Vincent Wagner
email: vincent.wagner@enit.fr

Maher Baili
email: maher.baili@enit.fr

Gilles Dessein
email: gilles.dessein@enit.fr

Daniel Lallement
email: daniel.lallement@messier-dowty.com

Experimental study of coated carbide tools behaviour: application for Ti-5-5-5-3 turning

Keywords: Ti-5-5-5-3, cutting force, cutting temperature, cutting conditions, tool geometry, wear

The goal of this paper is to study the relation between the input data (conditions and geometry of cut) and answers (wear of tool, forces and cutting temperatures) when machining the Ti-5-5-5-3 alloy treated. This study has shown that the cutting process is different and that the slip forces are preponderates. Compared with other materials, the specific cutting pressure is higher and does not vary according to the cutting speed but depend on feed rate. Moreover, both edge preparation and feed rate have an influence on cutting force direction. Besides, cutting temperatures are high and almost similar to those provided by high speed machining with low cutting speed. Finally, we have shown that failure modes are different from those obtained when machining other titanium alloys. Built-up edge is the most deteriorating phenomenon and no flank wear was met in our study context.

Introduction

The realisation of aeronautic parts with strong mechanical properties requires the use of high performance materials. Indeed, for the manufacture of their landing gears, Boeing proposed to move from one near β titanium alloy: Ti-10V-2Fe-3Al (Ti-10-2-3) to another: Ti-5Al-5Mo-5V-3Cr (Ti-5-5-5-3). This is because of the higher working strengths and wider processing window of Ti-5-5-5-3 [START_REF] Jones | Thermomechanical processing of Ti-5Al-5Mo-5V-3Cr[END_REF]. Currently, the available cutting tools and methods of industrialisation do not permit an interesting productivity and restrain possibilities of realisation of a part where the volume of chip is very important. So, the objective of improving the productivity for this material necessarily passes by understanding phenomena of its machining.

Nowadays, many researchers study machining of such materials (with poor machinability). They have shown that machining these materials leads to an unknown and new cutting process [START_REF] Ezugwu | Titanium alloys and their machinability -a review[END_REF]. [START_REF] Saglam | The effect of tool geometry and cutting speed on main cutting force and tool tip[END_REF] have studied the effect of geometry of cut and the cutting conditions on forces and cutting temperatures. [START_REF] Deshayes | Analysis of an equivalent tool face for the cutting speed range prediction of complex grooved tools[END_REF] have analysed the evolution of cutting force and wear of inserts with chip breaker. Other authors oriented their efforts toward the influence of the cutting edge geometry on cutting forces for machining conventional materials as aluminium and steel [START_REF] Fang | The effects of chamfered and honed tool edge geometry in machining of three aluminum alloys[END_REF][START_REF] Yen | A finite element analysis of orthogonal machining using different tool edge geometries[END_REF]. Furthermore, [START_REF] Ranganath | A model to calibrate and predict forces in machining with honed cutting tool or insert[END_REF] and Karpat and Ozel (2008b) investigated the relationship between honed edge tools and the cutting process. These papers used two different modelling and analysis. In Karpat and Ozel (2008a) and [START_REF] Ren | Mechanistic of machining with chamfered tools[END_REF], models of cutting forces and cutting temperatures for chamfered edge tools are provided. The first work is based on a finite element model, whereas, the second uses the shear band model.

Knowledge of tool failure modes involved during machining materials with poor machinability is thus needed in order to optimise the process. Some works allow to the understanding of tool failure modes when machining these new materials. Indeed, [START_REF] Ezugwu | Titanium alloys and their machinability -a review[END_REF] established the prominent failure modes when machining titanium alloys: flank wear, crater wear, chipping and catastrophic failure. These are caused by high cutting temperature, high cutting stresses and the strong chemical reactivity of titanium. [START_REF] Nabhani | Wear mechanisms of ultra-hard cutting tool materials[END_REF] explained that higher cutting temperature and quasi-static contact lead to adhesion and welding between the tool and the workpiece material. [START_REF] Corduan | Wear mechanisms of new tool materials for Ti6Al4V high performance machining[END_REF] mentioned that machining Ti-6Al-4V generates high cutting temperature producing notch for CBN inserts or crater and flank wear for PCD inserts and a phase changes of the material tools. Wear depends also on tool geometry, [START_REF] Manjunathaiah | A new model and analysis of orthogonal machining with an edge radiused tool[END_REF] establishes a new model that explicitly includes the effects of edge hone for orthogonal turning machining. [START_REF] Pawade | An investigation of cutting force and surface damage in high-speed turning for Inconel 718[END_REF] makes a link between using a honed plus chamfered cutting edge and reducing the cutting forces in the machining of Inconel 718.

The goal of this paper is to understand relations between input data (conditions and geometry of cut) and answers (wear of tool, forces and cutting temperatures) for turning Ti-5-5-5-3 without cutting fluid. This relationship is called the first level. On a second level, we had to understand interactions between phenomena of wear, forces and cutting temperatures. Finally, in the last level, forces, cutting temperatures and wear modify the geometry of cut that have influence on answers of the machining process. In term of cutting forces, we have shown that the machining of the Ti-5-5-5-3 generates cutting forces more important than the classic materials. The optimal choice is to use a tool with small value of honing, a rake angle of 20° and a feed rate superior to the honing value. The cutting speed is between 30 and 50 m/min. In addition to that, if we analyse wear phenomenon, we see that machining the Ti-5-5-5-3 does not generate the same modes of deterioration of tools as classic alloys of titanium (as depicted in [START_REF] Nouari | Mécanismes d'usure des outils coupants en usinage à sec de l'alliage de titane aéronautique Ti-6Al-4V[END_REF] or Ti-5-5-5-3 not treated (as detailed in [START_REF] Arrazola | Machinability of titanium alloys (Ti6Al4V and Ti555.3)[END_REF]. Indeed, no wear of clearance surface is obtained as affirmed in [START_REF] Nouari | Mécanismes d'usure des outils coupants en usinage à sec de l'alliage de titane aéronautique Ti-6Al-4V[END_REF] and [START_REF] Arrazola | Machinability of titanium alloys (Ti6Al4V and Ti555.3)[END_REF].

Experimental set-up

To characterise cutting forces, cutting temperature and wear of tool, a set of experiments was conducted on a CNC lathe without cutting fluid (Figure 1). For the cutting forces tests, a treated Ti-5-5-5-3 thin-walled tube has been used as a work piece. The thickness of the tube is 5 mm. The wear tests have been made in straight turning operation on a 50 mm diameter tube. The mechanical properties of the used material are: density = 4,650 kg/m3; ultimate strength = 1,240 MPa; elongation = 2%; thermal conductivity = 5.3 W/m°C; specific heat = 500 J/Kg°C. All tests have been made with coated carbide tool. The substrate is a carbide micro size grain. We have used a TiN coating for cutting force tests and a TiAlN coating for wear tests. For all tools, the coating method deposit is PVD. A dynamometer platform has been used for the cutting forces measurements (Kistler 9263-A). Signals have been analysed due to the software Dynoware © . In order to measure cutting temperature, Actarus © system was used. Concerning wear of tool, many images were taken by white light interferometry technics with optical metrology module Wyko © . The Wyko © interferometer uses light reflected from a reference mirror that is combined with light reflected from a surface under analysis to produce a series of interference fringes. The reference arm of the interferometer moves vertically to scan the surface at varying heights. A camera is used to capture frames of interference data that is analysed by a series of computer algorithm to produce a topological map of the tool. The input data are depicted in Figure 1.

For a turning experiment, input data (conditions and geometry of cut) are depicted in Figure 2. Among all these variables, a choice has been conducted and represented in Figure 2. Indeed, after a dialogue between the industrial, the tool manufacturer and by taking into account results provided by researches on similar alloys. These chosen variables are represented in Figure 2 in dark and bold characters.

Cutting speed

To define the cutting speed influence, we have used the French standard NF-E66-520-4. This standard presents a methodology to determine the minimum cutting speed V c,min .

The minimum cutting speed is used in industry to characterise the machinability of particular alloys. For the tests, a radius edge and a chamfer edge have been used. Figure 3 illustrates the cutting forces according to cutting speed. The cutting force (F) is defined in two components. The first component (F c ) represents the cutting force dropped on cutting speed direction (Figure 1). The thrust force (F t ) is parallel to the feed rate (Figure 1). In these experiments, the cross sectional area of cut is equal to 1 mm 2 , so the specific cutting force is equal to cutting forces. The V c,min is defined at the value corresponding to the lowest specific cutting force. For the two edge preparations, this value is difficult to identify clearly. We can define a minimum cutting speed at around 60 m/min. For this speed, the specific cutting force is the lowest. However, a wear appears rapidly and the tool is immediately failed. A notch and chipping occur after only six seconds of machining. For that reasons, we have defined, for our tests an optimal range between 30 m/min and 50 m/min. In these cutting speeds, the wear can be controlled.

The Figure 3 depicts that the cutting forces are not influenced by the cutting speed for a range between 30 and 80 m/min. For a value higher at 80 m/min, there is an increasing of F t component. The two components seem to become equal for a high cutting speed. An increasing of cutting speed increases the strain rate in the shear plan. So, the shear flow stress in primary and secondary plan increase. However, the cutting speed values are low and the thermal softening effect get with high values does not appear. The thermal softening effect can reduce the effective flow stress and results a decreasing of cutting forces. For Ti-5-5-5-3 machining, this effect is too difficult to get.

We can explain the poor variability of cutting force by the high Ti-5-5-5-3 mechanical properties. A higher cutting speed increases the effective strain rate and the cutting temperature. For standard material, the increasing of temperature reduces stress. However, the Ti-5-5-5-3 keep its mechanical properties in high temperatures. Hence, the cutting forces not decrease with cutting speed cutting speed increasing.

