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Monitoring integrity of operating structures has become a crucialneed in industrial applications while motivation for moresustainable materials pushes forward the development of naturalfibre composites (NFC). The complexity and variability of theirfailure behaviour and mechanical properties still limit theexploitation of their full potential. Acoustic Emission (AE) hasshown promising results to predict remaining service life ofstructures, allowing identification, localisation and assessment ofdamages in composite materials. Thus, embedment of AE sensors couldbe a reliable solution for real-time Structural Health Monitoring(SHM). This paper aims to quantify

Residual vibrations are a recurrent problem in MEMS actuators leading to a limitation of their dynamics and lifespan. This paper presents a voltage-controlled technique and its applicability to suppress residual vibrations in high voltage (about 100 V) electrostatic comb-drive actuators. The studied actuator is composed of a driving module and a clutch module devoted to the step by step driving of linear or circular gears. The proposed method is based on a command shaping technique well adapted to be integrated in an Application-Specific Integrated Circuit (ASIC), especially for nomadic applications. Firstly, an electrodynamic characterization is used to extract the key voltages of the optimized command signals. The key voltages correspond to the beginning and the end of the actual displacement. The command signals consist of two begin/end voltage jumps and a linear voltage rise/fall in an optimized time between the key voltages. Then, experimental validations investigated the influence of voltage fall time for a MEMS chip and the resulting dispersion for a set of 8 MEMS chips. The results show that residual vibrations are reduced by a factor higher than 20 in terms of the amplitude rate of bounces against back stop and the settling time of free oscillations. Finally, an analog circuit capable of building the optimized command signals is proposed and will be implemented in the next generation of the ASIC driving the electrostatic two-module actuator.

International audience; This paper presents a sensor using the mode localization phenomenon to detect a mass perturbation.It is composed of two cantilevers with different lengths and connected by a coupling beam. The shortcantilever is electrostatically actuated and by changing the applied DC voltage, we can reduce its stiffnessand reach the veering point, which corresponds to a balanced system. This principle allows us to overcomethe manufacturing defect which perturbs the initial system. An analytical model using the Euler-Bernoullibeam theory is developed for the design. The equation of the continuous system is discretized with theGalerkin method and simulations are performed. The designed device composed of polysilicon coupledmicrobeams is then fabricated with the Multi-User MEMS Processes and an experimental investigation iscarried out. Three devices with different coupling are considered with a length ratio of 0.98. This ratio issuitable to reach the veering point by using a DC balancing voltage around the half of the pull-in voltage.The comparison between theoretical and experimental results shows a good agreement for each device.

International audience; The preshaped curved beam is used as bistable mechanism in MEMS, its behavior is characterized by the snapping forces and stresses that evolve during deflection between the two sides of buckling. Based on analytical models, the influence of variating the material and dimension parameters on the behavior of the curved beam is analyzed. The limit of miniaturization of the beam based on the stress limits is identified. Further, a design optimization methodology is provided which allows selecting the miniature dimensions of the curved beam after defining the range of possible dimensions. This method ensures a required bistablity behavior in terms of stroke and holding forces, while respecting the design requirement. Several design constraints are defined including material and technological limitations and some other specifications. The results in different cases of optimization problems showed to be the same using a proposed optimization methodology and a standard gradient based optimization algorithm.

International audience; Current digital microrobots are mainly composed of bistable modules and flexible structures. Unlike conventional microrobots, they are based on mechanical stability instead of complex control strategies in order to reach precise and repeatable discrete positioning. By design, their number of stable positions depends on the number of bistable modules. As a consequence, increasing the robot workspace requires using several modules. In this case, the robot size increases and its miniaturization becomes complex and non-intuitive. To address this issue, a multistable module has been developed to reach several stable positions. In this letter, a new generation of digital microrobots is proposed. Based on two multistable modules and flexible structures, the new microrobot can reach a large workspace while having a small footprint. Concretely, the robot size decreases by 26% while the number of stable positions increases by 950% in comparison with the first generation of the digital microrobot. A prototype is designed, fabricated and characterized experimentally. Preliminary results show a good agreement between the expected and the achieved workspace. The robot achieves 169 stable positions with a discrete step of 4.125 μm and a resolution of 150 nm. With these capabilities, the robot paves the way for promising perspectives and applications, in particular precise micro-manipulation in confined environment such as a Scanning Electron Microscope.

