mixed global/local approach. Classical SHM methods use model driven data that require a high quality FE model and a damage model (delamination, crack etc…). Then the damage is identified from a metric corresponding to the distance between experimental and baseline numerical data (FRF, natural frequencies, modeshapes etc.) [START_REF] Farrar | Damage prognosis: the future of structural health monitoring[END_REF][START_REF] Schubel | Impact and post impact behaviour of composite sandwich panels[END_REF][START_REF] Abrate | Impact on composite structures[END_REF]. The localization problem can then be viewed as an inverse problem often solved using optimization and model updating process [START_REF] Petit | Impact and compression after impact experimental study of a composite laminate with a cork thermal shield[END_REF][START_REF] Cawley | The location of defects in structures from measurements of natural frequencies[END_REF][START_REF] Montalvão | A Review of Vibration-based Structural Health Monitoring with Special Emphasis on Composite Materials[END_REF][START_REF] Shahdin | Damage monitoring in sandwich beams by modal parameter shifts: a comparative study of burst random and sine dwell vibration testing[END_REF] for minimizing the error between experimental and numerical data. Our work is based on industrial needs:

• low cost instrumentation and easy connection,

• simple processing of data and reliable visualization of damage localization.

To achieve the first point, smart materials are commonly used. In particular, the electromechanical impedance (EMI) based SHM techniques have several advantages such as the use of non-intrusive piezoelectric transducers, simple measurements (only voltages and currents) and potentially low-cost implementations [START_REF] Frieden | Vibration-based characterization of impact induced delamination in composite plates using embedded FBG sensors and numerical modelling[END_REF][START_REF] Montalvao | A method for the localization of damage in a CFRP plate using damping[END_REF][START_REF] Morlier | Diagnosis of a portal frame using advanced signal processing of laser vibrometer data[END_REF][START_REF] Rucka | Application of continuous wavelet transform in vibration based damage detection method for beams and plates[END_REF][START_REF] Kim | Nondestructive damage evaluation of plates using the multiresolution analysis of two-dimensional Haar wavelet[END_REF][START_REF] Huang | Damage detection with spatially distributed 2D continuous wavelet transform[END_REF]. For the second point, damage indicators derived from the measured EMIs provide information about damage presence [START_REF] Giurgiutiu | Damage detection in thin plates and aerospace structures with the electro-mechanical impedance method[END_REF]. However, they fail to furnish enough information for damage localization. The originality of our approach is to propose a method based both on EMI damage indicators and on the acoustic attenuation level to localize damage.

ELETROMECHANICAL IMPEDANCE AND ACOUSTIC ATTENUATION FOR DAMAGE DETECTION AND LOCALIZATION

Principle of the method

The changes in local dynamics due to incipient structural damage affect the structure at ultrasonic frequencies. In a complex aeronautical structure like an aircraft door for example, such changes are too small to affect the global dynamics and hence cannot be readily detected by conventional low-frequency vibration methods. Experimental demonstrations have shown that the real part of the high frequency impedance spectrum is directly affected by the presence of damage in the structure [START_REF] Giurgiutiu | Structural health monitoring with piezoelectric wafer active sensors[END_REF]. The structure impedance spectrum can be measured indirectly by using piezoelectric transducers bonded on the mechanical structure under study. Indeed, piezoelectric transducers allow measuring high frequency electromechanical impedances resulting of the coupling between the transducers and the structure. Once the impedance spectrum is measured, the issue is to analyze this spectrum to detect and localize damage. For detection, the usual method is to compute electromechanical variation indexes and to compare them to thresholds whose levels are significant of damage existence. These thresholds have to be determined experimentally for each structure under study. To detect an impact that causes locally an increase of damping and a decrease of stiffness and thus a shift of the electromechanical impedance, usual variation indexes are: -the root mean square deviation RMSD: [START_REF] Farrar | An introduction to structural health monitoring[END_REF] where is the number of samples, the real part of impedance for the undamaged structure and the real part of impedance for the damaged structure -the mean frequency shift of modal peaks mean :

! "#$% &' ( ) ' ( *) & ' ( *) + , (-. / 012 344 (2)
where 5 is the modal frequency of the undamaged structure for mode n, 5 6 the modal frequency of the damaged structure for mode n and N pks the number of modal peaks in the studied frequency band.

For damage localization, analysing the index level is not sufficient. We propose to use, in addition to variation indexes, the acoustic wave attenuation level in materials. Acoustic attenuation is a measure of the energy loss of sound propagation in media and depends on several phenomena (diffraction, reflection, diffusion and absorption).

For laminated materials and unidirectional composite materials, acoustic waves propagation is strongly dependent of fibers spacing and orientation. The technique for measuring the acoustic attenuation coefficient is to evaluate the ratio between incident and transmitted wave level with an exponential decrease of wave energy: 7 + 8 9 :

(3) so 8 9 ;

<= 7 + > :

(4) where m( ) is the acoustic attenuation coefficient function of the considered frequency, ? the initial level of generated signal (incident wave) and @ the received signal after propagation at distance6A (transmitted wave).

The proposed method relies on the hypothesis that the electromechanical impedance variation is all the more important as the sensor is close to the damaged zone. So the method is based on an enhanced triangulation based on EMI spectrums and variation indexes weighted by inverse distance weighting (IDW) interpolation. The method is inspired from classical IDW (Inverse Distance Weighting) interpolation for 2D Reconstruction of scatter points. IDW methods are based on the assumption that the interpolating surface should be influenced most by the nearby points and less by the more distant points.

Description of the method

The proposed method for the construction of damage localization maps can be divided in 4 steps. Steps 1 and 2 are preliminary steps that are performed once before the monitoring. Step 3 is performed continuously. Step 4 is performed only if damage is detected in step 3.

Step 1: Instrumentation Depending on the size of structures, PZT transducers must be chosen large enough in order to give "good" EMI spectrums, that is to say with several modal peaks. They must be bonded with a rigid glue to get a good electromechanical coupling between the transducers and the structure. At least, they must also be bonded onto optimal placements that results from a trade-off between requirements for triangulation and EMI measurements.

Step 2: Map construction (meshing) using IDW for each piezoelectric transducer

The structure under study is meshed. Obviously the precision of localization is directly dependent on the mesh size. Each mesh has at least the piezoelectric transducers size. Then, for each transducer noted i, every mesh j is weighted by acoustic attenuation A ij defined by:

B CD E F G HI

(5) where x ij the distance between the transducer i and the center of each mesh j. Like in IDW methods, the acoustic attenuation is such that it decreases as the distance between the transducer and each scatter point increases. This method enables to generate weighted maps with a "physical" sense. There are as many weighted maps as transducers.

Step 3: EMI measurements and damage metrics EMI measurements must be performed regularly for monitoring. In this paper, 2 EMI spectrums are measured for pre and post impact. Then we can obtain RMSD and ∆f mean indexes for each transducer using Equation 3 and 4. These indexes are then compared to thresholds to determine if damage occurred. If yes, a damage localization map is built using step 4 of the method.

Step 4: Damage localization map computing

The damage indicators DI are computed using IDW interpolation and by taking the inverse in order to get a high value of the indicator in the damaged zone. For each mesh j, the damage indicator based on RMSD index is computed by:

JK L MNOP Q R MNO H S HI T U VWX Y X (6)
and the damage indicator based on mean index is computed by:

JK L Z' [\]( Q R Z' [\](^H S HI T U VWX Y X ( 7
)
where a is the number of transducers. For 1D structures, the number of transducers is at least two and, for 2D structures, it is at least three.

Plotting the values of a damage indicator for each mesh then gives one damage location map that enables to evaluate the damage position on the structure.

EXPERIMENTAL VALIDATION ON AN AIRCRAFT DOOR STIFFENER

Structure under study

The structure under study is a composite stiffener of an aircraft door. It has the shape of an I-beam of mean dimensions 66*89*1045mm. The beam web is composed of 16 plies of carbon/epoxy plus a peel ply of tissue (it is considered that it has an isotropic behaviour). The aircraft stiffener is a part of the door that is particularly monitored during the life cycle of the aircraft. Consequently, it is of high interest to find a method for real time SHM. The proposal method has been tested in two configurations: the stiffener is in laboratory conditions, alone and disassembled from the door, and in real conditions, fixed on the door.

Stiffener tested in laboratory conditions

Step 1: Instrumentation The stiffener is instrumented for damage localization: 2 PZT piezoelectric patches type DuraAct® (PI ceramics® PIC255) of dimensions 50*30*0.5mm are glued on to the beam web near each extremity of the beam with a structural glue type 3M DP460® (Figure 1). Step

2: Map construction (meshing) using IDW for each piezoelectric transducer

The stiffener web is meshed in equal parts (11 meshes of 95mm length and 89mm height). Preliminary tests demonstrate that the best EMI response (i.e. with detectable peaks) is between 8 and 16 kHz for the stiffener. In the beam web, the acoustic attenuation is measured in this frequency band and the mean value of the acoustic attenuation coefficient is 40e-3 [m -1 ]. Then, 2 maps, one for each transducer, can be computed with IDW interpolation and plotted (Figure 3). Step 3: EMI measurements and variation indexes Before and after damage generation, the real parts of the electromechanical impedances are measured (Figure 4). From EMI signatures, mean and RMSD indexes are computed. The variation indexes are presented in Table 1. Classically, indexes for damage n°2 are computed while considering the state with the first damage as the undamaged state (D1 replace UD as reference in equations 1 and 2). Table 1: RMSD and mean indexes for the composite stiffener and for the two transducers (pzt1 and pzt2)

Figure 4: Experimental EMI signatures (between 8 and 16 kHz) for the two transducers on the stiffener, for undamaged case (UD) and damaged cases after a first impact energy of 35J (D1 35J) and after a second impact energy of 35J (D2 35J)

Step 4: Damage localization map computing Figure 5 gives the damage localization maps plotted from RMSD and f mean variation indexes by using the method proposed in section 2. In Figures 5(a) and 5(b), the probability of damage is plotted for mean variation index. In Figures 5(c) and 5(d), the results for RMSD variation index are highlighted. For the first damage, the localization predicted by RMSD index is precise since the high probability zone indicates the real zone of impact. For the second damage, the predicted localization is less accurate. In comparison, the localization predicted by f mean index is excellent for every impact.