Rake angle

The Figure 4 shows the cutting force according to rake angle for three edge preparations [H0.04 (Figure 4(a) and H0.08 [Figure 4(b)] are the radius edge preparations. C0.15 [Figure 4(c)] is the chamfer edge preparation). The Figure 4 depicts that the increasing of rake angle decreases the cutting force. The observations are made on uncertainty area. This is the trend of cutting forces for all cutting speed. The reduction is only linear for the lowest edge preparation. In this case, the variation from 0° to 20° generates 35% decreasing cutting forces. With a bigger edge preparation a dead zone appears behind edge preparation. In this zone, the strain rate is very low. This phenomenon can explain the plot for the other edge preparation. [START_REF] Saglam | The effect of tool geometry and cutting speed on main cutting force and tool tip[END_REF] illustrates the cutting force reduction by a better edge penetration in material. A high rake angle generates a shear angle increasing and so a reduction of cutting force. This phenomenon can be illustrated by the [START_REF] Oxley | Predicting the strain rate in the zone intense shear in which the chip is formed in machining from the dynamic flow stress properties of the work material and the cutting conditions[END_REF] model.

To conclude, the rake angle choice must be a compromise between a cutting force reduction and a better tool mechanical aspect. An increasing of rake angle reduce the tool wedge angle and so the insert mechanical strength. The Ti-5-5-5-3 specific cutting force is higher than steel one. In this zone, the strain rates are very low generating a very high stress. Karpat and Ozel (2008a) demonstrate that increasing of honed edge radius generates higher effective stress and so higher cutting forces.

In comparison with some other materials, there is a difference. In steel machining, the difference obtained with the two types of edge preparations is not so important. This difference can be explained by the Ti-5-5-5-3 mechanical properties and the machining effective stress. In steel machining, the cutting speed is higher than in Ti-5-5-5-3 machining. The mechanical properties and thermal properties of steel permit to get thermal effect softening. The effective stress decreases and so the cutting force are reduced. In Ti-5-5-5-3 machining, the poor thermal properties and the cutting conditions do not permit to get thermal softening effect. [START_REF] Movahhedy | Numerical analysis of metal cutting with chamfered and blunt tools[END_REF] explains that dead metal zone is removed with the increasing of cutting speed (V c > 500 m/min) and a cutting force reduction appears.

The influence both of edge preparation and undeformed chip thickness modify the cutting force direction (equation 1). The cutting force direction is the ratio between the components.

c t F F μ = (1)
For a chamfered edge, the Figure 5(c) depicts that the evolution of the two components is the same if the undeformed chip thickness is less that 0.22 mm/rev. When feed rate is higher than 0.22 mm/rev, F c values became the highest. For the smallest edge honed radius, F t is the lowest when feed rate is lower than 0.13 mm/rev or three times the edge preparation. When feed rate exceeds this value, F c became the highest. For the 0.08 mm edge honed radius, F t is always superior. However, for the two components the slopes are different (Figure 5). So, Figure 5 depicts that the direction can be modified but for a high feed rate or a high ratio between feed rate and cutting edge. [START_REF] Movahhedy | Numerical analysis of metal cutting with chamfered and blunt tools[END_REF] explain that when undeformed chip thickness is less than cutting edge preparation the material is trapped under the edge preparation. The chamfer or honed radius becomes the principal cutting face. So, the cutting process becomes an over cut face. In this new process, the dead metal zone effect is predominant and creates an effect on F t . When feed rate is upper than width of cutting edge preparation, the dead metal zone becomes cutting face tool and the cutting geometry is modified. In this case, F c becomes higher compared at F t . This configuration is always true in steel machining. The reduction of cutting edge tool improves the tool penetration and reduces the dead metal zone. So, the cutting forces are low. [START_REF] Fang | The effects of chamfered and honed tool edge geometry in machining of three aluminum alloys[END_REF] show that transition is according to cutting speed and chamfer width. To conclude, the lowest cutting force is obtained with the smallest edge preparations. The using of a bigger edge honed radius edge increases the cutting force. We note an increase of over 50%. With a chamfer edge, the cutting forces are higher but allow a better control of cutting force direction.

Feed rate

There is a linear relationship between feed rate and cutting resultant. The increasing is function of cutting undeformed chip section. These observations are the same with the classical material like steel or titanium alloys (Ti-6-4) where cutting force is the product between undeformed chip section and specific cutting force. The increasing of specific cutting force is function to mechanical properties. For Ti-5-5-5-3 machining, the cutting force is higher than steel according to the mechanical properties.

Comparison between process machining of steel and Ti-5-5-5-3

We have chosen to compare Ti-5-5-5-3 and steel machining. This comparison has not physical sense because chip formation mechanisms are quite different. But, this work is made in industrial context and it's important to understand the difference between the new and the old landing gear machining process. This comparison allows us to understand the phenomena obtained in titanium alloy machining. The steel used is a non treated 300 M steel. The 300 M steel is used in old landing gear. The comparison is based on Merchant model. And the tests have been made with a minimal cutting edge preparation.

First, the tests depict that the cutting force obtained in Ti-5-5-5-3 machining are always the highest (Figure 6). These tests have shown also an effective primary shear plan shear stress similar in both materials. The difference is the cutting force direction (equation 1) which is the result of friction and effective shear stress. For steel, it's always higher than 1.5. For Ti-5-5-5-3 machining, the value varied and can be less than 1. To conclude, Ti-5-5-5-3 machining is very sensitive to cutting edge preparation and feed rate. These parameters modify the cutting force and especially its direction. Compared to steel, these influences are very important. In turning, these phenomena are not so important compared to milling where the surface roughness can be modified by the tool deflexion.

Cutting temperatures

The thermal conductivity and the heat capacity of Ti-5-5-5-3 are poor. [START_REF] Ezugwu | Titanium alloys and their machinability -a review[END_REF] demonstrate that 70% of heat is conducted in tool in Ti-6-4 machining. This value is caused by the poor titanium alloys thermal conductivity. Compared to Ti-6-4, the Ti-5-5-5-3 alloys thermal properties are quiet the same. We can think that [START_REF] Ezugwu | Titanium alloys and their machinability -a review[END_REF] conclusion can be the same for Ti-5-5-5-3 machining.

Moreover, the low thermal conductivity and the high mechanical properties do not allow the chip segmentation. Especially when the yield stress is high at 800°C. [START_REF] Poulachon | A contribution to the study of the cutting mechanisms in hard turning[END_REF] shows the chip segmentation formation is function of material deformation. An analysis can be achieved from [START_REF] Poulachon | A contribution to the study of the cutting mechanisms in hard turning[END_REF]. The large deformations area leads to an increase in temperature, which reduces the strength and increases the material ductility. For Ti-5-5-5-3, the combination of low thermal and high mechanical characteristics does not allow this softening even at high temperatures.

The tests have shown that friction forces are higher than Ti-6-4 machining. Whereas, the effective flow stress in primary shear plan remains the same between Ti-6-4 machining and Ti-5-5-5-3 machining. The distributed heat source is also different.

Cutting speed

Whatever the cutting edge preparation and the rake angle, the highest temperature is observed for the maximum cutting speed (Figure 7). A cutting speed increasing leads a highest strain rate and highest chip velocity. So, the cutting temperature depends on these values, we can observe an increasing of temperature.

Rake angle

Whatever the cutting edge preparation the lowest temperature is obtained with the maximal rake angle (10°) (Figure 7). The rake angle increasing reduces the effective shear angle and friction force. [START_REF] Oxley | Predicting the strain rate in the zone intense shear in which the chip is formed in machining from the dynamic flow stress properties of the work material and the cutting conditions[END_REF] demonstrate in their model that the cutting temperature is function of shear flow stress, friction force, chip velocity and shear velocity. In fact, whatever the cutting speed, a cutting forces decreasing reduce the cutting temperature.

Cutting edge preparation

We have seen that the combined effect of cutting edge preparation and feed rate modify the machining process. To limit the cutting edge preparation effect, all tests have been made with a feed rate that is two times the cutting edge width. The lowest temperature is obtained with the minimal cutting edge and the highest temperature with the maximal edge preparation. In using an equal factor between cutting edge width and feed rate for all tests, the undeformed chip section is different for each one. Karpat andOzel (2008a, 2008b) show that the difference between the two types of cutting edge preparation is not the cutting temperature but the distribution. They demonstrate that the raising of cutting edge preparation increases the cutting force, the cutting temperature and the heat transfer section. So, the heat is also modified, the cutting tool temperature is constant but the heat source is different. The cutting temperature is according to strain rate and cutting force then the maximal temperature is obtained with the highest cutting edge preparation. Figure 7 shows that the highest temperature is obtained with the biggest radius edge preparation. With this one, the temperature is increased of 25% for a cutting force which is two times higher. The smallest temperature appears with the sharpest cutting edge preparation.

To conclude, the decrease cutting edge reduces the cutting temperature and the tool chip area. The lowest temperature is obtained with the sharpest cutting edge preparation. Use a minimal cutting edge can enable us to reduce cutting force and cutting temperature. These observations can be used to control wear in titanium machining. Moreover, by reducing the cutting edge preparation, and so, the heat transfer section, enables us to keep heat in cutting zone and to obtain the thermal softening effect.

However, the thermocouple is placed at 0.3 mm from cutting edge. The observed temperatures are very high compared to steel cutting temperatures. This difference can be explained by the higher mechanical properties that increase the cutting temperature. But also, by the poor thermal conductivity that allows a rapid heat conducted into the tool.

Wear of tools

The goal of this section is to define the tool failure modes generated when machining the Ti-5-5-5-3 alloy. A set of experiments was conducted with two different cutting edge preparations and different cutting speed.