International audience; In this paper, the effects of initial deflection on the static and dynamic behaviors of circular capacitive transducers are experimentally investigated. The obtained results are in good agreement with numerical simulations. It is shown that the initial deflection has a major impact on the static response of the resonator by shifting the pull-in voltage, and on its dynamic response by increasing the resonance frequency and modifying the bifurcation topology from softening to hardening behavior. Moreover, the dynamic behavior of the microplate may display nonlinear periodic and quasiperiodic responses due to geometric and electrostatic nonlinearities.

Acoustic Emission (AE) is a non-destructive technique suitable for monitoring structural damages in complex systems. Interpretation of AE data is generally made by an unsupervised learning approach called clustering. A clustering method divides a dataset into different groups called clusters, which are expected to have a physical interpretation in terms of damages. The set of groups, called partition, is then evaluated through an external criterion. Since the number of clusters is a priori unknown, the standard approach for AE clustering consists in running a clustering method for different number of clusters and then to select the partition with the best value of the external criterion. The external criterion can vary across publications and aims at evidencing “natural” clusters. According to the criterion, the selected partition and its interpretation can be subjected to variability. In this publication, we explore a clustering method in which the parameters of clusters are optimised jointly with the number of clusters through an objective function and an optimisation procedure designed for that purpose. To our knowledge, it is the first method of this class in AE literature. Experimental results concern real data from tightening test.

International audience; The ability of Capacitive Micromachined Ultrasonic Transducer (CMUTs) todesign broadband sensors for Structural Health Monitoring (SHM) is studiedthrough both multi-frequency and bandwidth aspects. Elementary cells arecomposed of circular membranes fabricated using the standard MUMPS Process. The multi-frequency aspect, which involves different individual membranes from 50 µm to 250 µm radius, is theoretically addressed through anumerical modeling. The targeted frequency range, consistent with the SHMapplication, is then between 80 kHz and 2 MHz. Geometrical features induced by the manufacturing process greatly affect the dynamic properties ofthe membranes and this is experimentally validated. The bandwidth aspectis also addressed on an array of identical 100 µm radius membranes thusinvolving their intrinsic capabilities. Harmonic excitation with targeted frequencies 300 kHz, 530 kHz and 800 kHz, below and beyond the resonancefrequency of the membranes, are performed. The influence of the bias voltage VDC on the signal-to-noise ratio is studied according to the excitationfrequency. As a result, a signal-to-noise of 20 dB is achieved around the resonance frequency. Finally, the circular membranes array is tested for acousticemission sensing through a pencil lead break test. In spite of a low signal-tonoise ratio, acoustic events are clearly detected. The multi-frequency aspectand the large bandwidth capability of the CMUTs are hence demonstratedand highlight the adaptability of the sensor to its environment.

International audience; The present paper deals with the analysis of the squeeze film effects in circular capacitive micromachined ultrasound transducers (CMUT) operating in air, with emphasis on improved bandwidth. Firstly, a 1D analytical approach based on parallel plate approximation is recalled. The opposing influences of the electrostatic softening and the squeeze film stiffening make the resonant frequency dependent on the bias voltage with respect to air spring constant to mechanical spring constant ratio. In a second part, FEM models using COMSOL Multiphysics® are built to analyze the influences of the plate flexibility and the lateral venting boundary conditions on the squeeze film effects. The associated numerical results show that viscous losses are involved in sealed air-filled cavities. Moreover, the dimensionless elastic and viscous damping forces do not depend on the lateral venting boundary conditions for high squeeze number range, usual for CMUT operation. Finally, 2D full coupled simulations of flexible CMUT are compared with experimental data. Thus, the squeeze film damping increases bandwidth of air-coupled CMUTs with both sealed and laterally vented cavities.