Stiffener tested in real conditions

Step 1: Instrumentation The aircraft door comprises 6 stiffeners. As the number of impact tests is limited, tests will be performed for a single stiffener but 2 stiffeners and the inside of the door skin have been instrumented, each with 4 PZT piezoelectric patches type DuraAct® of dimensions 50*30*0.5mm, in order to study the problem of false detection (Figure 6). 4 PZT piezoelectric have been bonded onto the stiffeners and not only 2 as in the previous tests because of the door ribs. Figure 6 also shows the positions of the impacts that have been realised by a drop machine (Figure 7). Step 2: Map construction (meshing) using IDW for each piezoelectric transducer As previously, the stiffener web is meshed in equal parts (11 meshes of 95mm length and 89mm height) and the mesh of the stiffener with IDW interpolation computed by the exponential attenuation law for m=40e-3 is given in Figure 8 for each piezoelectric transducer. For the inside of the door, the skin is considered as a plate and is meshed with IDW interpolation computed by the exponential attenuation law for m=5e-3 as shown in Figure 8(e). Step 3: EMI measurements and variation indexes Before and after the damage, the real part of the electromechanical impedance is measured in the bandwidth 8-16KHz. The EMI spectrums are not given here since they are quite numerous. Tables 2 and3 give only mean and RMSD variation indexes for every piezoelectric transducer of the stiffener under study. Figure 9 summarize all the values for all transducers of the aircraft door. Step 4: Damage localization map computing Figure 9 gives the variations indexes computed for every piezoelectric transducer of both instrumented stiffeners and of the skin. Then, for every index, the four values obtained for the transducers have been summed for every stiffener and the skin. It is assumed that the impact is more likely to have occurred on the device for which the sum of the variation indexes is the highest. With this assumption, Figure 9 gives the damage localization maps for damage D1, D2 and D3 plotted from f mean and RMSD variation indexes by using the method proposed in section 2. Then, results show that both variation indexes localize the impact on the good stiffener for damage D1 and D3 and that only f mean variation index indicates the good stiffener for damage D2. Moreover, it can be observed that f mean variation index gives the position of the impact with a good precision for the first two impacts. For the third impact, the localization is not so accurate. It must be due to the presence of the rib. This result shows that, for complex structures, it may be necessary to increase the number of transducers.

CONCLUSIONS

The proposed method permits to detect and localize a single isolated damage in composite structures. The originality of the method is to generate a damage localization map based on both inverse distance weighting (IDW) interpolation and indicators computed from EMI spectrums measured by piezoelectric transducers bonded onto the structures. The weights for the interpolation have a physical sense and are computed according to an exponential law of the measured attenuation of acoustic waves. One of the main advantages of our method, so called data driven method, is that only experimental data are used as inputs for our algorithms. It does not rely on any model.

Figure 1 -

 1 Figure 1 -Instrumented composite stiffener in clamped configuration in the laboratory To test the proposed method of impact location, two positions of damage between the two transducers are investigated one after the other. Impacts (D1 and D2) are located on the flange of the beam (Figure 2a) and an example of damage is shown in Figure (2b).

Figure 2 :

 2 Figure 2: (a) positions of the impacts on the composite stiffener (b) example of typical damage

Figure 3 :

 3 Figure 3: Mesh of the stiffener with IDW interpolation computed by the exponential attenuation law for m=40e-3[m-1] (a) for transducer n°1 (b) for transducer n°2

Figure 5 :

 5 Figure 5: Damage localization maps for the two successive impacts (real in blue circle) for mean index (a,b) for RMSD index (c,d)

Figure 6 :Figure 7 :

 67 Figure 6: Schema of the door with the piezoelectric patches and the positions of the impacts

Figure 8 :

 8 Figure 8: Mesh of the stiffener with IDW interpolation (a) for transducer n°1 (b) for transducer n°2 (c) for transducer n°3 (d) for transducer n°4 and mesh of the inside of the door with IDW interpolation for the 4 transducers (e)

Figure 9 :

 9 Damage localization maps for damage D1 (a), D2 (b) and D3 (c) plotted from f mean and RMSD variation indexes

Table 2 :

 2 RMSD indexes for the composite stiffener and the four transducers

		RMSD pzt1 (%)	RMSD pzt2 (%)	RMSD pzt3 (%)	RMSD pzt4 (%)
	Damage n°1	3.64	3.3	2.87	7.08
	Damage n°2	0.74	0.70	0.84	1.01
	Damage n°3	3.55	3.13	2.79	2.72
		mean pzt1 (%)	mean pzt2 (%)	mean pzt3 (%)	mean pzt4 (%)
	Damage n°1	0.5109	0.2252	0.3055	0.2183
	Damage n°2	0.1097	0.1219	0.0862	0.2569
	Damage n°3	0.054	0.1953	0.1141	0.3006

Table 3 :

 3 mean indexes for the composite stiffener and the four transducers

ACKNOWLEDGMENTS

The authors would like to acknowledge the Région Midi-Pyrénées, the DIRRECTE as well as the FEDER for the funding of this study through EPICEA 2009 program related to the project titled "SAPES COMPOSITES". We also thank N. Boudjemaa, LATECOERE, for his insightful comments.


Xuan Wang

Valérie Budinger
email: bvalerie.budinger@isae.fr

Yves Gourinat
email: cyves.gourinat@isae.fr

A Specific Methodology of Creep Compensation for Piezoelectric Actuators by Open-loop control

Keywords: Creep, Relaxation, Piezoelectric actuator, Inverse model, Compensation, Open-loop control

Piezoelectric actuators exhibit creep behavior in open-loop operation, which may lead to unaffordable errors in high precision static positioning systems. An inversion-based compensation strategy by open-loop control is presented for reducing creep effect. The approach utilizes a nonlinear viscoelastic model to portray creep phenomenon, which consists of a linear spring, a nonlinear dashpot and a series of nonlinear Voigt elements. It is shown that for the presented creep model the step responses are very similar to the piezoelectric actuators. In order to compensate creep effect, a concept of voltage relaxation in piezoelectric actuators is proposed. And the voltage relaxation model tantamount to the inverse creep model is derived using a PID closed-loop control system. Experimental results prove that, by insertion of voltage relaxation model in open-loop operation, creep effect is attenuated markedly in piezoelectric actuators.

Introduction

Piezoelectric actuators, with their high resolution, fast response, high stiffness, no backlash, and no friction, are commonly used in a variety of applications such as microelectromechanical systems (MEMS), ultra-precision positioning, active structural vibration and shape control [1][2][3][4]. However, piezoelectric actuators also exhibit adversary nonlinear effect including creep, which can be seen as a slow drift of the displacement of piezoelectric actuator responding to a constant applied input voltage [START_REF] Osamah | America Control Conference[END_REF]. For example, the creep effect can cause loss in precision which becomes significant when positioning is required over extended periods time [START_REF] Robinson | [END_REF].

Because the closed-loop compensation methods for creep effect may not be easy to implement in many cases due to the high cost and space requirements of displacement sensors, the open-loop compensation method attracts more attention [7][8][9][10]. This method is to establish a model which can portray the creep phenomenon of piezoelectric actuators and then reshape the input voltage based on the inversion of the model [10]. Hence, many models have been developed for characterizing the creep behavior, including the logarithmic model [START_REF] Osamah | America Control Conference[END_REF]11], LTI model [START_REF] Osamah | America Control Conference[END_REF] and nonlinear viscoelastic model [12]. Among these models, except LTI model [START_REF] Osamah | America Control Conference[END_REF] that can be directly inversed, the other models [11,12] are difficult to be inversed for control purpose due to the nonlinear function existing in the models.

In this paper, a nonlinear viscoelastic model is adopted to describe creep behavior. It is shown that for the presented creep model the step responses are very similar to the piezoelectric actuators. To circumvent the difficulty of inversing the creep model, a concept of voltage relaxation in piezoelectric actuators is proposed in this contribution. And a PID closed-loop control system is developed to derive the voltage relaxation model tantamount to the inverse creep model. Finally, experimental results prove that, by insertion of voltage relaxation model in open-loop operation, creep effect is attenuated markedly in piezoelectric actuators.

Experimental Setup

All the parameters of the creep model are identified from experimental results. The experimental setup is shown in Fig. 1. A ring-type piezoelectric actuator (NCE41, Noliac) is used as study object. The displacement sensor is a noncontact capacitive sensor (D510.100, PI). Fig. 2 shows a functional diagram of experimental system. Creep Modeling and Identification Fig. 3 illustrates the typical response of piezoelectric actuators for a step input voltage. A nonlinear viscoelastic model is employed to characterize the creep response, as shown in Fig. 4, in which the linear spring corresponds to an instantaneous elastic response and the single nonlinear dashpot relates to the steady creep component for constant voltage, in addition, the series of nonlinear Voigt elements represents a transient component. Here it is considered that there is no instantaneous elastic component, as the subsequent expansion is slower than the initial fast transient by orders of magnitude [12].