For the two edge preparations, Figure 8 and Figure 9 illustrate the three steps where wear of tool occurs. For chamfered edge tool, edge geometry is strongly modified. The edge preparation disappears and is replaced by a smooth surface [see Figure 8(b) and Figure 9(b). This first mode of wear is caused by the abrasive process of cutting. Indeed, when machining, the friction of the chip not segmented with the cutting edge draws away coating particles. However, this phenomenon is less existent when using with honed edge tool. Indeed, this leads to a reducing of the cutting forces, so the sliding force on the cutting face and the grinding phenomenon decrease. Furthermore, this first mode appears quickly and its length depends on coating and the cutting conditions. The experiments were shown that this first mode represents 10% of the tool life.

The second mode of wear observed is a built-up edge [see Figure 8(c) and Figure 9(c)]. This built-up edge is created rapidly after the first wear mode. In general, built-up edge is a wear mode observed when machining ductile materials. However, the elongation of Ti-5-5-5-3 alloy is equal to 2%. Works conducted by [START_REF] Fang | Slip-line modelling of built edge formation in machining[END_REF] have proved that the built-up edge apparition can be closely related to the temperatures and slip stress on the tool-chip interface. We have shown that machining Ti-5-5-5-3 alloy leads to sliding force higher. Thus, knowledge of the sharing of the cutting force into normal and tangential forces permits us to control this wear mode.

Built-up edge is not a stable phenomenon and its presence is related to cutting process. It increases cutting forces and cutting temperature. Obviously, these phenomena will cause an increasing of stresses; hence, built-up edge will be extracted. This extraction will be accompanied with some particles of the insert. A progressive destruction of the cutting edge will be observed since the built-up edge is welded on the cutting face. As opposed to [START_REF] Arrazola | Machinability of titanium alloys (Ti6Al4V and Ti555.3)[END_REF], no flank wear was occurred. The non-existence of this wear mode can be explained by the mechanical properties of the Ti-5-5-5-3 alloy (poor ductility and strength mechanical resistance) and the direction of the cutting force. Indeed, no contact is possible between the machining surface and the flank tool. Nevertheless, we can see some burning marks on the flank owing to high cutting temperature.

To summarise, failure modes of tools when machining the Ti-5-5-5-3 alloy are different from failure modes of tools when machining other titanium alloys. As opposed to results found in our work, many researchers have reported that machining titanium alloys leads to some wear modes: notches wear, crater wear and flank wear. This difference can be explained by preponderant friction phenomena on one hand and by strength mechanical properties of the Ti-5-5-5-3 alloy on the other hand.

In this first work, the wear mode analysis is based geometrical analyse. To understand all wear phenomena, a MEB analysis must be made. With this, we can explain the physical and chemical aspects.

Conclusions

This paper has showed that machining Ti-5-5-5-3 alloy lead cutting forces superior to other material. The optimal choice found is to use tool with sharp edge, a rake angle = 20° and a feed value superior to edge preparation. In order to have an acceptable tool life and durable cutting process, the cutting speed is chosen to be between 30 and 50 m/min. Machining Ti-5-5-5-3 alloy leads important cutting temperatures values (similar to those provided by high speed machining processes). However, the high mechanical properties of Ti-5-5-5-3 alloy do not permit to benefit of these high temperatures in order to obtain the thermal softening. Moreover, the heat flow rate is shared differently compared with other materials (about 70% of heat is conducted in tool when machining titanium alloys).

Concerning wear, machining Ti-5-5-5-3 alloy generates different wear modes of tools from other titanium alloys or Ti-5-5-5-3 alloy none treated. Three steps were identified. The first one represents the elimination of edge preparation. During the second step, a built-up edge appears. Progressively, cycles of creation/extraction of this built-up edge contribute to destruction of the cutting edge. Yet, no flank wear was observed.

Only the first level relating the input data (cutting conditions and geometry of cut) and answers (wear of tool, forces and cutting temperatures) has been studied. The second level (interactions between phenomena of wear, forces and cutting temperatures) and the third level (forces, cutting temperatures and wear modify the geometry of cut that have influence on answers of the machining process) will be studied in future work. In order to define the optimal tool geometry and cutting conditions, a new experiment tests have to be done by considering the result tendencies provided by this paper.

Figure 1 -Figure 2

 12 Figure 1 Experimental set up and chosen input data (see online version for colours)

Figure 3

 3 Figure 3 Cutting forces according to cutting speed and edge preparation (see online version for colours)

Figure 4

 4 Figure 4 Cutting forces according to rake angle, cutting speed and edge preparation (a) honing 0.04 mm (b) honing 0.08 mm (c) chamfer 0.15 × 20° (see online version for colours)

Figure 5

 5 Figure 5 Cutting forces according to feed and cutting edge preparation (a) honing 0.04 mm (b) honing 0.08 mm (c) chamfer 0.15 × 20° (see online version for colours)

Figure 6

 6 Figure 6 Comparison between steel and Ti-5-5-5-3 (see online version for colours)

Figure 7

 7 Figure 7 Tool temperatures after eight seconds (see online version for colours)

Figure 8 Figure 9

 89 Figure 8 Wear modes for chamfered edge (see online version for colours)

Acknowledgements

This work was carried out within the context of the working group Manufacturing 21 which gathers 11 French research laboratories. The topics approached are as follows:

• the modelling of the manufacturing process

• the virtual machining

• the emerging of new manufacturing methods.


Éric Laporte

Nelson Rodrigues

Corpus linguistics and phraseo-paremiology

If corpus linguistics and phraseo-paremiology are two fields of linguistics, how are they interlinked and how can they interplay? In this paper, we defend the position that it is the interest of specialists of phraseo-paremiology to practice corpus-linguistic methods, without excluding other methods 1 .

Section 1 advocates that the practice of corpus analysis is a criterion of quality. In Section 2, we discuss the merits of various approaches to fact observation in phraseoparemiology. Section 3 is about forms of interaction between the different methods.

Corpus analysis as a criterion of quality

More and more linguists analyse corpora of preexisting texts in order to observe linguistic forms. This happens in several steps of their research, for example:

-during an initial step in which they collect facts; -or in order to check results or assessing formal representations such as lexicons and grammars: for example, in Laso (2009), grammatical knowledge about support verb constructions point to the phrases make/provide/produce contribution as possible paraphrases of contribute: the study investigates a corpus to control whether they do actually work as such in texts.

Corpus analysis ensures that linguistic facts in context of use are taken into account. Therefore, practice of corpus analysis during linguistic research has become a criterion of quality. And the advantages of implementing corpus-linguistic methods are at least as valid in phraseo-paremiology as in other fields of linguistics. The increasing practice of corpus analysis belongs to a historical progress, the advent of empiricalness in linguistics.

With all respect due to researchers" methodological freedom, this practice is so common and appreciated that it is now considered as almost indispensable for any linguistic research that involves observation of forms. In language processing, corpus-based assessment of results is practically imposed by journals and conferences.

Corpus linguistics has additional relevance to several notions which are difficult to formalize, such as semantic intensity. For example, the Brazilian Portuguese (BP) phrase de mão cheia (lit. with full hand) "excellent [at some human activity]" expresses intense admiration. Formal approaches to linguistics do not easily account for this scalable element of meaning. In contrast, scalable phenomena are familiar to the world of corpus linguistics, which efficiently collects and interrelates examples of them. Connotation is another example. By "connotation", we mean impressions and affective judgments which are felt as being only suggested by a linguistic form: the BP idiom quebrar um galho (lit. break a branch) "make do", "help out" implies an idea of relieved frustration2 . Stylistic preference is a third example: the phrases in terms of, in regard to, in/with respect to/of, in/with regard to, in/with reference to, concerning, regarding fit differently in document types and syntactic contexts [START_REF] Rankin | The use of marginal and complex prepositions in learner English[END_REF].

However, corpus-linguistics methods for phraseo-paremiology have limitations which can be surpassed with the aid of grammatical and formal methods. When we investigate variations of idioms, corpus-based studies rely on the actual occurrence of the variants in the corpus. For example, does the BP sentence As portas da companhia estão abertas ("The doors of the company are open", "The company is running") have syntactic variants? As some variants in use may be absent from the corpus being used, a practical alternative for native speakers is to generate forms according to grammatical models, e.g. the form with an agentive subject abrir as portas de uma companhia ("open the doors of a company", "start running a company"), and to check them with the aid of introspection (cf. Section 3.1).

As a matter of fact, corpus linguistics combines successfully with complementary methods through various intermethodological bridges. [START_REF] Seretan | An integrated environment for extracting and translating collocations[END_REF] studies collocations collected from a corpus and submits them to a process of formal description before integrating them into a lexicon for automatic translation. Laso (2009) uses introspective observation to select an initial set of paraphrases of contribute before examining their occurrences in a corpus. The advantages of a method compensate for the limitations of another.

Such intermethodological bridges are frequently and successfully crossed, but they are not mentioned often by corpus linguists or much valued in scientific discourse, as if to implicitly suggest that their favourite methodology should ideally be exclusive. This rhetorical option can be explained by too reasons. First, corpus linguistics has a prestigious, modern image. Second, it had to fight for its existence in the 1960s and 1970s in the context of its rivalry with generative grammar, and has developped a tradition of pugnacious rhetoric in this context.

From a scientific point of view, the fact of resorting exclusively to corpus linguistics as a methodology of observation is a decision with scientific consequences. Let us return to the example of the variants of As portas da companhia estão abertas ("The company is running"). Depending on whether we limit ourselves to forms attested in a corpus, the research will produce divergent outcomes, since the variants in use may occur or not in the corpus.

Summing up our position, all linguists should be corpus linguists; but do they need to give up their other activities, methods and tools?

Corpus linguistics and other approaches

Our point of departure in this section are a few expressions frequently used to emphasize merits of corpus linguistics. We discuss whether these merits offer a ground to exclude other approaches: are there any empirical approaches besides corpus linguistics? do they take into account contexts of use? are they based on a real facts? are they good at achieving recall?