International audience; Among the various experimental techniques used for Structural Health Monitoring (SHM), acoustic emission (AE) allows real time monitoring of large structures with the possibility to detect, characterize and locate damages. In practice, its implementation can be complex, especially for mobile structures. The main difficulties are related to the integration of sensors in the structure, electricalinterconnections, different sources of noise, the detection and processing of transient signals and the management of massive data streaming. Generally, AE are collected through piezoelectric sensors. We propose an AEbased SHM methodology using an alternative technology in breakthrough with piezoelectric transduction: Capacitive micro-machined ultrasonic transducers (CMUTs). These sensors have many advantages. One advantage lies in the size of the sensors allowing a little intrusive integration into the material. Following a previous work published in EWSHM’̣18, we report on the design, fabrication, and experimental demonstration of a new CMUT transducer specifically designed for the measurement of AE events. A data preprocessing methodology dedicated to interpret the obtained AE streaming is presented. We compare the results with standard piezoelectric sensors to detect damages during tensile tests on composite plates.

This paper investigates the mass sensing in a mode-localized sensor composed of two weakly coupled MEMS cantilevers with lengths 98μm and 100μm. The two resonators are connected by a coupling beam near the fixed end, and the shortest cantilever is electrostatically actuated with a combined AC-DC voltage. The DC actuation voltage is tuned to compensate the length difference and geometrical imperfections in order to dynamically equilibrate the system. An analytical model of the device using the Euler Bernoulli beam theory is presented and the required DC voltage to reach the balanced state is used. A mass perturbation is then added on the long cantilever and the eigenstate shifts and amplitude ratios in each mode are calculated for different couplings. Results show that the amplitude ratio of the second mode is the best output metric for the mass detection. For the validation of the model, an experimental investigation is carried out by using devices fabricated with the Multi-User MEMS Processes. Three different couplings are considered and the long cantilever is designed with a mass attached at its end. Instead of adding a mass on the device, we remove this part with a probe to introduce the perturbation. When the mass is removed, the experimental frequency responses of the device show localized vibrations, which are in good agreement with the theoretical results.

The ability of capacitive micromachined ultrasonic transducer (CMUTs) to design broadband sensors for structural health monitoring (SHM) is studied through both multi-frequency and bandwidth aspects. Elementary cells are composed of circular membranes fabricated using the standard MUMPS Process. The multi-frequency aspect, which involves different individual membranes from 50 to 250 μ m radius, is theoretically addressed through a numerical modeling. The targeted frequency range, consistent with the SHM application, is then between 80 kHz and 2 MHz. Geometrical features induced by the manufacturing process greatly affect the dynamic properties of the membranes and this is experimentally validated. The bandwidth aspect is also addressed on an array of identical 100 μ m radius membranes thus involving their intrinsic capabilities. Harmonic excitation with targeted frequencies 300, 530 and 800 kHz, below and beyond the resonance frequency of the membranes, are performed. The influence of the bias voltage VDC on the signal-to-noise ratio is studied according to the excitation frequency. As a result, a signal-to-noise of 20 dB is achieved around the resonance frequency. Finally, the circular membranes array is tested for acoustic emission sensing through a pencil lead break test. In spite of a low signal-to-noise ratio, acoustic events are clearly detected. The multi-frequency aspect and the large bandwidth capability of the CMUTs are hence demonstrated and highlight the adaptability of the sensor to its environment.

International audience; In order to overcome mode aliasing limitation in linear mode-localized sensors, we demonstrate in this letter both theoreticallyandexperimentallyhowtotunetheelectrostaticnonlinearityandexceedtheultimatesensitivityreachable in the linear regime. A mass sensor composed of two coupled micro-cantilevers with different lengths is considered and the results show that the sensitivity can be significantly enhanced up to 67 %. By performing experiments on the fabricated device and depositing the mass perturbation with a focused ion beam, the same phenomenon has been observed. These promising results open the way towards ultrasensitive multimodal microsensors with functionalized nonlinearities.

International audience; This paper presents a nonlinear analytical model of MEMS mass sensor, which is composed of two cantilevers of 98 μm and 100 μm length, 20 μm width and 1.3 μm thick. They are connected by a coupling beam and only the shortest cantilever is actuated by a combined AC-DCvoltage. The DC voltage is used to equilibrate the system and the phenomenon of mode localization is investigated when a mass perturbation is applied. The sensor is modeled as a continuous system with beam theory and non-ideal boundary conditions are considered by using flexible supports. With a low AC voltage of 10 mV, a DC voltage of 5.85 V can counterbalance the length difference. This DC voltage decreases at 5.60 V when we increase the AC voltage, due to the effect of electrostatic nonlinearities. For a relative added mass of 0.1%, the amplitude change in the two cantilevers is more important when the coupling is weaker.