=- + = n n p v v p v v p v n v n n m s V ε λε λr μ V ε λ ε λ r μ V ε λ ε λ r μ V ε η                                               (1)
And output equation is:

= + + + + . n v v v s ε ε ε ε ε  (2)
In the above equations, λ i and r i are respectively characteristic frequencies and weights [12]; ε is the position voltage obtained from capacitive sensor shown in Fig. 1. After substituting the integration of Eq. 1 into Eq. 2 the creep model can be described as:

  - =1 = 1 - + . i p m n λ t i i V V ε r e t μ η              (3)
The model parameters consisting of the n characteristic frequencies λ i and their weights r i , and the parameters from voltage power laws, namely, p, m, μ and η. They are estimated from a series of step voltage responses, as shown in Fig. 5. First, the steady rates of creep are estimated using a regression line fitted to the last portion of the step response shown in Fig. 3. And the slopes of the regression lines corresponding to steady constant rates are fitted to the voltage power law in the last equation of Eq. 1 using another linear regression, as depicted in Fig. 6 that shows the good fitness of the parameters from voltage power laws. Second, extracting the steady component of creep from the overall creep response, only the transient components shown in Fig. 3 with zero rates and final values are left for parameter estimation. Using linear regression again, the final values of the transient components depicted in Fig. 3 are fitted to the voltage power law in the first n equations of Eq. 1. The fitness is very good as shown in Fig. 6. Third, for obtaining the characteristic frequencies and their weights, the normalized transient response of 220 V input is chosen as the reference one. Four frequencies and their weights fitting very well with the reference response are found as a first approximation, resulting in a model of order five, via "Procedure X" [START_REF] Tschoegl | The Phenomenological Theory of Linear Viscoelastic Behavior: An Introduction[END_REF], an iterative process that estimates characteristic frequencies and their weights from a step response. These parameters are further refined by employing a standard nonlinear least-squares fitting, as shown in Table 1. A comparison between measured responses to a set of constant voltages input in experiments and predicted curves employing the presented creep model as Eq. 2 is shown in Fig. 5, demonstrating the predicted curves fitting the measured responses very well. According to Eq. 3, ε (V, t) is not only the nonlinear function of time but also of the input voltage for the creep model. As a result, it is very difficult to derive the unique inverse model directly from the creep model mathematically. However, it is known that the inverse creep model should have the form of V(ε, t). Corresponding to creep, a slow drift of the displacement of piezoelectric actuator responding to a constant applied voltage, the form of inverse creep model V(ε, t) can be seen as the input voltage changes with a fixed displacement of piezoelectric actuator. Obviously, the relationship between creep and its inversion in piezoelectric actuator is analogous to the pair behaviors of creep and relaxation in viscoelastic material. Hence voltage relaxation, namely the pairing behavior of creep in piezoelectric actuators, is proposed to describe the inverse behavior of creep. That is to say, the problem of inversing creep model can be replaced by the issue of developing the voltage relaxation model of piezoelectric actuator. A PID closed-loop control system is developed using MATLAB to keep constant displacement. Fig. 7 shows the block diagram of the PID closed-loop control system that leads to a set of voltage relaxation curves can be obtained as depicted in Fig. 8.

From Fig. 8 and [START_REF] Tschoegl | The Phenomenological Theory of Linear Viscoelastic Behavior: An Introduction[END_REF], the relaxation model can be presumed as:

  - =1 =1 =- 1- - + + . i a c a b n n τ t i i i i ε ε ε ε V f e t f ρ α β ρ                           (4)
The first two terms in Eq.4 is same as Eq.3 in form except the negative sign, so the parameters from displacements power laws, namely, ρ, a, α, b, and characteristic frequencies τ i and their weights f i can be estimated via the similar process of parameter identification in creep modeling. The parameters β and c from displacement power laws in the third term of Eq.4 can be identified using linear regression once again. Fig. 8 illustrates the very good agreement between the voltage relaxation curves obtained from PID closed-loop system and from the model.

Fig. 1

 1 Fig. 1 The experimental setup Fig. 2 Functional diagram of experimental system

Fig. 3

 3 Fig. 3 Experimental creep response Fig. 4 The phenomenological model for creep The creep model shown in Fig. 4 can be described in state-space form using the n transient components and the steady creep component as states:

Fig. 5 Fig. 6

 56 Fig. 5 Responses to a series of input voltages and their predictions

Fig. 7

 7 Fig. 7 Diagram of PID closed-loop system Fig. 8 Voltage relaxation curves

Table 1

 1 Characteristic frequencies and weigths

	Char. Freq. λ	Weight r	Char. Freq. λ	Weight r
	Proc. X	Proc. X	NLSQ	NLSQ
	0.1129	0.024	0.197	0.0706
	0.2166	0.0495	1.1594	0.0282
	1.0817	0.0388	2.4434	0.0261
	10.8936	0.3755	17.7129	0.863
	Voltage Relaxation Modeling


Stanislas Lagarde
email: stanislas.lagarde@ap-hm.fr

J-C Lagier

R Charrel

G Quérat

J Vanhomwegen

P Desprès

J Pelletier

E Kaphan

JAPANESE ENCEPHALITIS IN A FRENCH TRAVELLER TO NEPAL

Keywords: Japanese encephalitis, Nepal, Travel Medicine, Vaccination, Viral Encephalitis, Flavivirus, arthropod-borne virus

Japanese encephalitis is frequent in Asia, with a severe prognosis, but rare in travelers. Culex mosquitoes transmit Japanese encephalitis virus. Risk factors are destination, duration of stay, summer and fall seasons, outdoors activities and type of accommodation. We report the case of a French traveler to Nepal with neutralization-based serological confirmed Japanese Encephalitis. He presented classical clinical (viral syndrome before an encephalitis status with behavioral disorder, global hypotonia, mutism, movement disorders, seizure and coma), radiological (lesions of thalami, cortico-spinal tracts and brainstem) and biological features (lymphocytic meningitis). Nowadays, the presence of Japanese Encephalitis Virus in Nepal, including mountain areas is established but Japanese Encephalitis remains rare in travelers returning from this area and neurologist physicians need to become familiar with this. We recommend vaccination for travelers spending a long period of time in Nepal and having at-risk outdoor activities.

Introduction

Japanese encephalitis (JE) is an arboviral disease due to the eponymous Flavivirus (JEV) transmitted by Culex mosquitoes. JEV is prevalent throughout Eastern, Southern Asia and Pacific Ring Area [START_REF] Misra | Overview: Japanese encephalitis[END_REF]. JE is one of the most frequent causes of encephalitis in the world with about 67,000 cases (incidence: 1.8/100.000) leading to 15,000 deaths annually [START_REF] Campbell | Estimated global incidence of Japanese encephalitis[END_REF]. JE leads to a severe prognosis: mortality approaches 30% of symptomatic cases and 50% of survivors present severe sequelae [START_REF] Oya | Japanese Encephalitis for a Reference to International Travelers[END_REF].

The risk of infection by JEV is low for travelers visiting Asia: less than one case per one million travelers [START_REF] Hatz | Japanese Encephalitis: Defining Risk Incidence for Travelers to Endemic Countries and Vaccine Prescribing From the UK and Switzerland[END_REF]. This risk depends on the duration of stay, the season of travel, the practice of outdoor accommodations/activities, and the itinerary [START_REF] Hills | Japanese Encephalitis in Travelers from Non-Endemic Countries, 1973-2008[END_REF]. No specific treatment exists, but a vaccine is available. We report a case of JE in a French traveler returning from travel in Nepal.

Case description

Our case concerns a 22-year-old man who stayed in Nepal as a volunteer for a non-governmental organization from May to September 2012. His past medical history included heavy smoking and HIV seropositivity (without immunodepression [533 CD4/mm3] and no antiretroviral therapy). Seven days after trekking in Pokhara area he presented a viral syndrome (headache, hyperthermia, asthenia) quickly followed by an agitated delirium (Day 9), then multiple seizures (Day 11) leading to coma. Routine biological tests were normal. The CD4 lymphocyte count was 892/mm 3 . Cerebral tomodensitometry (CT) showed diffuse edema. Magnetic Resonance Imaging (MRI) revealed brainstem and di-encephalic lesions (thalami, cortico-spinal tracts, and corpus callosum on T2 & FLAIR sequences) without any evidence of either bleeding (T2 EG) or enhancement after gadolinium injection (fig. 2a). Cerebro-Spinal Fluid (CSF) analysis showed lymphocytic meningitis (452 cells/mm 3 ), slightly elevated protein rate (56 mg/dl) and normal glucose rate. Extensive plasma analyses (including serology for Hepatitis A- Multiple antibiotics regimens (Amoxicillin, Amikacin 500mg daily, Ceftriaxone 2g daily, Meropenem, Itraconazole 100mg daily, Zithromax 500 mg daily, Acyclovir 750 mg daily, Rifampicine 600 mg daily, Isoniazide 300 mg daily, Pyrazinamide 500 mg daily, Ethambutol 1g daily) and Corticosteroid (1 mg/kg/day) had remained inefficient despite 3 weeks of treatment. The patient was repatriated from Nepal to Bangkok one week after the onset and back to France two months after the onset. He was hospitalized in our neurology department with poor clinical condition: drowsiness, mutism, quadriparesia, axial hypotonia (seated position was impossible), paroxysmal axial dystonia, and severe swallowing disorders (needing a gastrostomy). He presented multiple episodes of partial complex seizures.

