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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

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.

International audience; In this article, an original approach to model the squeeze film effects in capacitive circular microplates is developed. The nonlinear von Kàrmàn plate theory is used while taking into consideration the electrostatic and geometric nonlinearities of the clamped edge microplate.The fluid underneath the plate is modeled using the nonlinear Reynolds equation with a corrected effective dynamic viscosity due to size effect. The strongly coupled system of equations is solved using the Differential Quadrature Method (DQM) by discretizing the structural and the fluid domains into a set of grid points. The linear effects of the squeeze film on the microplate have been investigated based onthe complex eigenfrequencies of the multiphysical problem. It is shown that the air film can alter the resonance frequencies by adding stiffness as well as damping to the system. The model has been validated numerically with respect to a Finite Element Model (FEM)implemented in ANSYS and experimentally on a fabricated circular microplates. The nonlinear effects of the squeeze film have been studied by determining the steady state solution of the system using the finite difference method (FDM) coupled with the arclength continuation technique. It is shown that the decrease of the static pressure shifts the resonance frequency and leads to an increase of the vibration amplitude due to the reduction of the damping coefficient, while the increase in the pressure enlarges the bistability domain. The developed model can be exploited as an effective tool to predict the nonlinear dynamic behavior of microplates under the effect of air film for the design of Capacitive Micromachined Ultrasonic Transducers (CMUTs).

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; This paper focuses on the dynamic characterization of an electrothermal actuator devoted to discrete MEMS positioning. Based on U-shape structure, such actuator has been employed in several MEMS applications where fine and repeatable positioning is required. The studied electrothermal actuator here is microfabricated on a doped SOI substrate and its dynamic response, during heating and cooling cycles, is recorded using precise and high-speed camera. To explain its dynamic behavior, FEM simulations, using Comsol multiphysics software facility, are carried out. The result of this numerical analysis shows a strong relationship between the temperature distribution and the displacement provided by the actuator. Finally, the influence of the dynamic behavior on the control of the actuator is discussed using experimental characterizations of its displacements under several voltage pulses with different frequencies.

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.

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.

International audience; In medical ultrasound imaging, mostly piezoelectric crystals are used as ultrasonic transducers. Capacitive micromachined ultrasonic transducers (CMUTs) introduced around 1994 have been shown to be a good alternative to conventional piezoelectric transducers in various aspects, such as sensitivity, transduction efficiency or bandwidth. This paper focuses on a fabrication process for CMUTs using anodic bonding of a silicon on insulator wafer on a glass wafer. The processing steps are described leading to a good control of the mechanical response of the membrane. This technology makes possible the fabrication of large membranes and can extend the frequency range of CMUTs to lower frequencies of operation. Silicon membranes having radii of 50, 70, 100 and 150 µm and a 1.5 µm thickness are fabricated and electromechanically characterized using an auto-balanced bridge impedance analyzer. Resonant frequencies from 0.6 to 2.3 MHz and an electromechanical coupling coefficient around 55% are reported. The effects of residual stress in the membranes and uncontrolled clamping conditions are clearly responsible for the discrepancies between experimental and theoretical values of the first resonance frequency. The residual stress in the membranes is determined to be between 90 and 110 MPa. The actual boundary conditions are between the clamped condition and the simply supported condition and can be modeled with a torsional stiffness of 2.10−7 Nm rad-1 in the numerical model.

Computer chips and other electronics have been getting faster and cheaper for a long time. But, often overlooked, they are also getting smaller and therefore allowing wearable high-technology to be developed on the near future. Current miniaturization and integration trends will result in the need to assemble ever-smaller systems and to handle smaller and smaller chips and other components that will be too small for humans to manipulate and assemble. As a consequence, the automation infrastructure of the future electronics assembly industry will have to scale with the technology, using fully and entirely automated micromachines such as manipulators and intelligent conveyance systems. The paper investigates, consequently, rapid and accurate positioning systems that will allow very small chips and components to be conveyed and automatically sorted. High-density arrays of silicon-based electrostatic actuators particularly are analyzed, according to their ability to move parts locally with high speed and sub-micrometer positioning accuracy. Actuator densities as high as 1000 actuators/mm are investigated, therefore allowing micrometer silicon based components to be manipulated. Lower densities involving larger actuation cells are also investigated in order to move millimeter size components on a few cm square motive surface. Very first design steps toward future smart surfaces are finally discussed. The paper shows how arrayed silicon based sensors, that can be easily distributed along with arrayed actuators, may provide tactile information from the external world, therefore allowing intelligent conveyance systems to be realized on the near future.