International audience; The Acoustic Emission (AE) technique is widely used in Structural Health Monitoring (SHM) in order to detect, in real-time, small-scale damages and to evaluate their kinetics. The piezoelectric transduction is generally favored for detecting the subnanometric displacements of the surface of a material induced by the propagation of the elastic waves generated by the sudden release of energy during damage. Yet highly sensitive, this type of sensor has a great impact on the signals collected introducing interpretation biases when assigning damage families to AE signals. In this paper, new Capacitive Micromachined Ultrasonic Transducers (CMUT) have been designed and tested with the objective to decrease the footprint of AE sensors compared to conventional piezoelectric sensor. CMUT allow getting AE sensors with wide-band and flat response making them of great interest for the collection of AE signals in SHM solutions.

International audience; This paper presents a compliant monolithic multistable actuator which is able to switch its moving part between several stable positions linearly in one dimensional direction. The number of stable positions can be increased by extending the range of displacement of the moving part. The transition in each step of displacement is made to the nearest stable position in the direction of motion. Upward and downward steps are made by a specific sequence of moving, using a bistable module, opening and closing two internal clamps which are actuated by U-shaped electrothermal actuator using three subsystems. The principle and the design of each subsystem in the discrete acruator, fabrication process and experimental results are presented. The fabricated prototypes of the discrete actuator showed a proper functioning. The mean achieved displacement is 120.67±0.08 µm over 12 upward steps with a mean step of 10.06 µm, which is very close to the designed performance.

International audience; Capacitive micromachined ultrasonic transducers (CMUTs) are microelectromechanical systems used for the generation of ultrasounds. The fundamental element of the transducer is a clamped thin metallized membrane that vibrates under voltage variations. To control such oscillations and to optimize its dynamic response it is necessary to know the modal parameters of the membrane such as resonance frequency, damping and stiffness coefficients. The purpose of this work is to identify these parameters using only the time data obtained from the membrane center displacement. Dynamic measurements are conducted in time domain and we use two methods to identify the modal parameters: a subspace method based on an innovation model of the state-space representation and the continuous wavelet transform method based on the use of the ridge of the wavelet transform of the displacement. Experimental results are presented showing the effectiveness of these two procedures in modal parameter identification.

International audience; This paper describes numerical and experimental characterization of capacitive micromachined ultrasonic transducers (CMUTs) for ultrasound transmission. Simulations based on a finite elements method to model the electromechanical behaviour of CMUTs and to determine the dimensions of elementary cells are presented. In particular, we analyse how the collapse voltage and the capacitanceare affected by different parameters of a circular cell and by different bias voltages. The fill factor is defined as the ratio of the top electrode radius to the membrane radius and we study the influence of the fill factor in the performances of CMUTs. The fabrication process of a CMUT uses anodic bonding of a SOI wafer on a borosilicate glass substrate and we compare experimental results with numericalresults in terms of eigenfrequencies, bandwidth, quality factor and capacitance for non-metallized and metallized membranes.

In this paper, we develop original analytical electro-thermal and thermo-mechanical models for the U-shaped electro-thermal actuator. The dynamics of the temperature distribution and displacement are obtained as a direct relationship between the system's dimensions, material properties and electrical input. The electro-thermal model provides an exact solution of the hybrid partial differential equations that describe the electro-thermal behaviour for each of the actuator's three connected arms. The solution is obtained using a new calculation method that allows the representation of an integrable function by a hybrid infinite sum of sine and cosine functions. The displacement at the actuator's tip is then calculated using a quasi-static model based on the superposition and virtual works principles. The obtained temperature and displacement solutions are then discussed and compared with finite element method simulations via ANSYS® and experimental results. Comparisons showed good agreement making the proposed modelling a reliable alternative which paves the way for improving the design and optimising the dimensions of U-shaped micro-actuators.