Progressively he improved, and after 2 months of reeducation he was able to walk, to swallow and talk normally.

He was free of seizure but he conserved a mild distal motor deficit of right upper limb and a total amnesia of the episode. Six months after the onset the patient was showing important behavioral disorders (impulsivity, intolerance to frustration, difficulties in social relationship); and one year later only mild impulsivity, moderate attention disorders and memory recall deficiency persisted. In accordance with this evolution, the MRI showed dramatic regression of lesions (Fig 1)

Discussion

JEV is transmitted between infected vertebrates through the bite of mosquitoes (mainly Culex tritaeniorhynchus); pigs and aquatic birds play pivotal roles as amplifying hosts. Therefore, incidence rates are much higher in rural areas, where pigs, aquatic birds and paddy-breeding vector mosquitoes are prevalent. Direct diagnosis of JE is seldom made since early clinical manifestations lack specificity. We report a case with a diagnosis of JE laboratory-confirmed by demonstration of the presence of neutralizing antibodies specific to JEV in a previously non-exposed French traveler. Our case presents classical characteristics of JE (displayed in Figure 2 and Table 1). The clinical features include an incubation period (5-15 days) followed by an aspecific viral syndrome (headache, fever), then an encephalitis syndrome (seizure, motor and sensorium deficit, movement disorders, Parkinsonism, behavioral troubles, coma). Seizures are often multiple, mostly generalized tonico-clonic and possibly leading to status epilepticus. Motor deficits are quadriplegia more than hemiplegia, upper limbs more than lower limbs, and can be due to lower or upper motor neuron lesion. Movement disorders include tremor, chorea, focal dystonia, myoclonus and oro-facial dyskinesia. [START_REF] Misra | Overview: Japanese encephalitis[END_REF][START_REF] Basumatary | Clinical and radiological spectrum of Japanese Encephalitis[END_REF]. Radiological explorations, based on MRI, are abnormal in 80% of cases with T2-hypersignal and T1 iso/hyposignal (topographies are resumed in Table 1). Bilateral thalami lesions are highly evocative. [START_REF] Kalita | Comparison of CT scan and MRI findings in the diagnosis of Japanese encephalitis[END_REF]. CSF analyses are abnormal in more than 80% of cases. The clinical course consists of an acute shortlived illness and a prolonged convalescence.

Imported cases of JE remain rare but their incidence has been increasing for the last three decades [START_REF] Hills | Japanese Encephalitis in Travelers from Non-Endemic Countries, 1973-2008[END_REF][START_REF] Buhl | Japanese encephalitis in travelers: review of cases and seasonal risk[END_REF]. Whereas JE mostly concerns the pediatric population in endemics areas, residents of all ages of developed countries have no immunity to JEV and are susceptible to severe infection.

However, short-term tourism has not been associated with a significant risk for contracting JE. The Katmandu Valley is now well-known for endemic transmission of JEV (Thakkur et al, 2012;[START_REF] Impoinvil | The spatial heterogeneity between Japanese encephalitis incidence distribution and environmental variables in Nepal[END_REF][START_REF] Bhattachan | Japanese encephalitis in hill and mountain districts, Nepal[END_REF][START_REF] Partridge | Endemic Japanese encephalitis in the Kathmandu valley, Nepal[END_REF]. For travelers to rural areas where JE is endemic, the estimated risk for JE during the transmission period is 1 per 20,000 per week [START_REF] Halstead | Japanese encephalitis vaccines[END_REF]. Because of the availability of a vaccine, the nature of the stay of this patient was in adequation with immunization recommendations (long stay, endemic geographic area, HIV+, outdoor professional and leisure activities, travel itinerary including rural areas) [START_REF] Burchard | Expert opinion on vaccination of travelers against Japanese encephalitis[END_REF].

Conclusion

This case of JE in a French traveler returning from Nepal reemphasizes the need to disseminate revised recommendations on travel vaccines towards general practitioners and the general population. In the context of the recent increase in incidence of imported JE cases [START_REF] Buhl | Japanese encephalitis in travelers: review of cases and seasonal risk[END_REF][START_REF] Tappe | Two laboratory-confirmed cases of Japanese encephalitis imported to Germany by travelers returning from Southeast Asia[END_REF][START_REF] Werlinrud | Japanese Encephalitis in a Danish Short-Term Traveler to Cambodia[END_REF], it is crucial for neurologists to be aware of the possibility of JE in their at-risk population. They also must become familiar with the epidemiological, clinical and paraclinical features consistent with the diagnosis of JE.

E

  viruses, Epstein-Barr virus, Rubella, Dengue virus, Measles, Chikungunya, Treponema, Rickettsia species, Tularemia, Bartonella, Ehrlichia, Mycoplasma, Legionella, Thick blood smear) and CSF analyses (direct exam, bacteriological, parasitological cultures and viral PCR [HSV, VZV, Measles, Toxoplasmosis, JC virus, HHV-6, Toscana virus, Mumps virus, Lymphocytic Choriomeningitis virus, Adenovirus, Listeria, Cryptococcus, Leptospirosis, Tropheryma whipplei, Borrelia] ) were negative or not conclusive. Unfortunately neither plasma nor CSF pan-flavivirus PCR was available during the acute stage of the disease. After his repatriation into France, the definitive diagnosis was established by demonstrating that the serum contained neutralizing antibodies at a titre of 2,500 using the classic neutralization methods with JEV, West Nile virus, the four dengue viruses, and Yellow fever virus. An innovative fluorescent bead-based multiplex serological assay using the Luminex xMAP method and associating up to 20 types of recombinant domain III (rEDIII) from flavivirus (Vanhomwegen and Desprès, unpublished data) confirmed the results.

Figure 1 :

 1 Figure 1: T2 sequences of cerebral MRI 15 days (A) and 5 month (B) after the onset.

Figure 2 :

 2 Figure 2: JE Clinical Features

Table 1 : JE CSF Analysis, MRI and EEG Features CSF Analysis (Median value) MRI Lesions (Prevalence) EEG

 1

	 Mild Protein rates elevation (70mg/dl)	 Thalami (94%)	 Diffuse background activity slowing
	 Pleiocytosis (66,5/mm3)	 Midbrain (58 %)	 Spike-Wave discharges
	 Lymphocytic predominance	 Basal ganglia (38,5%)
		 Cerebellum (25.8%
		 Pons (19%)
		 Cortex (19%)

Acknowledgements: the work on rEDIII-based serodiagnosis and neutralization test was supported by the project PREDEMICS (EU FP-7-HEALTH-2011 grant agreement n°:278433). Many thanks to Stephanie François, Pierre Willaume, Gregory and Irene Grefenstette for reviewing the English of the manuscript.

Funding statement: This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors

Competing Interests Statement: All the authors (Drs Lagarde, Lagier, Charrel, Quérat, Vanhomwegen, Desprès, Pelletier, Kaphan) declare that they have no conflict of interest.


Leonardo Meli
email: meli@diism.unisi.it

Claudio Pacchierotti
email: claudio.pacchierotti@irisa.fr

Gionata Salvietti
email: salviettigio@diism.unisi.it

Francesco Chinello
email: chinello@btech.au.dk

Maurizio Maisto
email: maisto@diag.uniroma1.it

Alessandro De Luca
email: deluca@diag.uniroma1.it

Domenico Prattichizzo
email: prat-tichizzo@diism.unisi.it

Combining wearable finger haptics and Augmented Reality: User evaluation using an external camera and the Microsoft HoloLens

Keywords: Haptics and Haptic Interfaces, Wearable Robots, Human-Centered Robotics. I. INTRODUCTION

Augmented Reality (AR) enriches our physical world with digital content and media, such as 3D models and videos, overlaying in real time the camera view of our smartphone, tablet, laptop, or glasses. Despite the recent massive interest for this technology, it is still not possible to receive rich haptic feedback when interacting with augmented environments. This lack is mainly due to the poor diffusion of suitable haptic interfaces, which should be easy to wear, lightweight, compact, and inexpensive. In this paper, we briefly review the state of the art on wearable haptics and its application in AR. Then, we present three AR use cases, considering tasks of manipulation, guidance, and gaming, using both external cameras with standard screens as well as fully-wearable solutions, using the Microsoft HoloLens. We evaluate these tasks enrolling a total of 34 subjects, analyzing performance and user experience when using a 3-DoF wearable device for the fingertip, a 2-DoF wearable device for the proximal finger phalanx, a vibrotactile ring, and a popular sensory substitution technique (interaction force displayed as a colored bar). Results show that providing haptic feedback through the wearable devices significantly improves the performance, intuitiveness, and comfort of the considered AR tasks.

et al. [START_REF] Frisoli | A fingertip haptic display for improving curvature discrimination[END_REF] presented a device composed of a parallel platform and a serial wrist; the parallel platform actuates a translation stage for positioning the plate with respect to the fingerpad, while the wrist is in charge of adjusting its orientation. Gabardi et al. [START_REF] Gabardi | A new wearable fingertip haptic interface for the rendering of virtual shapes and surface features[END_REF] further improved this device by replacing sheathed tendon actuation with DC motors mounted directly on the joints. Prattichizzo et al. [START_REF] Prattichizzo | Towards wearability in fingertip haptics: A 3-DoF wearable device for cutaneous force feedback[END_REF] developed a wearable 3-DoF fingertip device consisting of two platforms: one located on the nail side of the finger, housing three DC motors, and the other one located in contact with the finger pulp. The two platforms are connected by three cables. A new version of this device featuring three articulated rigid legs connecting the two platforms has been presented in [START_REF] Chinello | Design and development of a 3RRS wearable fingertip cutaneous device[END_REF], [START_REF] Chinello | A three revolute-revolute-spherical wearable fingertip cutaneous device for stiffness rendering[END_REF]. More recently, Leonardis et al. [START_REF] Leonardis | A 3-RSR haptic wearable device for rendering fingertip contact forces[END_REF] developed a 3RSR wearable skin stretch device for the fingertip. It moves a rigid tactor in contact with the skin, providing skin stretch and making/breaking contact sensations. Girard et al. [START_REF] Girard | Haptip: Displaying haptic shear forces at the fingertips for multi-finger interaction in virtual environments[END_REF] developed a wearable fingertip device capable of rendering 2-DoF skin stretch stimuli. Two DC motors move a tactor in contact with the finger pulp, achieving a maximum displacement of 2 mm in both directions. Schorr and Okamura [START_REF] Schorr | Three-dimensional skin deformation as force substitution: Wearable device design and performance during haptic exploration of virtual environments[END_REF] presented a wearable device composed of a delta parallel mechanism, capable of making/breaking contact with the fingertip, as well as rendering shear and normal skin deformation stimuli. The device has three translational DoF, enabling normal, lateral, and longitudinal skin deformation.