Empiricalness

Corpus linguistics claims, obviously with good cause, to be a discipline for empirical observation. As Garrão (2011) puts it, for example, "o critério estatístico a partir de corpus (...), como alternativa a uma abordagem fortemente baseada na intuição do pesquisador, [é] altamente promissor" (the corpus-based statistical criterion (...), as an alternative to an approach strongly based on the researcher"s intuition, [is] highly promising).

This naturally leads us to wonder whether approaches based on the researcher"s intuition are also promising or not, and if they are valid empirical processes? This opens the long-debated epistemological question of introspective linguistics, i.e. linguistics relying on self-examination of one"s linguistic competence. Clearly, the answer depends on how introspective approaches are implemented: there exist bad and good practices. Among good practices [START_REF] Laporte | Exemplos atestados e exemplos construídos na prática do léxicogramática[END_REF], we can cite:

-study of languages by trained native speakers with a regular descriptive activity, -collective control by peers, -use of formal criteria, -assessment of reproducibility of criteria, -taking into account contexts of use. Such provisions are typical of empirical investigation. When they are put to practice, they justify at least some trust in a specific form of empirical observation. Condemning any form of introspective linguistics because of bad practices would be a hasty generalizationin other words, a prejudice.

Consideration of contexts of use

A locus communis of corpus linguistics is an opposition between "contexts of use", or "language in use", and "Chomskyan introspection", which is of a "grammatical" nature. Sure, the observation of corpora provides researchers with contexts of use, and with examples of language in use.

The opposition which has become a locus communis raises a question: is introspection also able to take into account contexts of use? Again, this depends on the skill, training and motivation of introspective linguists. Our experience is that the ability to take into account contexts of use depends on memory, because native speakers have to recall linguistic forms which they have been exposed to, and on imagination, since contexts of use correspond to human situations: linguists equipped with memory and imagination can get trained to relate linguistic forms and contexts of use.

The opposition mentioned above, as it mentions Chomsky, raises another question: is introspection exclusively Chomskyan?

The main type of introspective judgment used in generative grammar is grammaticality, which assumes the existence of a speaker-internal grammar. [START_REF] Chomsky | Syntactic Structures[END_REF] stated that grammar is autonomous and independent of meaning, illustrating this with the meaningless sequence Colorless green ideas sleep furiously which is deemed grammatical. This approach tends to consider contexts of use as irrelevant. [START_REF] Harris | Discourse analysis[END_REF] has another view of introspective judgment, and uses another term, acceptability: native speakers can assess to which point a form is in use, taking into account contexts of use, among other constraints; the acceptable forms are those in use. With this definition, *Colorless green ideas sleep furiously receives the mark of inacceptability, "*". In this perspective, grammar is not an autonomous entity postulated by the theory, but a target of empirical investigation. Harris" conception of syntax and semantics inspired a large enterprise of description of languages, the Lexicon-Grammar [START_REF] Gross | A linguistic environment for comparative Romance syntax[END_REF], which remained almost only introspective until the 1990s.

In other words, introspection in linguistics is not exclusively a Chomskyan notion, and does not necessarily disregards contexts of use.

Reality of facts

A massive merit of corpus analysis is that it grounds investigation on real, authentic data: "[com] recursos estatísticos (...), contamos com um valioso aliado (...): dados reais da língua" ([with] statistic resources (...), we have a valuable ally (...): real language data) [START_REF] Garrão | A identificação de expressões fixas verbais com base em corpora[END_REF]. What about approaches without statistics? can they take advantage of real data?

One of the well-known motivations for adopting corpus linguistics is the common observation that many dictionaries and grammars contain errors or lack important information, which makes them imperfect sources. However, corpus-based statistical data have well-known limitations and imperfections too. In addition, the existence of errors is not sufficient to invalidate the methods of construction of these dictionaries and grammars: what should be incriminated might also be the way these methods have been implemented, rather than the methods themselves.

Another observation in favour of the use of corpora is that dictionaries (for human readers) made with the aid of corpus-based statistical data are often of a better quality, because such data provide abundant facts to authors, which facilitates their work. But, again, this does not rule out the relevance of other approaches when they bring complementary information.

The question of grammars containing errors is worth looking deeper into. Matuda (2011) warns about "formas permitidas pela gramática mas que não ocorrem" (forms allowed by the grammar, but which do not occur). As a matter of fact, if the grammar does not predict which forms occur, it is invalid or incomplete, since grammars are precisely made to this end3 . However, this naturally raises another question: is it possible to complete or improve grammars so that they predict which forms occur? Before trying to answer, observe that Matuda"s warning above is about "a gramática" ("the grammar"), whereas our own question mentions "grammars". In fact, what does Matuda mean by "a gramática" ("the grammar")?

-a concept preexisting to linguistic research? but then, which concept? if she means the specifically generativist concept of grammar, only generativist approaches are concerned by her criticism; -a result of investigation? but then, writing "uma gramática" ("a grammar") or "gramáticas" ("grammars") would be more accurate, since a grammar usually does not predict exactly the same forms as another.

Thus, a relevant question is: is it possible to complete or improve grammars so that they predict which forms occur?

Answering this question is equivalent to assessing a whole field of linguistics: manual construction of formal grammars, the research goal of a number of scholars from the 1950s until now. If a grammar is formalized and readable, there is no reason why it would be impossible to correct an error in it. This is what grammar authors do. Among the major challenges they are collectively facing are… phraseological units.

To take on this challenge, they need to rely on real facts. Corpus linguistics is one of the approaches that can massively provide reliable data, but statistical data do not automatically answer all the questions. Grammar authors seek complementarity in introspective sources, and an important part of their activity consists in turning examples into rules, i.e., assessing how general is each piece of information, which often requires introspection too.

Recall

Corpus linguistics is reputed to contribute to extending the range of forms taken into account in linguistic research, increasing the recall of descriptions. As a matter of fact, in terminological and phraseological studies, corpus analysis easily provides numerous expressions and collocations, many of which prove to be relevant [START_REF] Teixeira | Vocabulário para culinária inglês-português, Série Mil & um Termos[END_REF]. Hence the following question: can methods without statistics achieve high lexical coverage?

The answer is complex and requires reviewing literature and current practices.

As regards inventories of phraseological units, corpus analysis is a time-effective option, widely adopted, for example, for collocations in terminology. However, it is not strictly indispensable: comprehensive phraseological inventories have been compiled for natural language processing lexicons without the aid of any corpus [START_REF] Gross | Lexicon-grammar. The representation of compound words[END_REF]. In addition, inventorying syntactic variants of phraseological units is almost as useful as registering the most common form, and this is what is done in lexicology with a commitment to quality. Corpus linguistics, which is more efficient for frequent forms than for rare forms, encounters here an inherent limitation: data sparseness. Technically, data sparseness corresponds to the situation in which numbers of occurrences of forms under study in the corpus are insufficient to ensure statistic significance. Such a situation can stem from variation in form: if an expression occurs in numerous forms, occurrences are scattered over variants, and this reduces numbers of occurrences. The rarest variants are not sufficiently numerous. They can even be absent from the corpus. For example, [START_REF] Fritzinger | Automatic grouping of morphologically related collocations[END_REF] address a problem of data sparseness affecting German collocations:

Verfahren einleiten

"file a sue" eingeleitetes Verfahren "sue being filed" Einleitung des Verfahrens "filing the sue" Verfahrenseinleitung "sue filing"

In their study, corpus linguistics benefits from grammatical and formal knowledge of syntactic and morphological variation: such knowledge allows for aggregating forms, "undoing" the variation and thus contributing to resolve the problem. In that case, data sparseness is resolved with the aid of data of introspective origin. Introspection, strangely, does not have an analogous problem with rare forms: it is almost as reliable for rare forms as for frequent forms.

Let us mention another facet of lexical coverage: inventorying senses of words. Corpus analysis, here again, provides valuable help in the form of examples of uses of a given word in various contexts. But, here again, good-quality lexicology always inventoried word senses, with or without the aid of corpora.

Interaction with other linguists

The main point of the preceding sections was to highlight the complementarity between corpus linguistics, introspective observation and formalization. More and more researchers take advantage of this complementarity: for example, corpus linguists do use data of introspective origin (e.g. [START_REF] Fritzinger | Automatic grouping of morphologically related collocations[END_REF][START_REF] Martín | A corpus-based study of the phraseological behaviour of abstract nouns in medical English[END_REF][START_REF] Rankin | The use of marginal and complex prepositions in learner English[END_REF][START_REF] Seretan | An integrated environment for extracting and translating collocations[END_REF].

The contrasts and interactions between grammar and corpus linguistics suggest a duality between them, and situate phraseo-paremiology in the heart of this duality. This duality should be taken as an opportunity of fertile collaboration, rather than of competition between scientific communities. Corpus linguistics is now a respected field which does not need to gain approbation to flourish.

In this section, we discuss forms of effective collaboration. The main force of corpus linguistics is fact observation: can other linguists "farm out" observation activities to corpus linguists? Is it better for corpus linguistics to remain separate, or to integrate with other activities?

Description

Take the example of a descriptive activity: construction of lexicons for language processing. Such lexicons are a crucial resource for machine translation with symbolic approaches, and for other much-demanded computer applications such as search engines and information management [START_REF] Tolone | Using Lexicon-Grammar tables for French verbs in a large-coverage parser[END_REF]. In this context, describing phraseological units is one of the problems faced by language-processing lexicologists. They meet huge needs for observation of facts.