The dynamic behavior of a U-shaped electrothermal actuator is investigated in this paper based on experimental findings and FEM simulations. Few previous works are found in the literature that have addressed the dynamic response of U-shaped actuators. A lack of data on the transient displacement of the actuator in the heating and cooling cycles is noted. Experiments are made in order to characterize the dynamic response of the actuator. The actuators are micro-fabricated on doped SOI wafers and the displacement is recorded with a high speed camera. FEM simulations are in good agreement with the experimental findings. Simulations and experiments show that the shape of displacement follows evolution of the temperature distribution inside the actuator. The influence of the dynamic behavior on the control of the actuator is finally discussed.

The following work investigates output mechanical power limits of electrostatic micromotors using stator/rotor contact interactions. Loading characteristics of micrometer size motors monolithically coupled with elastic polysilicon torque sensors are measured through an high-frequency complementary metal oxide semiconductor (CMOS) camera combined with a specific image processing software. Short duration video sequences recorded during the rotor braking are analyzed by tracking of a target pixel attached to the tested devices. Output mechanical power limits are measured through electrostatic probing on the wafer level within a few milliseconds. Experiments point out an unusual mechanical power per mass unit which is as high as 100 W/gr, considering driving frequencies on the order of 100 kHz.

Thick soft and hard lead zirconate titanate (PZT) films are fabricated on alumina substrates using the screen-printing technique. The geometries of the substrates are suited to three different mechanical characterization tests devoted to the determination of the Young’s modulus of the films: a dynamic, a quasi-static and a nanoindentation test. Values of 52±8 and 20±7 GPa for the Young’s modulus are, respectively calculated for the soft- and hard-PZT. These values are strongly influenced by porosity in the films. The piezoelectric coefficient d 31 is also investigated. It is found to be near −120±5 pC/N for the soft-PZT and it is close to −25±3 pC/N for the hard-PZT. Motivated by experimental observations, a non-linear piezo-mechanical model predicting the electromechanical behavior of a cantilever bimorph is developed.

The present work deals with experimental and theoretical analyses of mechanical energy transductions in standing wave ultrasonic motors. A piezoelectric translator prototype previously developed is tested with regard to both out-of-plane and tangential mechanical behaviours. Influences of the vibration amplitude, the normal pre-load and the dynamic friction coefficient at the stator/frame interface are pointed out through the acquisition of speed-driving force characteristics. In the main part of the article, theoretical approaches assuming the decoupling of the out-of-plane and tangential behaviours are proposed: the `complete' model takes into account transient phenomena and tangential inertia effects, and the `simplified' model supposes that the steady state is achieved. In both models, equivalent mass-spring systems allow the intermittent stator/frame contact to be characterized with regard to the vibration amplitude and the normal pre-load. Successive contact and flight periods are clearly shown. During contact periods, the sequences of stick-slip phases are at the origin of the driving mechanism. They are theoretically discriminated through the study of their behaviour equations. Finally, experimental and theoretical data fitting proves the validation of analytical analyses and allows the future optimization of standing wave ultrasonic motors to be envisaged.

International audience; A mechanical stop mechanism is developed in order to compensate MEMS fabrication tolerances in discrete positioning. The mechanical stop mechanism is designed to be implemented on SOI wafers using a common DRIE etching process. The various fabrication tolerances obtained due to the etching process are presented and discussed in the paper. The principle and design of the mechanism are then presented. Finally, experiments on microfabricated positioning prototypes show accurate steps unaffected by the fabrication tolerances.

International audience; In this paper, we investigate the effect of high modes of buckling on the mechanical behavior of a pre-shaped curved beam. In a first stage, the presented modeling develops further the snapping forces solution and bistability conditions in order to include high modes of buckling. In a second stage, we develop the analytical solution of the stresses inside the beam during deflection between the two sides of buckling. The buckling with or without mechanical conditions on antisymmetric modes, the force characteristics, bistability conditions and stresses are described in this paper based on mathematical approach in order to provide a clear physical understanding of the curved beam behaviorand its design parameters. The accurate knowledge of the design parameters is important in order to achieve the best integration of the curved beam in a complete microstructure. The analytical results are compared with and without considering high modes of bucklingand have shown to be in excellent agreement with FEM simulations. The results show the importance of the high modes in calculating stresses and snapping forces.