While most of the above wearable haptic devices have been evaluated and tested in VR, the use of wearable haptics in AR is still at a very early stage. This is mainly due to the fact that many wearable devices are designed for the fingertip [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: Taxonomy, review, and perspectives[END_REF], impairing the users when they need to interact with real objects. For this reason, few haptic feedback systems have been used in AR so far. For example, Scheggi et al. [START_REF] Scheggi | Shape and weight rendering for haptic augmented reality[END_REF] used a grounded haptic interface and two wearable fingertip devices to provide, at the same time, haptic information about the shape and the weight of a virtual object superimposed on a real object. More recently, Maisto et al. [START_REF] Maisto | Evaluation of wearable haptic systems for the fingers in augmented reality applications[END_REF] presented the experimental evaluation of two wearable haptic interfaces for the fingers in AR. The first one is a 3-DoF fingertip device, which is similar to that presented in [START_REF] Prattichizzo | Towards wearability in fingertip haptics: A 3-DoF wearable device for cutaneous force feedback[END_REF] and applies tactile stimuli through a moving platform; the second one is a 2-DoF skin stretch device for the finger and applies tactile stimuli through a soft belt. De Tinguy et al. [START_REF] Tinguy | Enhancing the stiffness perception of tangible objects in mixed reality using wearable haptics[END_REF] used a similar 2-DoF skin stretch device to alter the perceived stiffness of tangible objects in AR. Other haptic technologies used in AR include electrostimulation [START_REF] Bau | REVEL: Tactile feedback technology for augmented reality[END_REF], haptic vibratory belts [START_REF] Joseph | Semantic indoor navigation with a blind-user oriented augmented reality[END_REF], [START_REF] Buchmann | Directional interfaces for wearable augmented reality[END_REF], and string-based haptic device [START_REF] Tsetserukou | ExoInterfaces: Novel exosceleton haptic interfaces for virtual reality, augmented sport and rehabilitation[END_REF].

In this paper, we evaluate the role that wearable finger haptics may play in augmented reality. We consider different tasks involving manipulation, guidance, and gaming, using three wearable haptic interfaces providing haptic stimuli either at the fingertip or at the finger proximal phalanx. With respect to [START_REF] Maisto | Evaluation of wearable haptic systems for the fingers in augmented reality applications[END_REF], in this work we evaluate the role of wearable haptics in a broader set of tasks and AR settings. Enrolling 34 different subjects in three different experiments, this work presents one the most extensive evaluations of wearable haptics in AR applications.

The main contributions of our work are:

• design of new modular settings and use cases in Augmented Reality, using either standard cameras and displays or fully-wearable solutions (i.e., MS HoloLens); • design of a wireless wearable vibrotactile ring for the proximal phalanx; • design of three user studies assessing the effectiveness of wearable haptics in the different AR contexts, considering for the first time tasks involving manipulation, guidance, and feedback for mid-air gestures; • comparison, through statistical analysis, of the performance of a wearable device for the fingertip, a wearable devices for the proximal finger phalanx, and a sensory substitution technique vs. not providing any kind of haptic feedback.

II. HAPTIC FEEDBACK METHODS We evaluated our AR scenarios using a wearable device for the fingertip (see Sec. II-A), a wearable device for the proximal finger phalanx (see Sec. II-B), a wearable vibrotactile ring (see Sec. II-C), and a sensory substitution technique (see Sec. II-D), comparing them with conditions providing no haptic feedback information. The characteristics of these devices are summarized in Tab. I.

A. 3-DoF wearable device for the fingertip

This device has been preliminary presented by Chinello et al. [START_REF] Chinello | Design and development of a 3RRS wearable fingertip cutaneous device[END_REF], [START_REF] Chinello | A three revolute-revolute-spherical wearable fingertip cutaneous device for stiffness rendering[END_REF] and it is shown in Fig. 1a. It consists of two sides: a static upper body located on the nail side of the finger and a mobile end-effector in contact with the finger pulp. The upper body part supports three light motors, while a FSR 402 Short sensor (Interlink Electronics, USA) is placed on the platform to detect contact with the finger pulp. The two parts are connected by three articulated legs, actuated by the motors. The end-effector can tilt and apply forces on the user's fingertip so to simulate a contacts with arbitrarily-oriented surfaces. A Fingertip deformation and applied wrench can be related by a non-linear impedance model, that depends on the fingertip specific features (e.g., geometry of the contact surface, subject's age). In this work, we assume a simplified linear fingertip impedance model representing the relationship between the resultant wrench and the platform displacement. Thus, we consider the platform configuration ∆ξ ξ ξ proportional to the wrench -w p , w p = K∆ξ ξ ξ, where K ∈ R 6×6 is the fingertip stiffness matrix, as defined in [START_REF] Chinello | A three revolute-revolute-spherical wearable fingertip cutaneous device for stiffness rendering[END_REF]. From a desired wrench w p,d , we can therefore compute the corresponding desired platform configuration ξ ξ ξ d = ξ ξ ξ 0 + K -1 w p,d which is then obtained controlling the positions of the servo motors.

B. 2-DoF wearable device for the finger, "hRing"

This device has been preliminary presented by Pacchierotti et al. [START_REF] Pacchierotti | The hRing: A wearable haptic device to avoid occlusions in hand tracking[END_REF] and it is shown in Fig. 1b. It is composed of an ABS support that houses two servo motors and two pulleys, and a fabric belt that can apply normal and shear forces on the finger pulp. A strap band is used to secure the device on the finger proximal phalanx. The working principle of the device is similar to the principle proposed by Minamizawa et al. [START_REF] Minamizawa | Gravity grabber: wearable haptic display to present virtual mass sensation[END_REF]. When the two motors rotate in opposite directions, the belt is pulled up, providing a force normal to the finger; when the motors spin in the same direction, the belt applies a shear force to the finger. The device weighs 15 g for 31×28×12 mm dimensions.

As for the 3-DoF RRS fingertip device, since the servomotors are position controlled, it is only possible to command them with a desired angle. The relationship between the commanded angle and belt displacement for each motor is ∆b i = r∆θ i , i = 1, 2, where r = 5 mm is the radius of the servo motor pulley, ∆b i the commanded belt displacement due to the motion of motor i, and ∆θ i the i-th motor commanded angle expressed in radians. Consider a reference frame Σ h placed on the middle of the belt with the z-axis pointing toward the finger, the x-axis aligned with the belt and the y-axis defined accordingly. When the two motors rotate of the same amount (|∆θ 1 | = |∆θ 2 |) in opposite directions, the belt is displaced along the z-axis of ∆b i . When the two motor rotate of the same amount in the same direction ∆θ 1 = ∆θ 2 the belt is displaced of ∆b i along the x-axis. To relate the belt displacement to the desired wrench to apply on the finger proximal phalanx w p,d , we assume

w p,d = [n t] T ∈ R 2 as w p,d = n t = k n 0 0 k t ∆θ 1 r + ∆θ 2 r ∆θ 1 r -∆θ 2 r , (1)
where K = k n 0 0 k t is the phalanx stiffness matrix [START_REF] Chinello | A three revolute-revolute-spherical wearable fingertip cutaneous device for stiffness rendering[END_REF].

C. Wireless vibrotactile ring for the finger

This device is a novel vibrotactile ring for the proximal finger phalanx and it is shown in Fig. 2. It is composed of an ABS plastic ring, which houses a vibrotactile motor. The motor used is an eccentric rotating mass vibrotactile motor (Precision MicroDrives, United Kingdom). The ring is also equipped with an RFduino, a fingertip-size Arduino-compatible microcontroller (RFduino, USA) that enables Bluetooth Low Energy (BLE) communications with any compatible device, e.g., an external computer or the Microsoft HoloLens. The powering is guaranteed by an embedded 3 V Lithium Coin Battery, that guarantees long lifetime. A tiny blue LED light is placed on the side of the device's case, to visually show when the vibrotactile motor is active (see the supplementary video). Although other vibrating rings have been presented in [START_REF] Ariza | Ring-shaped haptic device with vibrotactile feedback patterns to support natural spatial interaction[END_REF], [START_REF] Freeman | Tactile feedback for abovedevice gesture interfaces: adding touch to touchless interactions[END_REF], this is the first time a similar device is used in an immersive scenario.