For each phraseological unit, meanings must be inventoried. Idioms are less ambiguous than simple words, but some do have several senses, to be represented as independent lexical items [START_REF] Xatara | Dictionnaire multilingue des expressions figées[END_REF]:

(1)

O empreendedor abre as portas de sua loja "The entrepreneur opens the doors of his shop" (2)

Aquilo me abriu as portas para a carreira de atriz "This opened doors for me on a career as an actress"

For each item, syntactic variants must be inventoried. In practice, this involves checking all known variation types, because even idioms with similar grammatical structures do not necessarily have the same variants [START_REF] Xatara | Dictionnaire multilingue des expressions figées[END_REF]:

O empreendedor abre as portas de sua loja "The entrepreneur opens the doors of his shop" (3)

As portas da sua loja vão se abrir "The doors of his shop will open" (4) Sua loja abre as portas "His shop will open its doors"

Dunga abaixou a bola do adversário "Dunga reduced the pretensions of his opponent" *A bola do adversário se abaixou "The pretensions of his opponent were reduced" O adversário abaixou a bola "His opponent reduced his own pretensions"

The inventory of syntactic variants is part of the description of the formal behaviour of an idiom. In addition, it may determine its assignment into a category. For example, BP constructions with the verb fazer and a noun may or may not be support verb constructions, depending on whether a variant of the construction with the same meaning without fazer may occur [START_REF] Langer | A linguistic test battery for support verb constructions[END_REF]:

O Munícipe fez questão de parabenizar publicamente a cidade [START_REF] Garrão | A identificação de expressões fixas verbais com base em corpora[END_REF] "The citizen was keen on publicly congratulating the town" Depois da questão que fez o Munícipe de parabenizar publicamente a cidade... "After the citizen keenly congratulated the town publicly…"

This expression allows for forming a relative clause, but the reduction of the relative with deletion of fazer does not work:

*Depois da questão do Munícipe de parabenizar publicamente a cidade... *After the citizen"s keen on publicly congratulating the town… Since no other operation erasing fazer is applicable, the phrase cannot be classified as a support verb construction, but rather as an idiom. In contrast, the following support verb construction admits the corresponding variants:

A Igreja Adventista do Sétimo Dia fez acordo teológico com a Igreja Católica [START_REF] Garrão | A identificação de expressões fixas verbais com base em corpora[END_REF]) "The Seventh-day Adventist Church made a theological agreement with the Catholic Church" Depois do acordo teológico da Igreja Adventista do Sétimo Dia com a Igreja Católica... "After the theological agreement of the Seventh-day Adventist Church with the Catholic Church…"

The inventory of phraseological units, and for each of them the inventory of senses and of syntactic variants, make up a descriptive work that involves essentially fact observation, which suggests that corpus analysis is relevant. However, the target of this research is a multitude of highly specific pieces of information, and not all of them automatically emerge from raw statistics, for two reasons. First, some pieces of information depend on the identification of a meaning among several ones: for example, distinguishing between examples (1) and ( 2) above requires an introspective examination of context. Second, some syntactic variants in use, and even some phraseological units at all, may happen to be absent from the corpus used for the research, perhaps examples (3) or (4).

In such complex descriptive work, corpus analysis is welcome or even required, but needs close coordination with the other tasks at stake: prediction of potential syntactic variants (with the aid of formal grammatical knowledge), assignment of occurrences to senses (with the aid of introspection). It is really difficult to believe that such close interaction could be achieved by "farming out" observational tasks between separate groups of linguists.

Formalization

When the descriptive activity mentioned in the preceding section produces lexicons and grammars to be used in computer applications with symbolic approaches, the process needs to include a formalization step. Formalization consists in adjusting the linguistic information to a simplifying formal model, which can be handled by software.

Many linguistic phenomena can be situated on a continuous scale, for example that of more or less strict selection restrictions: one end of the scale corresponds to highly frozen phrases, and the other to compositional constructions. For machine translation, in the frozen end, what is recorded is the translation of a word sequence: lip service/aprovação fingida; in the free end, it is the translation of a word: challenge/desafio/défi, or bike/bicicleta. For intermediate cases, there are intermediate solutions, for example recording the translation of a word in presence of others: ride [a bike]/andar [de bicicleta], or rise to [a challenge]/responder [a um desafio]/relever [un défi]. But these formal solutions are in finite number. They do not make up a continuous scale, but a discrete one. In such cases, language processing requires the choice of a formal model. [START_REF] Seretan | An integrated environment for extracting and translating collocations[END_REF] addresses this problem by distributing lists of collocations into a finite number of types. Corpus-derived statistical facts are useful for this task, but the transition from raw statistical facts to a model is not automatically provided by corpus linguistics: it belongs rather to formal grammar. Here again, an integration of methods between corpus linguistics and formal grammar seems relevant.

Conclusion

The conclusions of this reflection about the relations between corpus linguistics and phraseo-paremiology are not surprising. The two fields have strong natural connections. There

By "connotation" we do not mean the figurative meaning of a lexicalized idiom. One of the effects of lexicalization is that the figurative meaning, here "make do", "help out", becomes the strict, explicit meaning of the idiom. In the BP sentence Dilma tomou posse como presidente "Dilma was inaugurated as president", the expression has a connotation of an active part, whereas in English and in the French translation Dilma a été investie dans ses fonctions de présidente, it has a connotation of a passive part, though the strict, explicit meaning is the same.

The problem of errors in grammars are precisely the forms allowed by grammars, but which do not occur. (And the forms excluded by grammars, but which occur.)

We acknowledge the invitation of the Organizing Committee of the International Congress of Phraseology and Paremiology, and the support of the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Board for


Soukeyna Bouchebout

Aude Bolopion

Mohamed Kharboutly

Ioan Alexandru Ivan

Joël Agnus

Stéphane Régnier

Design and first experiments on MagPieR, the magnetic microrobot

This article deals with the design, the actuation and the control of magnetic microrobots. The interest in such microscale robots actuated by remote force fields is increasing since a large range of application fields could benefit from these small size manipulators. However major scientific challenges such as the optimization of the actuation platform, or the reduction of the adhesion between the robot and the substrate must still be overcome to perform complex micromanipulations. This article presents an example of a magnetic microrobot, MagPieR. Its design, fabrication, actuation and control are detailed. First experiments are presented, and the issues that must still be overcome are highlighted.

I. INTRODUCTION

The manipulation of micrometer size objects is of utmost importance for a wide range of application domains, from electronics to biology. Two different approaches, contact and non contact manipulation, are currently considered. Contact manipulation uses tools, such as tips or grippers, directly inspired from macroscale systems. However, due to the scale reduction adhesion forces become predominant over gravity and limit the precision of object release. Non contact manipulation consists in controlling objects with force fields created by remote sources. The most common techniques are laser trapping [START_REF] Arai | Multi-beam laser micromanipulation of microtool by integrated optical tweezers[END_REF], dielectrophoresis [START_REF] Subramanian | Dielectrophoretic micro/nanoassembly with microtweezers and nanoelectrodes[END_REF], [START_REF] Kharboutly | Modeling the trajectory of a micro particle in a dielectrophoresis device[END_REF] and magnetic actuation [START_REF] Kummer | OctoMag: an electromagnetic system for 5-DOF wireless micromanipulation[END_REF], [START_REF] Diller | Control of multiple heterogeneous magnetic microrobots in two dimensions on nonspecialized surfaces[END_REF]. Among them magnetic actuation is promising since high forces can be generated, and only magnetic objects are subject to magnetic fields. Magnetic microrobots can thus be used to selectively manipulate objects.

In this article a magnetic microrobot, MagPieR, is presented. The main issues that must be addressed for the design, the actuation and the control of such microrobotic platforms are detailed. The current system and the first experiments are presented in Sec. II. Challenges that must still be overcome are highlighted in Sec. III. Sec. IV concludes the paper.

II. THE MAGPIER MICROROBOT

The MagPieR microrobot is rectagular, with dimensions 388µm × 300µm × 224µm (fig. 1(a)). It is composed of two different layers:

• a 24µm thick layer of nickel, which is a ferromagnetic material that enables to move the robot using magnetic fields

• a 200µm thick layer of PMN-PT, which is a piezoelectric layer The robot is fabricated using classical cleanroom microfabrication process. The PMN-PT substrate is cut into half-a-cm2 plates. A layer of chromium and copper is sputtered to ensure the conductivity for the electrodeposition of the nickel and the mechanical coherence. The nickel layer is then electroplated. The microrobots are obtained by saw dicing the whole substrate into rectangular samples of the desired dimensions. Trenches are made by saw on top of the nickel layer.

When a magnetic field is applied on the robot it experiences a force F and a torque T :

F = V (M • ∇)B (1)
T = V M × B ( 2
)
where V is the volume of the ferromagnetic part of the robot, M is its internal magnetization and B is the magnetic field.

The force tends to attract the robot towards strong values of magnetic field. The torque aligns the magnetization of the robot along with the magnetic field lines.

To generate a controlled magnetic field an actuation platform composed of four coils is used (fig. 1(b)). The coils require less than 1 A to activate the micro-robot. The coils generate a magnetic field that allows 3 DoF to the microrobot: 2 translations in the x and y directions, and a rotation in the -x,y-plane. The MagPieR moves inside a capacitor. The bottom electrode is the arena substrate itself, and the top electrode is an optically transparent conductive glass (ITO glass) allowing a visualization of the scene from above. The observation of the MagPieR is done by a top situated high speed camera (1000 frames per second). The micro-robot then moves in a dry environment. A detailed description of the whole system is available in [START_REF] Ivan | First experiments on magpier: A planar wireless magnetic and piezoelectric microrobot[END_REF]. ). It takes 28ms to the robot to go through a 2mm distance. The second experiment consists in performing an ∞ shape trajectory in automated mode. To control the microrobot the workspace has been divided into eight different areas. A sequential command has been implemented to move the microrobot from one of the area to the next one. Good results have been obtained since the trajectory has been performed in less than 0.5s. However this approach does not enable a precise trajectory tracking. III. FUTURE DIRECTIONS Several challenges must still be addressed to perform complex micromanipulation tasks and to ensure precise positionning.