The material properties of silicon, as well as the planar and monolithic nature of the microstructures make electrostatic field energy conversion the most suitable driving principle on the micrometer scale. Moreover, compared with most other actuation principles, the scaling of electrostatic forces is particularly suitable for actuator downsizing. In spite of the advantages, it is still difficult to obtain appropriate driving characteristics because of silicon based actuator limitations such as small structural height, micrometer gap requirements and material limitations in the shaping process. Actuators require specific tools to verify that their mechanical properties and motions obey the designer’s intent. In this paper capabilities of future direct-drive electrostatic actuators SDA (Scratch Drive Actuators) are investigated through the characterisation of their out-of-plane displacements by interferometry. The actuation involves contact interactions by using flexible polisilicon elementary actuator plate. The region of the physical contact is measured using Twyman-Green interferometer incorporated within a metallurgical microscope. The shapes and out-of-plane displacements of microstructures are extracted from interferograms by temporal phase shift method (TPS). Additionally, the results from interferometric method are compared with numerical simulations given by finite elements software - ANSYS. Keywords: electrostatic actuators, contact interactions, loading characteristics, interferometry, out-of-plane displacements measurements

This paper is devoted to the design, the fabrication and the characterization of a new generation of direct-drive silicon based electrostatic micromotors. The following work points out the main torque limitations that occur in the design of conventional electrostatic actuators using electrostatic field interactions. Stator/rotor contact interactions are then considered, in order to provide larger external torque. Distributed motion systems are also investigated, because of significant driving forces that are expected through cooperative arrayed microactuators. According to this analysis, annular-type micromotors having a (500 Atm external diameter are proposed. The motor actuation involves stator/rotor contact interactions by using a flexible polysilicon rotor which combines arrayed elementary bending actuators. Moreover, the rotor integrates a self-adjusted torque sensor that allows the driving torque to be directly measured. Experimental characteristics point out the relevance of the proposed design rules. Unusual driving torque, one thousand times higher than ones obtained using conventional electrostatic wobble motors, have been already measured. Rotation speed ranging from 0.001 up to 750 rpm is also reported, according to the driving voltage frequency which was tuned from 1 Hz up to 500 kHz.

The purpose of the present paper is to develop methods allowing the design and optimization of traveling wave type ultrasonic micrometers. These methods are based on both mechanical models. The first one concerns the contact mechanism and allows us to perform the analytical calculation of the contact rate and the pressure distribution at the stator-rotor interface for a given external preload. Then, these results are included in a second model describing the driving mechanism from kinematic considerations. Both models have been validated and applied to design and manufacture micrometers fabricated from watch-making technologies. The mechanical potentialities of such piezomotors have been confirmed. Finally, a concept for batch-fabricated piezoelectric micrometers is exposed and different induced problems are discussed as perspectives.

The authors have developed new multidegrees of freedom (dof) piezoelectric micromotors using both stationary bending modes and traveling wave technology. These devices are respectively based on the conversion, through frictional contact, of bending vibrations sustained in one or several vibrators into rigid body displacements of a moving element. They have allowed the design and the development of a micromanipulator with five degrees of freedom composed of stationary wave type linear translators (2 dof) and traveling wave type spherical micromotors (3 dof). These two motors can be driven reversibly in both directions with linear speeds between one mm/s-1 and few 10-1 ms-1. The paper shows however that micrometer steps can be activated for micropositioning applications without displacement limitations. A prototype of an autonomous mobile minirobot integrating a behavior-based intelligence is realized in order to study the behavior designing problem of a multiple robot system incorporating a behavior-based dynamic cooperation strategy for multiple behavior-based robots.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this study, we investigate new-generation high-torque/low-speed electrostatic micromotors as well as original on-chip testing methods for the mechanical characterization of silicon-based microactuators. Torque/speed characteristics of micrometer-size electrostatic actuators are measured for the first time using real-time elastic braking video analysis of polysilicon rotors that are monolithically coupled with elastic mechanical sensors. Loading characteristics measured through on-chip electrostatic probing indicate the potentialities of emerging actuator design methodologies involving frictional stator/rotor contact interactions on the micrometer scale.