D. Sensory substitution of haptic feedback

Sensory substitution is the use of one human sense to receive information usually received by another sense. In our case, we want to provide haptic information through a non-haptic feedback modality. In the literature, there are several examples of such sensory substitution approach, especially in the field of robotic surgery [START_REF] Kitagawa | Effect of sensory substitution on suture-manipulation forces for robotic surgical systems[END_REF], [START_REF] Gwilliam | Effects of haptic and graphical force feedback on teleoperated palpation[END_REF].

In our experiments, we changed the color of the considered virtual object to provide the user with the corresponding haptic information. For example, if a user touches a virtual object, the color of the object changes according to the amount of force exerted on it, from red to black. A red shade of the object indicates a light grasp, while a black shade indicates a strong grasp. In this condition, no haptic device is worn by the user.

III. FINGERS TRACKING METHODS

To achieve compelling and timely haptic feedback, it is paramount to guarantee a reliable and accurate tracking of the fingertips position and orientation. In the first two experiments described in Secs. IV-A and IV-B, the images were registered by an external camera and the tracking was carried out using the ARToolKit 5.3 library. The quality of the tracking is affected by several factors, such as marker shape, sharpness, smoothness, dimension, and positioning. To maximize the quality of our tracking, we followed the design suggestions presented by Khan et al. [START_REF] Khan | Factors affecting the design and tracking of artoolkit markers[END_REF]. To evaluate the performance of this tracking approach, we asked three human users to sit in front of a table and wear visual markers on their thumb and index fingers (as in Fig. 3). Then, we asked them to move freely in 3dimensional space for 5 minutes, moving around four cubes placed on the table. As done in Khan et al. [START_REF] Khan | Factors affecting the design and tracking of artoolkit markers[END_REF], we evaluated (i) the number of true identifications, (ii) the number of false identifications (i.e., inter-marker confusion), and (iii) the confidence factor, which shows the tracking system confidence in the marker identification. We registered an average of 2867 true identifications, 1012 false identifications, and a 0.7602 confidence factor for the true identifications. These results are in agreements with the ones registered in [START_REF] Khan | Factors affecting the design and tracking of artoolkit markers[END_REF] and guaranteed a quite reliable and stable tracking.

In the third experiment of Sec. IV-C, we wanted to focus on a fully-wearable solution. For this reason, we used the Microsoft HoloLens to register the real scene and render the augmented scenario to the user. The gesture recognition API provided by the Microsoft HoloLens was in charge of detecting when the user was performing the "air tap" gesture. So, in this case, users did not have to wear any visual marker. To evaluate the performance of this tracking approach, we asked the same three human users to wear the HoloLens and our vibrotactile ring (as in Fig. 2). Then, we asked them to move freely in 3-dimensional space for 5 minutes, performing the "air tap" gesture as they pleased. The gesture was correctly recognized 91% of the times.

IV. EXPERIMENTAL EVALUATION We carried out three experiments. A video showing all experiments is available as supplemental material.

A. Experiment #1: box and block

This experiment is inspired from the "box and block" test, which is a functional test used in upper limb rehabilitation to measure the gross manual dexterity of a patient or of a person using an upper limb prosthesis [START_REF] Mathiowetz | Adult norms for the box and block test of manual dexterity[END_REF].

1) Setup and Subjects: The experimental setup is composed of a video camera, three (or four) visual markers, a screen, and one object (virtual or real), as shown in Fig. 3. Two markers are worn by the subjects on the thumb and index middle phalanges, one marker is placed on the real object, and one marker is attached on the table supporting the environment. From each marker worn by the subjects, the AR system creates a proxy point, roughly positioned at the fingertip of the corresponding finger. All the interactions between the finger and the virtual objects are mediated by these points. Target positions for both real and virtual objects are represented as transparent volumes. When at least half of the virtual object is inserted into the corresponding volume, the latter turns green to indicate a correct positioning.

Fourteen participants (10 males, 4 females, age range 22 -33) took part to the experiment.

2) Methods: The task starts with one object, either virtual or real, placed randomly on the table in front of the subject. The task consists of moving this object to a random target position, indicated by a transparent volume. Once the object is correctly positioned inside its target volume, it is removed from the scene. If the object is virtual, it simply disappears from the scene; if the object is real, it is removed by the experimenter. As soon as the environment is clear, a new object -virtual or real -is again randomly placed on the table. If the new object is virtual, it simply appears on the scene; if the new object is real, it is placed on the table by the experimenter. The subject is then asked again to move the new object to its target volume, and so on. The task consists of correctly placing as many objects as possible in 60 seconds [START_REF] Mathiowetz | Adult norms for the box and block test of manual dexterity[END_REF].

As the index and thumb fingertips touch the virtual objects, a suitable amount of force f p is provided to the user, according to the feedback modality considered. This interaction force is evaluated according to the god-object model [START_REF] Zilles | A constraint-based god-object method for haptic display[END_REF]. We modeled the virtual object-fingertip interaction with a spring of stiffness N/m, a friction coefficient of 0.75, and a weight of 150 g. Each participant execute twelve trials of the boxes and blocks task, with three randomized repetitions for each feedback condition proposed: (CR) haptic feedback provided by the hRing wearable device (see Sec. II-B), (CF) haptic feedback provided by the 3-DoF wearable device (see Sec. II-A), (V) sensory substitution via visual feedback (see Sec. II-D), (N) no force feedback.

In condition CR, subjects are required to wear two visual markers and two hRing devices, on the thumb and index fingers (as in Fig. 3). As the user touches a virtual object, the hRing devices provide a suitable amount of cutaneous feedback, as described in Sec. II-B: pressure stimuli to render the stiffness of the object and skin stretch stimuli to render friction/weight information [START_REF] Minamizawa | Gravity grabber: wearable haptic display to present virtual mass sensation[END_REF]. In condition CF, subjects are required to wear two visual markers and two 3-RRS devices, on the thumb and index fingers. As the user touches a virtual object, the 3-RRS devices provide a suitable amount of cutaneous feedback, as described in Sec. II-A: the platforms move towards the skin to render the stiffness of the object and tilt to render friction/weight information [START_REF] Chinello | A three revolute-revolute-spherical wearable fingertip cutaneous device for stiffness rendering[END_REF]. In condition V, subjects are required to wear two visual markers but no cutaneous devices. As the user touches a virtual object, the color of the object changes according to the amount of force exerted on it, as described in Sec. II-D. A red shade of the object indicates a light grasp, while a black shade indicates a strong grasp. In condition N, subjects are required to wear two visual markers. No devices are worn on the fingers, and no information about the interaction forces are provided to the user. In all conditions, no force feedback is provided when interacting with the real cube.

3) Results: To compare the different metrics, we ran oneway repeated-measures ANOVAs or Friedman tests.

Fig. 4a shows the number of boxes correctly placed in the given time. A Friedman test showed a statistically significant difference between the means of the four feedback conditions (χ 2 (3) = 22.397, p < 0.001, a = 0.05). Post hoc analysis with Bonferroni adjustments revealed a statistically significant difference between CR vs. N (p = 0.005), CR vs. V (p = 0.003), CF vs. N (p = 0.020), CF vs. V (p = 0.013). Fig. 4b shows the force exerted by the users on the virtual objects, calculated as the root mean square of f p (see Sec. IV-A2). The data passed the Shapiro-Wilk normality test. Mauchly's Test of Sphericity indicated that the assumption of sphericity had been violated (χ 2 (5) = 12.416, p = 0.030). The one-way repeatedmeasure ANOVA with a Greenhouse-Geisser correction revealed statistically significant difference between the feedback Post hoc analysis with Bonferroni adjustments revealed a statistically significant difference between conditions CR vs. V (p < 0.001), CR vs. N (p < 0.001), CF vs. V (p < 0.001), CF vs. N (p < 0.001), V vs. N (p < 0.001). In addition to the quantitative evaluation reported above, we also measured users' experience. At the end of the experiment, subjects were asked to rate, on a slider going from 0 to 20, the effectiveness of each feedback condition in completing the given task. Fig. 4c shows the perceived effectiveness for the four feedback conditions. The data passed the Shapiro-Wilk normality test and the Mauchly's Test of Sphericity. The one-way repeated-measure ANOVA revealed statistically significant difference between the feedback conditions (F 3,39 = 81.364, p < 0.001, a = 0.05). Post hoc analysis with Bonferroni adjustments revealed a statistically significant difference between all conditions: CR vs. CF (p = 0.004), CR vs. V (p < 0.001), CR vs. N (p < 0.001), CF vs. N (p < 0.001), CF vs. V (p = 0.004), V vs. N (p < 0.001). Finally, we asked which of the two considered wearable haptic systems was more unobtrusive. Thirteen out of fourteen subjects chose the hRing.

B. Experiment #2: guidance

This second experiment considers a guidance task, which is particularly relevant in industrial training applications. The trainees can directly look at the console/machine they need to command, while the AR and wearable haptic systems provide them with specific and unobtrusive information about the task to learn (e.g., how to correctly place some material inside a broaching machine).

1) Setup and Subjects: The experimental setup is composed of a video camera, two visual markers, and a screen, as shown in Fig. 5. One marker is worn by the subjects on the right index middle phalanx and one marker is attached on the table supporting the environment. From the marker worn by the subjects, the AR system creates a cube (yellow object in Fig. 5), roughly positioned at the finger middle phalanx.

Ten participants (10 males, age range 23 -33) took part to the experiment.