The first issue is the optimisation of the actuation system. The dimensions of the coils should be determined to ensure that a high force can be applied on the robot. The time response should be minimized to enable high frequency operations. In addition, the position of the coils should be optimized to ensure that any area of the workspace is reachable. It is also important to avoid overwhelming non linearities of the magnetic field. To perform complex manipulation 3 degrees of freedom of displacements are necessary. At least two additional coils should be added to perform controlled up and down motion.

Compared to most of the current magnetic microrobot systems MagPieR is actuated in air, and not in a liquid environment [START_REF] Kummer | OctoMag: an electromagnetic system for 5-DOF wireless micromanipulation[END_REF], [START_REF] Diller | Control of multiple heterogeneous magnetic microrobots in two dimensions on nonspecialized surfaces[END_REF]. This enables to reach higher velocities since the drag force is considerably reduced, however the adhesion between the robot and the substrate becomes a major issue, and limits the positioning precision. To reduce this phenomena it is possible to the use the piezoelectric effect. High frequency, high voltage impulses on the bottom electrode can actuate the piezoelectric effect to free the micro-robot from adhesion. However experiments should be carried out to precisely determine the influence of each parameters (intensity and frequency of the impulses) to control the vibrations.

The control law should also be improved since in the current system only one coil at a time is actuated. The control of several coils simultaneously will enable to perform more complex trajectories. To ensure a stable control law it is necessary to track the robot at high frequency. The use of an event based camera [START_REF] Ni | Asynchronous event-based visual shape tracking for stable haptic feedback in microrobotics[END_REF], that records only changes of contrast will enable a fast tracking (frequencies higher than 1kHz can be reached for this sensor).

IV. CONCLUSION

This paper presents a magnetic microrobot, MagPieR. The design, actuation and control of this robot are detailed. First experiments have been performed, and show promising results. In particular they confirm that due to the small inertia at this scale high velocities can be obtained. However the precision of trajectory tracking is still limited.

Future works include the optimisation of the actuation platform to ensure that high forces can be applied on the robot, while decreasing the time response of the system and improving the linearity of the magnetic field. Adhesion between the microrobot and the substrate should be reduced, and adapted control laws should be implemented to enable complex micromanipulations.

Fig. 1 .

 1 Fig. 1. The magnetic micromanipulation platform

Fig. 2 .

 2 Fig.2. '2mm dash' experiment performed for the 2010 NIST IEEE Mobile Microrobotics Challenge[START_REF] Ivan | First experiments on magpier: A planar wireless magnetic and piezoelectric microrobot[END_REF]

Institut des Systèmes Intelligents et de Robotique, Université Pierre et Marie Curie, CNRS UMR 7222, 4 Place Jussieu, 75005 Paris, France.bouchebout@isir.upmc.fr

Institut FEMTO-ST, UFC, ENSMM, UTBM, CNRS UMR 6174, 24 rue Alain Savary, 25000 Besanc ¸on, France.

Valahia University of Targoviste, Electr. Eng. Fac. B-Dul Unirii 18-20, Targoviste 130082, Romania

ACKNOWLEDGMENT This work was supported by the EU FP7-SP3-People Program under Grant No: PIEF-GA-2008-219412 (NewMicro-Robotic Systems featuring Piezoelectric Adaptive Mi-croStructures for Sensing and Actuating, with Associated Embedded Control: MicroPAdS) and by the French National Agency of Research, through the LEMA project.


The paper documents the principle and experiments of the "2mm dash" winner at NIST IEEE Mobile Microrobotics Challenge held at ICRA2010 in Alaska [1]. Submission is made for the special session "ICRA Robot Challenge: Advancing Research Through Competitions". The new MagPieR microrobot was specially designed for breaking the speed record, providing a planar magnetic actuation with an optimised coils setup and a subsequent piezoelectric actuation for improved sliding condition. The paper describes the principle of actuation, the microrobot manufacturing flowchart and the assembly setup. Some simulations are provided with a first series of experimental data and conclusions.

I. INTRODUCTION

ecent progress in micro-/nanotechnologies enabled micromechanical systems (MEMS) sensors and actuators. A new emerging field has recently appeared in the micro and nanotechnologies, the untethered, submillimeter, micro or nano-sized robots [START_REF] Sitti | Miniature devices: voyage of the microrobots[END_REF]. They present potential applications in biomedical diagnosis, targeted drug delivery or microassembly. Many issues including system design, microfabrication, remote power sourcing and control must still be considered and developed towards real applications.

Micron-size robots can also solve one of the most important problems of the micromanipulation at this scale. Their abilities of manipulating micron scale objects are very promising but challenging. Untethered microrobot approaches offer a valuable alternative to conventional micro-robotic systems based on tethered manipulation systems. They are not kinematically constrained, thus they allow a larger manipulation workspace, access to unstructured environment and even liquid environments in which can be immerged for avoiding adhesion or sticking effects. Ioan Alexandru Ivan, Micky Rakotondrabe, Joel Agnus and Nicolas Chaillet are with FEMTO-st Institute, UMR CNRS-6174 / UFC / ENSMM / UTBM Automatic Control and Micro-Mechatronic Systems department (AS2M department) 25000 Besancon -FRANCE (phone: +33-381-402-789; fax:

+33-381-402-809; e-mails: alex.ivan@femto-st.fr, joel.agnus@ens2m.fr, mrakoton@femto-st.fr, nicolas.chaillet@femto-st.fr).

Gilgueng Hwang and Stephane Regnier are with University Pierre et Marie Curie, Institut des Systèmes Intelligents et de Robotique Paris, FRANCE (e-mails: hwang@isir.upmc.fr, stephane.regnier@upmc.fr).

The major current challenges consist in innovating efficient remote power sourcing and proper locomotion mechanisms. Drive and propelling such tiny devices require overcoming the nonlinear physics at this scale. Then, the ability to accurately maneuver them represents the next challenge. Current wireless mobile micro-robots have been designed with the power source from electrostatic, electromagnetic, thermal and electrokinetic force (e.g. bacteria propelled systems). For the biological approaches, harnessing biological organisms were demonstrated to give the power to propulsion [START_REF] Darnton | Moving fluid with bacterial carpets[END_REF][START_REF] Behkam | Design methodology for biomimetic propulsion of miniature swimming robots[END_REF][START_REF] Steager | A novel method of microfabrication and manipulation of bacterial teamsters in low Reynolds number fluidic environments[END_REF][START_REF] Martel | Flagellated magnetotactic bacteria as controlled MRI-trackable propulsion and steering systems for medical nanorobots operating in the human microvasculature[END_REF]. These have a great potential due to the fact that the energy can be harvested from everywhere through chemical reactions abundant in the environment. However controlling their propulsion is very challenging. As with inorganic approaches, an electrostatic scratch drive actuator-based microrobot was shown in [START_REF] Donald | An untethered, electrostatic, globally controllable MEMS micro-robot[END_REF]. It has the advantage of using the most dominating controllable force type in the micro-scale. However, its application range is limited due to the need for a specific patterned substrates and high electrical potentials. Other energy conversion mechanisms such as thermal actuators [START_REF] Yamagata | A micro mobile mechanism using thermal expansion and its theoretical analysis[END_REF] and piezoactuators [START_REF] Yamagata | Ultrahigh vacuum precise positioning device utilizing rapid deformations of elements[END_REF] were demonstrated. Although these principles are well developed and are supported by many physics works, they cannot meet the requirements of the fully untethered microrobots. Among all these approaches, the electromagnetic forces still remain the most reasonable power sources for building untethered microrobots. Electromagnetic stick-slip force was used to drive micron size robots in [START_REF] Pawashe | Modeling of experimental characterization of an untethered magnetic micro-robot[END_REF]. Resonating magnetic field was utilized to actuate small mass-spring microrobots in a linear motion [START_REF] Frutiger | Small, fast, and under control: wireless resonant magnetic micro-agents[END_REF]. Artificial bacteria were actuated by external rotating magnetic field [START_REF] Zhang | Artificial bacterial flagella: fabrication and magnetic control[END_REF] and electroosmotic force [START_REF] Hwang | Remotely powered propulsion of helical nanobelts[END_REF]. Multi-DOF electromagnetic levitation force principle was successfully presented in [START_REF] Kummer | OctoMag: An Electromagnetic System for 5-DOF Wireless Micromanipulation[END_REF].

Although such a dramatic advancement in powering remote propulsion, the range of applications remains still limited. It is mainly due to the lack of functions to be applied to any complex tasks. Real breakthrough must overcome the current maneuvering ability limits by combining the available technologies. Another challenge in micro-robotics is the control of multiple untethered robots for conducting collaborative manipulation tasks. Control of decoupled motion, strategies of manipulation, multirobot cooperation [START_REF] Pawashe | Multiple magnetic microrobot control using electrostatic anchoring[END_REF] [START_REF] Becker | Motion planning under bounded uncertainty using ensemble control[END_REF] In 2007 NIST launched a competition, the NIST Mobile Nicrorobotics Challenge. Since 2010, this challenge got IEEE support and will occur during the ICRA Conference. A French team, composed by the two institutes FEMTO-ST and ISIR, decided to participate in 2010 to this competition, aiming for the first participation an as fast as possible "2mm dash race". In the 2mm dash race [1], a microrobot smaller than 600 µm in its largest dimension must cross a distance of two millimeters in a time as short as possible.