Polysilicon deposited by low pressure chemical vapor deposition is the most widely used structural material for surface-micromachined microelectromechanical systems. Thus, investigations are still needed in order to identify polysilicon mechanical characteristics as a function of the deposition conditions. The following work focuses on new micrometer size monolithic test cells combining polysilicon tensile specimens and ultra-high driving force electrostatic actuators fabricated from a common process flow. The proposed approach allows intrinsic material properties such as polysilicon tensile fracture to be measured through electrostatic probing, using on-chip micrometer size uniaxial tensile test cells.

This paper concerns a physical limits evaluation with respect to the miniaturization of traveling wave type ultrasonic motors (TWUM) down to the millimeter scale. According to present knowledge in research dedicated to centimeter scale TWUM, two different prototypes, whose external diameters are φ 5 mm and φ 2 mm, have previously been designed through numerical calculations, and fabricated using watch making technologies. An experimental characterization has been carried out in order to identify the main physical and technological limitations specific to millimeter scale traveling wave motors. The very first numerical results presented in this paper show important outcomes related to the miniaturization of TWUM. In particular, the limits of current mechanical assumptions such as those related to the two-dimensional mechanical behavior of the stator/rotor interface and also to the rotor's rigidity are clearly shown by a three dimensional finite element calculation. New design rules are thus given in order to allow the mechanical architecture of TWUM to be optimized. The corresponding results indicate that powerful torque/speed characteristics can be expected, and therefore show the adequacy of traveling wave ultrasonic motors for direct-drive actuation down to the millimeter scale.

International audience; This article deals with the optimization of the main element of a digital microrobot, which is a bistable module. the bistable module consists of curved beams and electrothermal actuators. A design study of the curved beam and the electrothermal actuator is presented in order to achievethe limits in line with miniaturization. Finally, An optimization for the dimensions is proposed respect to microfabrication and elastic limits.

A number of topics concerning digital microrobotics were addressed in this thesis. A new generation of the digital microrobot ”DiMiBot” was proposed with several advantages making the DiMiBot more accurate, more controllable and smaller. The new structure consists of only two multistable modules which adds some important features such as increasing the number of positions with smaller size and the ability to realize complex trajectories in the workspace. The principle of the new multistable module combines the advantages of the stepping microactuators in terms of the principle and of the digital concept in terms of the repeatability and robustness without feedback. The accuracy is ensured with an accurate positioning mechanism that compensate the fabrication tolerances. In parallel, analytical models was developed for the main components in the DiMiBot: preshaped curved beams and U-shaped electrothermal actuators. Subsequently, design methods were developed that allow choosing the optimal dimensions that ensure the desired outputs and respecting the design specifications and limitations. Multistable module prototypes, fabricated in the clean room MIMENTO, showed a proper functioning in the experiments.; Un nombre de sujets concernant la microrobotique numérique ont été abordés dans le cadre de cette the` se. Une nouvelle génération du microrobot numérique ”DiMiBot” a e´ te´ proposé ce qui rend le DiMiBot plus précis, plus contrôlable et plus petit. La nouvelle structure est formée de deux modules multistables seulement, ce qui ajoute des fonctionnalités´ s importantes comme l’augmentation du nombre de positions avec une taille plus réduite et la capacité´ de réaliser des trajectoires complexes dans l’espace de travail. Le principe du nouveau module multistable combine les avantages des microactionneurs pas à pas en termes du principe et du concept numérique en termes de la répétabilité et la robustesse en boucle ouverte. Un mécanisme de positionnement précis, capable de compenser les incertitudes de fabrication a e´ te´ développé et utilise´ pour assurer un positionnement précis. En parallèle, des modèles analytiques ont e´ te´ développés pour les principaux composants dans le DiMiBot: poutres flambées préformées et actionneurs e´ électrothermiques en U. Des méthodes de conception ont été développées par la suite qui permettent de choisir les dimensions optimales garantissant les performances requises en respectant les spécifications et limites de design. Des prototypes de modules multistables, fabrique´ s dans la salle Blanche MIMENTO, ont montré´ un bon Fonctionnement dans les expériences.