2) Methods: The task consisted of placing the cube rendered on the subject's index finger in a target position, as precisely and as fast as possible. Differently from the previous experiment, the target position was not visible on the screen, and subjects had to solely rely on the haptic guidance information provided by the system, according to the feedback modality considered. Each participant carries out nine trials of the guidance task, with three randomized repetitions for each feedback condition: (CR) guidance haptic cues provided by the hRing device, (CF) guidance haptic cues provided by the 3-DoF device, (V) sensory substitution via visual feedback. In Fig. 6 we show how the guidance is obtained in the three conditions. We consider the planar case, where the height of the hand with respect to the table is not taken into account. In other words, the user is guided toward a certain goal position p goal ∈ R 2 starting from an initial position p int ∈ R 2 defined w.r.t. the marker placed on the table, see Fig. 6a. In condition CR, once the hRing is worn, the belt is placed at the middle of the possible displacement along the z-axis (see Sec. II-B). From there, the displacement of the belt is generated proportionally to the position error computed as p err = p goal -p int . In particular, the error along the y-axis is mapped onto a displacement along the normal to the finger pad, whereas the error along the x-axis is mapped onto a shear displacement, see Fig. 6b.

In condition (CF), subjects are required to wear one 3-RRS devices. Similarly to the previous condition, the position error is mapped onto platform rotations as reported in Fig. 6c: when the fingertip is pushed on the right side tilting the platform it means the user has to move to the right, and so on. Finally, in condition V, subjects do not wear any wearable haptic device.

The color of the object changes according to the distance from the target position, from yellow to black. A yellow shade of the object indicates that the target position is very near, while a black shade indicates that the target position is still far, see Fig. 6d. We tested conditions CR and CF with the devices worn either on the left index finger (controlateral to the finger with the visual marker, conditions CR l and CF l ) or on the right index finger (on the same finger with the visual marker, conditions CR r and CF r ). Considering the three feedback conditions and the two positions for the wearable devices, we ended up with five different experimental modalities.

3) Results: All subjects succeeded in placing the virtual object within 1 cm from the target location in less than 1 minute in all conditions. Fig. 7a shows the task completion time. The data passed the Shapiro-Wilk normality test and the Mauchly's Test of Sphericity. The one-way repeated-measure ANOVA revealed statistically significant difference between the feedback conditions (F 4,36 = 10.675, p < 0.001, a = 0.05). Post hoc analysis with Bonferroni adjustments revealed a statistically significant difference between conditions CR l vs. CR r (p = 0.002), CR l vs. CF r (p = 0.030), CR r vs. CF l (p = 0.007), CR r vs. V (p = 0.035), CF l vs. CF r (p = 0.015), and CF r vs. V (p = 0.018).

After the experiment, subjects were asked to rate (i) the effectiveness of the guidance system, (ii) its wearability, and (iii) its intuitiveness on a slider going from 0 to 20. Fig. 7b shows the perceived effectiveness. The data passed the Shapiro-Wilk normality test and the Mauchly's Test of Sphericity. The one-way repeated-measure ANOVA revealed statistically significant difference between the feedback conditions (F 4,36 = 5.806, p = 0.001, a = 0.05). Post hoc analysis with Bonferroni adjustments revealed a statistically significant difference between conditions CR l vs. CR r (p = 0.028), CR l vs. CF r (p = 0.027), and CF l vs. CF r (p = 0.015). Fig. 7c shows the perceived wearability. The data passed the Shapiro-Wilk normality test and the Mauchly's Test of Sphericity. The one-way repeated-measure ANOVA revealed statistically significant difference between the feedback conditions (F 4,36 = 39.577, p < 0.001, a = 0.05). Post hoc analysis with Bonferroni adjustments revealed a statistically significant difference between CR l vs. CR r (p = 0.008), CR l vs. CF l (p = 0.006), CR l vs. CF r (p < 0.001), CR l vs. V (p = 0.027), CR r vs. V (p < 0.001), CF l vs. CF r (p = 0.005), CF l vs. V (p < 0.001), and CF r vs. V (p < 0.001). Fig. 7d shows the perceived intuitiveness. The data passed the Shapiro-Wilk normality test and the Mauchly's Test of Sphericity. The one-way repeated-measure ANOVA revealed statistically significant difference between the feedback conditions (F 4,36 = 11.288, p < 0.001, a = 0.05). Post hoc analysis with Bonferroni adjustments revealed a statistically significant difference between CR l vs. CR r (p = 0.009), CR l vs. V (p = 0.028), CR r vs. CF l (p = 0.032), CF l vs. CF r (p = 0.002), and CF l vs. V (p = 0.012).

C. Experiment #3: gaming

While the first two experiments employed an external camera to capture the real environment and an LCD screen to show the augmented scenario to the user, in this third experiment we wanted to employ a fully-wearable and portable solution. The external camera and LCD screen are substituted by the Microsoft HoloLens, which seamlessly superimposes the virtual scene onto the real environment. To evaluate wearable haptics in this new context, we considered a gaming experience. The gaming business is powering an increasing amount of money, but the haptic feedback provided by commercially-available gaming solutions is still very limited and rudimentary [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: Taxonomy, review, and perspectives[END_REF].

1) Setup and Subjects: The experimental setup is composed of a Microsoft HoloLens and the vibrotactile ring, as shown in Fig. 2. Ten participants (8 males, 2 females, age range 23 -34) took part to the experiment.

2) Methods: The task consisted of playing the mixed reality first-person shooter RoboRaid game for 5 minutes in endless mode. The game consists of defending your home against a (virtual) robot/alien invasion. Users can aim the weapon via gaze and fire using the "air tap" gesture, which consists of holding the hand straight out in a loose fist with the index finger straight up toward the ceiling, tapping your finger down and then quickly raising it back up again. We started recording the time as soon as the subject was able to shoot (we skipped the introductory part). Whether a "game over" occurs within the 5 minutes, we stop the stopwatch, restart the game session, and then restart the stopwatch again (without reset), until reaching the predefined playing time.

Each participant carries out two sessions of the RoboRaid task, one for each feedback condition proposed: (CV) gesture haptic feedback provided by the wearable vibrotactile ring (see Sec. II-C), (N) no haptic feedback.

In condition CV, subjects are required to wear one wearable vibrotactile ring on the right index finger and play the game. Every time the HoloLens recognizes the "air tap" gesture, it makes the weapon fire and the vibrotactile ring provides a 100-ms-long vibration burst. In condition N, subjects do not wear the wearable vibrotactile ring and do not receive any haptic feedback when shooting the weapon.

3) Results: We evaluated the number of robots destroyed during the game session in the two conditions. The data passed the Shapiro-Wilk normality test. A paired-samples t-test determined that the number of robots destroyed did not differed statistically significantly between the two conditions (CV: 66.9±7.4; N: 64.7±7.0 robots, mean±std. deviation). Moreover, we evaluated the immersiveness of the haptic-enabled virtual reality scenario through the Usefulness, Satisfaction, and Ease of use (USE) questionnaire [START_REF] Lund | Measuring usability with the use questionnaire[END_REF], asking subjects to compare the experience while wearing the wearable vibrotactile ring vs. bare hand. Our USE questionnaire is composed of 21 Likerttype questions. It contained a set of assertions, where a score of 9 was described as "completely agree" and a score of 1 as "completely disagree" with the assertion. Questions and results are reported in Table II. The first set of questions (QA in Table II) considered the usefulness of having the wearable vibrotactile devices (CV) with respect to not having any haptic feedback (N). The other three sets of questions (QB, QC, and QD in Table II) were asked separately for both conditions. In order to determine whether the data registered in sets B, C, and D differ between the two feedback conditions, we ran seventeen Wilcoxon signed-rank tests (significance level alpha = 0.05), one for each question. The analysis revealed significant statistical difference between answers QB6 (p = 0.034), QD1 (p = 0.035), QD2 (p = 0.011), and QD4 (p = 0.031). Answers that resulted statistically significantly different are reported in red in Table II.

We also asked which condition the subjects preferred. All subjects preferred the condition where they were wearing the wearable vibrotactile ring (condition CV).

V. DISCUSSION AND CONCLUSION

In this paper we studied and evaluated how wearable finger haptics can enhance user experience in Augmented Reality. Sec. IV-A presents the first experiment, considering a manipulation task where subjects had to re-locate as many objects as possible in a given time frame. We compared task's performance when providing the user with haptic feedback through the hRing, haptic feedback through the 3-DoF device, sensory substitution via visual cues, and no haptic feedback at all. Results showed that providing haptic feedback through the wearable devices significantly improved the performance of the considered task, in terms of number of placed objects, force, and perceived effectiveness. We chose the interaction force as a measurement of the device transparency, because less force applied to the virtual object means that users have promptly stopped their fingers when in contact with the object. This latter metric is also useful to understand how well the display of virtual stiffness works: if users keep moving their fingers after contact, the virtual object may indeed be perceived as softer than it really is. It is interesting to also highlight that we gave no explicit indication regarding the amount of force to apply when grasping an object. The fact that users anyway applied less force during the haptics conditions shows that wearable haptic feedback is able to elicit a natural and compelling feeling of contact. We registered a difference between the two wearable devices in the perceived effectiveness metric only. In fact, subjects found the hRing more effective and, in general, more "wearable". This result was expected since the hRing is significantly smaller and lighter and its position at the proximal finger phalanx enables a direct contact with the real environment. The capability of the hRing to leave the fingertip free to interact with real objects was indeed well appreciated by the subjects, confirming the results of [START_REF] Maisto | Evaluation of wearable haptic systems for the fingers in augmented reality applications[END_REF]. On the other hand, the rigid platform of the 3-DoF device severely impaired the users when they had to interact with the real environment, since it remained between the fingertip and the object to grasp. However, the hRing has only 2-DoF and therefore it cannot provide the same information as the 3-DoF fingertip device. For this reason, if we need to provide richer information, the 3-DoF fingertip device may still be a better choice.