A new microrobot called MagPieR (Magnetic -Piezoelectric microrobot) was designed for this challenge. The MagPieR mobile microrobot is composed of two heterostructure layers: ferromagnetic material (Nickel) layer for magnetic driving on top of a bulk piezoelectric material (PMN-PT). The microrobot is intended to overcome the surface friction. It can move on the horizontal plane of a capacitor whose bottom electrode is the arena substrate itself and the top electrode is optically transparent conductive glass (ITO glass). A series of high voltage periodic pulses provide out-of-plane piezoelectric strain, making the robot loose adhesion contact with the arena surface for short amounts of time. Subsequently, fast in-plane motions (translation and steering) are insured by external magnetic field gradients provided by closely coupled four electromagnetic coil sets. With this proposed design, the French team won the competition with an RMS (root mean squared) score of 32 ms. The article will describe in details the overall structure of the robot and characterize in a first instance its linear propulsion performance.

The first part of this article deals with the description of the principle drive mechanisms of the microrobot, from process flowchart to overall system implementation. Then, a modeling section will follow with finite elements simulations. Finally the experimental results on a couple microrobot types are shown to reveal the propulsion characteristics such as linearity and the performances. Finally the paper discusses conclusions and further improvements.

II. SYSTEM OVERVIEW

A. Operating principle

The untethered MagPieR microrobot successfully competed to the ICRA 2010 Microrobotic challenge [1]. Given our first participation to this challenge we mainly focused on the 2 mm dash. In the attempt to provide faster actuation, we chose to combine a new mix of simultaneously piezoelectric and a magnetic type of actuation.

As well known, adhesion forces at microscale become predominant. A microrobot whose size is imposed to less than a half-millimeter will stay "sticked" to its contact surface regardless its inclination. One of the first experiments was investigating the possibility of reducing the adhesion effects by getting vibrating pieces of PZT and PMN-PT piezoelectric material. In Fig. 1 we inserted a piece of piezoelectric material between a silicon and a bulk aluminium plate that were inclined at 30 deg.. Adjusting the gap to a slightly superior value than to the piezo material thickness (200 µm) and upon applying high voltage square impulses (300 V, 100 Hz) and paying attention to the limit of the Paschen curve, we eventually noticed the piezoelectric material loosing its adhesion and starting to slide according to the gravitational laws.

Fig. 1 Early experiments on a 500x500x200 µm 3 piece of PMN-PT material validated sliding conditions upon applying step impulses of (0…+300)V at 100 Hz. The frames are taken at ~100ms interval.

Following the positive results in limiting adhesion forces upon vibrating a piezoelectric element, we considered the hybrid actuation principle from Fig. 2, which combines piezoelectric out-of-plane vibrations for improved friction-free conditions and magnetic field gradient actuation for in-plane displacement (x-y) and orientation (θ). For that purpose, the final microrobot was designed of two distinctive layers: a lower bulk piezoelectric layer and a ferromagnetic patterned layer (Fig. 4). The microrobot was positioned into a 3.5x2.0 mm 2 "challenge arena" whose dimensions were imposed by the competition organizers [1]. In order to be able to visualize the scene perpendicularly, an ITO conductive glass was considered for the top electrode (Fig. 2). Further details about the arena setup are provided in Section-II.D. Details about the coils setup and driving electronics are presented in Section-II.B and in Section-III.

B. Electronic driving circuit

The electronic circuit was custom-made, microcontroller-based and embedded. For the "2 mm dash" a position feed-back control was not necessary, although after the competition we started studying a couple vision-based approaches.

Fig.3 Block diagram of embedded electronic circuit

Once configured through a USB or serial interface (current values, high voltage, impulse times) the system is able to operate without computer connection. We designed high-power op-amp based voltage-to-current converters able to continuously operate up to 1A and in impulses up to 4A. Attention was paid to the coils overheating protection, a feedback voltage intended for the coils temperature evaluation was put in place. However, given optimized coils setup (1300 turns, 17 mm long, 4 mm diameter ferromagnetic core), currents of ≤1A were largely sufficient for the application. An analog switch controlled the high voltage amplifier type KH7602 was connected to the electrodes, in order to activate the piezoelectric effect. A four-directions keypad with some extra buttons (reset, high speed command) served to manually control the microrobot back and forth in any directions, to position it close to the starting line and to launch the 2mm dash. Results will be presented more detailed in Section-IV. The models I.1 and II.1 were considered from the beginning with two and one trench, respectively (Fig. 5). That feature allowed a faster and more oscillatory-free orientation along magnetic field lines. We equally tested bulk samples type II.2 (without trenches) like in Fig. 11 which naturally showed fastest speeds due to the increased amount of Ni, but they performed in a more unstable manner and we didn't compete with.

C. Microrobot design and process flow

D. Arena microfabrication and assembly

The rules for arena dimensions and tolerances were provided by NIST organizers [1], each team feeling free to use its own materials. We chose a dry environment made of four distinctive layers of glass and silicon (Fig. 2) cut at convenient dimensions for a rapid assembly. Figures 6 and7 illustrate how the robot is encapsulated in the arena assembly by means of two specially designed teflon flanges. Once assembled, preferably in a clean room environment, the microrobot setup can be furthermore attached and detached from the coils base (Fig 7 .d). The four arena layers were 6.5x6.5 mm 2 in the lateral size but cut in a manner that allowed the electrical access to the electrodes. The layers are as follows: 1. A top conductive ITO glass acting as transparent top electrode and glued to a wire (up-left in fig. 7.c).

2. An intermediate glass layer of 150 µm that was cut annularly using an ultrasound machine. This layer served as the dielectric separation.

3. An intermediate silicon layer of 110 µm that was microfabricated at both sides using metal sputtering, photolithography and DRIE (deep reactive ion etching) techniques. A thin aluminium layer was used as DRIE mask while a gold film to trace the marks. This layer insures the borders and the marks of the virtual starting and finish lines. 4. A bottom piece cut from a regular silicon layer whose surface was metallized by Cr-Al sputtering. This layer served as the base and as the ground electrode, a wire being bonded accordingly.

III. MAGNETIC FIELD MODELLING

A. Equation of the motion

Three natures of forces act on the microrobot: mechanical (from friction and adhesions between the robot and the base), electrical (from the electrical field) and magnetic (from the electromagnets) forces. Hence: ( )

, mec m F F E B Γ = + r r r r r (1)
where Γ r is the acceleration of the microrobot and m is its mass. ( ) , F E B r r r includes the magnetic force acting in the volume, the piezoelectric stress and the eventual Lorentz force w.r.t. the trajectory .

Since the paper is particularly dedicated for the analysis of performances upon executing the "2mm dash" as required in [1], we assume that:

-for large magnetic field values the contribution of piezoelectric effect is less predominant. Therefore for 2 mm task we assume that electrical field is null, -only one electromagnet is used to actuate the robot.

The magnetic force is dependant on the volume V of the ferromagnetic layer and proportional to the gradient of the magnetic field at its location. We assume that during the challenge the axis of the microrobot is very closed to the symmetry axis denoted x of the electromagnet, -the tangential part of the mechanical force comes from friction during the motion. The normal forcesfrom the weight and from adhesion forces [START_REF] Gauthier | Forces analysis for micromanipulations in dry and liquid media[END_REF] -are compensated by the reaction of the base. thus:

( ) ( )

0 0 adh µ mg F m V M B   - +   Γ = + ⋅ ∇       r r r (2) where 2 10 m g s   ≅    
and where the adhesion forces adh F can be approximated by using plane-to-plane contact in dry media condition (see for example [START_REF] Gauthier | Forces analysis for micromanipulations in dry and liquid media[END_REF]). There are a lot of approaches to model the friction coefficient µ . The choice mainly depends on the conditions and on the expected applications. At the microscale where unwanted stick-slip phenomena often happens during the motion, the elasto-plastic friction model can be efficiently used [START_REF] Dupont | Single state elastoplastic models[END_REF], for a purpose of modelling and control.

B. Magnetic force analysis

The magnetic force

( ) mag F V M B = ⋅ ∇ r r r
, where the magnetization M r of the robot can be assumed uniform, is volumetric and therefore the required field and field gradient to exert a certain force increases rapidly with the decrease of the sizes of the robot [START_REF] Yesin | Modeling and Control of Untethered Biomicrorobots in a Fluidic Environment Using Electromagnetic Fields[END_REF].

To estimate and analyze the magnetic field density, we use finite-element-modelling (FEM) software from Comsol Multiphysics. Because of the symmetry geometry of the electromagnet (described in Section II.B), of the negligible robot sizes relative to it and their small separating distances, we decide to employ 2D-axisymmetrical analysis in the FEM software (Fig. 8). This allows an ease of computation during top electrode wire bottom wire the processing task. In the software, the coil is represented by a cylindrical (copper) block (Fig. 8.a). So, to introduce the electrical excitation, we apply a current density J . This density can be computed from the current i that we want to apply to the coil such that , where w d is the diameter of the used copper wire. In this analysis we consider i = 0.2A , 0.5A and 1.0A . The streamline of the magnetic field w.r.t. current density appears as in Fig. 9. As we can see, the lines can be assumed to be parallel within the dimensions of the microrobot. This is due to small sizes of the robot and the fact that it is very closed to the symmetry axis x during its motion. Under this configuration, with the electromagnets in close vicinity to the arena, we gain in field gradients but we loose in the orthogonality of the field. The trade-off consists in increasing the distance between coils with the cost of a lower dragging force. The setup allows the mechanical re-configuration of the coils position, as noticeable in Fig 7 .d. Research on this topic will be published on a separate paper regarding microrobot in-plane control. Fig. 10 shows the simulation results of the magnetic field along 3mm, which includes the 2mm challenge dash (coils in close vicinity to the arena). The field and its gradient increase in an exponential manner as the microrobot moves towards the electromagnet (increasing acceleration).

IV. EXPERIMENTS

In this section, we demonstrate a few experiments of MagPieR, in order to characterize the propulsion performance and prove eventually the design efficiency. The aim of this section is to analyze the motion of MagPieR in the 2mm dash task and try to further improve its design.

A. Propulsion in 2mm dash task

First of all, we describe briefly the 2mm dash task demonstrated in the section. The task is to measure the travel time from start to complete stop by a highspeed camera analysis during the MagPieR motion from start to goal line. The surface friction are assumed to be constant. Then two more issues should be considered to accomplish the task. Firstly, a straight linear propulsion increase the energy transfer efficiency of the microrobot. Furthermore, the stopping at the closest point from the goal line should also be important to avoid any redundant motion which still is counted in the travel time. The assembled arena with two different types of MagPieRs were used for the 2mm dash task experiments. The specifications of four types of MagPieR and minimum corresponding scores are summarized in Table-I. We chose to compare the fastest types II.1 and II.2 that have similar geometries except one has a trench crossing the longitudinal axis on the ferromagnetic nickel layer while the other present a smooth surface without the trench. Then we aim to reveal the effect of this trench correlated to the propulsion linearity and the performance, thus can validate the proposed design. For a better comparison in propulsion linearity, a high-speed camera with 1000 frames per second (fps) was used to record the videos during the tasks.

B. Propulsion performance characterizations

We further compare the propulsion performance between two different types of MagPieRs. As summarised in the Table I, the type II.1 measures the average record at 14.8 ms from four different trials (the standard deviation is 4.2 ms). And the type II.2 took average 9.4 ms with 2 ms standard deviation. It should be noted that pure travel time between start and goal lines was measured to exclude the effect from collision to wall. Although the propulsion linearity is assured better in the type II.1 by the better alignment of the trench to the external magnetic field, such deviation of measured time in the type II.1 is attributed to the initial alignment condition. When the trench is initially well aligned through the field, additional motion required for the alignment (a damped oscillatory rotation) that adds friction can be avoided, thus relatively fast enough propulsion can be achieved. Moreover the etched away ferromagnetic layer in the type II.1 reduces the magnetically driven propulsion force. Assuming that the overall inertial effect from slight mass changes is ignorable, the magnetic propulsion force (Eq. 2 in the modeling section) is volumetric thus the type II.1 loses 49% of the driving force compared to the type II.2. This can be confirmed from the consistency to the reduction of maximum acceleration in the type II.1 which is 49.7%.

For more accurate analyses of their propulsion, the velocity and the acceleration curves are displayed on fig [START_REF] Zhang | Artificial bacterial flagella: fabrication and magnetic control[END_REF]. The rapid drop of each curve clearly shows when the MagPieRs were collided to the wall behind the goal line. Therefore it is inevitable to find the optimal input impulse parameter to properly make the MagPieRs stop at the closest to the goal line without any collision.

C. Improvement of the propulsion

To avoid the collision to the wall and make the MagPieR stop at the closest distance from the goal line, we aim to find the optimal input step pulse. For this purpose, we have characterized the travel distances of the type II.1 at each input impulse time in the range of 4 and 223 ms. The measured result is displayed in the Figure 13. As a result, a 14 ms of input impulse time stopps MagPieR at the closest to the goal line. This parameter could reduce the travel time by avoiding the redundant travel behind the goal line from collision and bouncing. Further surface optimization can even enhance the current propulsion performance. V. CONCLUSION, CURRENT AND FUTURE WORKS We proposed a new microrobot system by combining external piezoelectric and electromagnetic forces. The principle has the advantage of partly overcoming the surface adhesion, allowing faster planar propulsion especially under low field gradients. The microrobot system parts were fabricated using MEMS process and assembled in the clean room conditions. We have demonstrated the fastest travel time at the 2mm dash task by the proposed MagPieR robot and external field controller.

To evaluate the propulsion efficiency, highspeed video analyses were performed. They revealed the better aligning motion of the type II.1 compared to the type II.2, the horizontal trench improving the propulsion linearity with the cost of a slightly slower actuation. The redundant travel after the collision to the wall behind the goal line was observed. Since these can slow down the propulsion, we achieved the proper input impulse to make a complete stop close to the goal line.

Further improvement of the propulsion performance will be attempted with more advanced surface adhesion studies in different environments (dry and wet). Incorporating piezoelectric effect will help to overcome these unavoidable frictions. Then the ability to maneuver them should be the next challenges. In the future works, and for the 2011 Challenge, we plan to analyze the behaviour of the microrobot like a multivariable system manner (Fig. 14), performing x y θ --planar motions. Using a high sample rate reference camera and the derived behaviour analysis, we will design a closed-loop control law able to provide high performance velocity and trajectory tracking for the microrobot. The friction between the microrobot and the base is considered as disturbance since it generates unwanted stick-slip phenomenon during the movement. If it is modelled, it can be accounted inside the closed-loop control design or for a feedforward disturbance rejection. An efficient friction model that can be used is the elastoplastic model that depends on both velocity and relative displacement of the microrobot. By this way, MagPieR could be controlled in closed loop in order to achieve other challenges of the 2011 competition.

  Manuscript received September 15, 2010. This work was supported in part by the EU FP7SP3-People Program under Grant No: PIEF-GA-2008-219412 (NewMicro-Robotic Systems featuring Piezoelectric Adaptive MicroStructures for Sensing and Actuating, with Associated Embedded Control: MicroPAdS).

Fig. 2

 2 Fig.2 MagPieR Microrobot Principle of actuation and arena schematic.

  The final microrobot structure consisted of a top Ni layer electrodeposited on a PMN-PT substrate. Piezo PMN-PT material was preferred to the classical PZT ceramic due to the very high longitudinal d 33 piezoelectric coefficient (3100 pC/N compared to 590 pC/N) that finally allowed a significantly reduced voltage to get the same strain (vibration) conditions. Microrobot flowchart consisted of the following stages: Cutting PMN-PT substrate into half-a-cm 2 plates; Sputtering a Cu-Cr layer on the top side; Sputtering a Ti-Cr layer on the bottom side; Spin coating teflon-based resin on the bottom (optional); Ni electroplating of top electrode; Saw dicing into small square and rectangular samples; Saw trenching the top Ni layer (optional). The final structures were 224 µm thick; of which 200 µm the PMN-PT layer and 24 µm the Ni layer, as in Fig 4. We used two different geometries, square and rectangular ones as summarized in the Table1.

Fig. 4

 4 Fig.4 SEM of a MagPier microrobot type I.2 (trenches are 50µm wide).

Fig. 5

 5 Fig.5 SEM of a MagPier microrobots a) type I.1 and b) type II.1

Fig. 6

 6 Fig.6 Schematic of the packaged arena consisting of the following layers (from up to bottom): ITO top electrode, glass spacer, silicon border, silicon base.

Fig. 7 a

 7 Fig.7 a,b) CAD of the arena assembly, c,d) photos of the arena and of the complete system. Total system size is 11x11 cm 2 .

Fig. 8 :

 8 Fig.8: Comsol Multiphysics -FEM analysis. (a): scheme showing the geometrical characteristics and distance between the microrobot and the electromagnet. (b): 2D-axisymmetrical meshing.

Fig. 9

 9 Fig.9 Streamline of the magnetic field.

Fig. 10

 10 Fig.10 Magnetic field along the distance travelled by the microrobot, during the 2mm dash challenge.

Fig. 11

 11 Fig. 11 The captured motions during 2mm dash task and their tracked trajectory plots for the two different types of MagPieR. Type II.1 is shown in the upper part and type II.2 in the lower part. Current imposed value was 1.2A. Aligning motion in the type II.1 and unstable motion in the type II.2 were observed (the scale bar is 500 µm).

Figure 11

 11 Figure 11 shows a series of captured images at 3 ms interval, captured from the original movie taken at 1000 fps. As shown in the Figure 11, while the MagPieR type II.1 revealed a propulsion linearity, the type II.2 showed a faster motion. The type II.1 with a trench on top of the ferromagnetic layer took longer time for alignment during their propulsion. It should be noted here that the motion of MagPieR was demonstrated in four different distinctive steps. Firstly, it aligns through the electromagnetic field gradient, continues the linear propulsion then passes the goal line, collides to the wall behind the goal line and finally stops. It implies that we can further improve the propulsion performance either by modifying the input pulse time and controlling the propulsion linearity from design parameters.

Figure 11

 11 Figure 11 also depicts the tracked trajectories of the propulsions of two different types. As shown in the planar trajectory plots, the type II.2 (right plot) loses the propulsion stability by rotating right after the collision to the wall. The travel time estimation in 2 mm dash task measures the time between the start line and a complete stop behind the goal line. Therefore unnecessary motions can be avoided to further improve the record by about 25-30%.

Fig. 12

 12 Fig. 12 Velocity characterizations of MagPieR type II.1 and II.2

Fig. 13

 13 Fig. 13 Propulsion characteristics of stopping distances depending on the input pulse steps. MagPieR type II.1.

Fig. 14

 14 Fig.14 Block diagram of the complete system to be controlled.

First experiments on MagPieR: a planar wireless magnetic and piezoelectric microrobot

  are on-going projects and the first results show promising results. Therefore, untethered microrobots with multiple functions could open a new era. Our approach is to

	develop a new hybrid piezo-magnetic powered microrobot to
	demonstrate a higher planar propulsion performance.
	Ioan Alexandru Ivan, Member, IEEE, Gilgueng Hwang, Member, IEEE, Joel Agnus, Micky
	Rakotondrabe, Member, IEEE, Nicolas Chaillet, Member, IEEE, Stéphane Régnier, Member, IEEE
	R

Table I .

 I MagPieR types used for experiments. Type II.1 competed.

	Type	Length	Width	Trench	2mm dash score (ms)
		[µm]	[µm]	width [µm]	and observations
	T I.1	388	388	50 (two)	25 ms
	T I.2	388	388	0	not tested, unstable geom..
	T II.1	388	300	50 (one)	15 ms, competition type
	T II.2	388	300	0	9 ms, but unstable