The second experiment of Sec. IV-B considers a guidance task, where subjects had to follow haptic cues to place a virtual object. We compared task's performance when providing the user with haptic cues through the hRing, haptic cues through the 3-DoF device, and sensory substitution via visual cues. We also compared conditions while wearing the wearable devices either on the right or left index finger. Results showed again that providing haptic feedback through the wearable haptic devices significantly improved the performance of the considered task, in terms of completion time, and perceived effectiveness, wearability, and intuitiveness. Moreover, results improved when haptic feedback was provided on the same hand which moved the virtual object (i.e., the right hand). Sensory substitution via visual feedback also showed a rather high performance. This result is quite surprising considering that sensory substitution only provided 1-D information about the distance from the target position, while the haptic conditions conveyed higher-dimensional information. In the future, we plan to test this experiment while also providing 2-D visual feedback.

In the first two experiments, an external camera was in charge of capturing the real environment, the augmented scene was shown on a LCD screen, and the wearable haptic devices were wired to an external unit in charge of providing power and communication with the computer. While this setup is acceptable for a proof of concept and first evaluation, it is unsuitable to demonstrate the applicability, effectiveness, and readiness of wearable haptics for use in everyday life and the entertainment industry. The use of this non-wearable setup was also the main limitation of the work presented by Maisto et al. [START_REF] Maisto | Evaluation of wearable haptic systems for the fingers in augmented reality applications[END_REF]. For this reason, in this work we also considered a third experiment which explores a simpler, but fully-wearable and portable solution. The external camera and LCD screen were substituted by the Microsoft HoloLens, which seamlessly superimposed the virtual scene onto the real environment. To evaluate wearable haptics in this new context, we considered a gaming task, in which users had to play a first-person shooter game and fire their weapon using the "air tap" HoloLens gesture. We compared user performance and experience when providing haptic feedback through a lightweight wireless vibrotactile ring vs. no haptic feedback at all. While the presence of haptics did not affect the player performance, results showed that providing haptic feedback through the wearable haptic device significantly improved the immersiveness and satisfaction of the task. Moreover, all subjects greatly preferred using the vibrotactile ring and rated it as very wearable and comfortable. These results prove that we can improve the immersiveness of AR systems even with very simple, market-ready devices such as our vibrotactile ring, taking advantage of their extreme wearability, comfort, and ease of use. Indeed, haptics seems to play a primary importance role in improving one of the major factor for videogames: the immersiveness. Of course, more complex devices are expected to enable richer interactions. A final important consideration is the price tag on our system. The vibrotactile ring, hRing, and 3-DoF devices can be assembled with less the one hundred dollars, making our method especially attractive for all those applications where grounded kinesthetic interfaces are too expensive or too complicated to use (e.g., gaming, at-home rehabilitation).

In the next future, we will extend this evaluation to include even more wearable devices, able to apply different cutaneous stimuli to different part of the body. Specifically, we will study more in detail the effect of vibrotactile stimuli in AR, and we will consider haptic devices for the wrist and arm (e.g., [START_REF] Aggravi | Design and evaluation of a wearable haptic device for skin stretch, pressure, and vibrotactile stimuli[END_REF], [START_REF] Chinello | Design and evaluation of a wearable skin stretch device for haptic guidance[END_REF]). Finally, we intend to better investigate the role of sensory substitution of visual feedback, combining visual and haptic information together to achieve higher performance.

Fig. 1 .

 1 Fig. 1. Two of the wearable haptic devices employed in our study.

Fig. 2 .

 2 Fig. 2. The vibrotactile ring worn on the proximal phalanx of the index finger is connected to the Microsoft HoloLens headset via BLE. It provides vibration bursts according to the information received from the HoloLens.

Fig. 3 .

 3 Fig.3. Experiment #1: box and block. Subjects move an object (virtual or real) from a random spawning point on the table to a random given target position, indicated by a transparent volume. The task consists of correctly placing as many objects as possible in 60 seconds.

  Perceived effectiveness. Fig. 4. Experiment #1. Mean and standard deviation of (a) boxes correctly placed, (b) force exerted on the virtual objects, and (c) perceived effectiveness of the four feedback conditions are plotted.

Fig. 5 .

 5 Fig. 5. Experiment #2: guidance. Subjects need to place the virtual cube in a random target position in the space, which is not visible. This picture shows condition CF l , in which the user wore the 3-DoF wearable device on the contralateral (left) index finger. conditions (F 1.802,23.425 = 345.555, p < 0.001, a = 0.05).Post hoc analysis with Bonferroni adjustments revealed a statistically significant difference between conditions CR vs. V (p < 0.001), CR vs. N (p < 0.001), CF vs. V (p < 0.001), CF vs. N (p < 0.001), V vs. N (p < 0.001). In addition to the quantitative evaluation reported above, we also measured users' experience. At the end of the experiment, subjects were asked to rate, on a slider going from 0 to 20, the effectiveness of each feedback condition in completing the given task. Fig.4cshows the perceived effectiveness for the four feedback conditions. The data passed the Shapiro-Wilk normality test and the Mauchly's Test of Sphericity. The one-way repeated-measure ANOVA revealed statistically significant difference between the feedback conditions (F 3,39 = 81.364, p < 0.001, a = 0.05). Post hoc analysis with Bonferroni adjustments revealed a statistically significant difference between all conditions: CR vs. CF (p = 0.004), CR vs. V (p < 0.001), CR vs. N (p < 0.001), CF vs. N (p < 0.001), CF vs. V (p = 0.004), V vs. N (p < 0.001). Finally, we asked which of the two considered wearable haptic systems was more unobtrusive. Thirteen out of fourteen subjects chose the hRing.

Fig. 6 .

 6 Fig. 6. Experiment #2: guidance. In (a) the proposed scenario. In (b) and (c), how the 2-DoF and the 3-DoF devices suggest the directions. In (d), the principle used for sensory substitution: the color of the cube changes from black to yellow according to the distance to the goal.

Fig. 7 .

 7 Fig. 7. Experiment #2. Mean and standard deviation of (a) task completion time, (b) perceived effectiveness, (c) wearability, and (d) intuitiveness of the five feedback conditions are plotted.

TABLE I SPECIFICATIONS

 I FOR THE PROPOSED 3-DOF AND 2-DOF DEVICES.

	3-DoF RRS device (Sec. II-A)	2-DoF device "hRing" (Sec. II-B)
	Dimension	35x50x48 mm	Dimension		31x28x12 mm
	Weight	38 g	Weight		15 g
	Max. vertical displ.	15 mm	Max. vertical displ.	13 mm
	Max. normal force	4.7 N	Max. normal force	1.4 N
	Maximum Pitch	π/6 rad	Max. horizontal displ.	13 mm
	Maximum Roll	π/5 rad	Pulleys radius	5 mm
		Vibrotactile ring (Sec. II-C)
	Dimension			22x24x20 mm
	Weight			11 g
	Actuator vibrations frequency range	110 -240 Hz
	Actuator vibrations amplitude range	0.5 -1.7 G
	RRS (Revolute-Revolute-Spherical) kinematic chain constitutes
	the legs connecting the end-effector and to the upper body
	part. The wearable device weighs 38 g for 35×50×48 mm
	dimensions.

TABLE II EXPERIMENT

 II #3. USEFULNESS, SATISFACTION, AND EASE OF USE (USE) QUESTIONNAIRE RESULTS (STAT. DIFFERENT VALUES IN RED).

	QA. Usefulness (condition CV with respect to condition N)
	1. It helps me be more effective.		7.2 ± 0.78
	2. It helps me be more productive.		7.0 ± 0.94
	3. It is useful.		7.8 ± 0.63
	4. It makes the things I want to accomplish easier to get done.	6.7 ± 0.67
	QB. Ease of use	CV	N
	1. It is easy to use.	8.2 ± 0.78 8.1 ± 0.73
	2. It is user friendly.	7.7 ± 0.67 7.9 ± 0.73
	3. Using it is effortless.	7.2 ± 0.79 7.9 ± 0.74
	4. I can use it without written instructions.	7.2 ± 0.78 7.1 ± 0.73
	5. I don't notice any inconsistencies as I use it.	7.1 ± 0.73 7.7 ± 0.67
	6. Both occasional and regular users would like it.	8.1 ± 0.73 7.9 ± 0.73
	7. I can use it successfully every time.	7.4 ± 0.69 7.6 ± 0.51
	QC. Ease of learning	CV	N
	1. I learned to use it quickly.	8.0 ± 0.94 8.2 ± 0.78
	2. It is easy to learn to use it.	7.9 ± 0.73 8.0 ± 0.66
	3. I quickly became skillful with it.	8.2 ± 0.78 8.0 ± 0.81
	QD. Satisfaction	CV	N
	1. I am satisfied with it.	8.1 ± 0.74 7.3 ± 0.67
	2. It is fun to use.	7.8 ± 0.63 7.0 ± 0.67
	3. It works the way I want it to work.	7.5 ± 1.08 7.3 ± 0.67
	4. It is wonderful.	8.3 ± 0.67 7.2 ± 0.79
	5. It is pleasant to use.	7.3 ± 0.82 7.3 ± 0.67

research leading to these results has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement n • 688857 of the project "SoftPro".

1 Meli, Salvietti, and Prattichizzo are with Dept.