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 paper presents a macroscopic description for the simulation of the two way shape memory effect (TWSME) of shape memory alloys (SMA), and is an extension of a previous work dealing with the global thermomechanical behavior of SMA. The model is set in the framework of the thermodynamics of irreversible processes. Two internal variables are taken into account : the volume fraction of self-accommodating (pure thermal effect) and oriented (stress-induced) product phase. A term dedicated to the modeling of the trained SMA is added to the specific free energy derived for the non-trained SMA. This term φ ed , depends on the thermomechanical procedure of training submitted to the sample. In the present case, it accounts for the stress tensor σ ed applied during thermal cycling. The parameters of the model have been identified for a Cu–Zn–Al SMA, and the simulated results show good agreement with experiments.

A new generation of electrostatic micro-motors is investigated using cooperation of arrayed direct-drive actuators. Electrostatic scratch-drive actuators (SDA), which combine active frictional contact mechanisms with electrostatic actuation, are particularly analyzed. Active polysilicon sheets of 2*3 mm2 that integrate up to several thousands of electrostatic scratch drive actuators are fabricated by silicon surface micro-machining process. Each elementary actuator provides its contribution according to the driving force superposition principle, with internal forces as high as 105uN are available from this sheet. According to their natural flexibility, active polysilicon sheets can be coated onto large surfaces. A new generation of self-assembled tubular electrostatic micromotors is developed using this concept. A prototype of a cylindrical micromotor, whose external diameter and length are 1 mm and 2 mm, respectively, has been realized through the insertion of a flexible active polysilicon sheet at the rotor/motor- frame interface. After final assembling, the sheet has to be jammed onto the chassis, in order to allow the rotor to be moved with respect to the motor frame. Thus, the sheet must be in close contact with both the rotor and the motor frame, whatever the gap, which separates the two macroscopic parts. The problem related to the micro/macro world interfacing is solved during the design of sheet in allowing an out-of- plane motion of SDA in order to provide a self gap compensation, whatever both the thermal expansion effects and the macroscopic machining tolerances. The expected driving characteristics show the interest of both cooperative arrayed microactuators and direct drive frictional mechanisms.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Capabilities of future direct-drive electrostatic actuators are investigated through the mechanical characterization of annular-type polysilicon micromotors. Contact interactions are provided at the stator/rotor interface in order to allow high integrated speed reduction to be performed. The driving torque is therefore considerably amplified, as a consequence of the torque/speed duality. Direct torque measurements are performed through a self-adjustable torque sensor that is integrated in the polysilicon rotor. The first experimental results presented here give rise to the expectations of numerous direct drive applications down to the micrometer scale.

This paper investigates millimeter scale electrostatic motors designed through a new "bottom-up design approach." Active polysilicon sheets involving thousands of elementary scratch drive actuators are analyzed, according to the driving force superposition capabilities of distributed electrostatic actuators. Driving forces as high as 1 00,000 pIN are reported in this paper, using 6 mm2 polysilicon sheets that integrate up to 1430 elementary scratch drive actuators. The mechanical power transmission from the sheet to the external load is also investigated. Because of structural height limitations, active polysilicon sheets do not allow significant driving forces to be transmitted to the external world. Therefore, original architectures, such as tubular electrostatic motors, are proposed using the insertion of a flexible polysilicon stator within the gap of a macroscopic cylindrical joint. An original self-assembling method is used in order to develop a first generation of millimeter scale electrostatic actuators. Expected performances that are evaluated from the in situ sheet characterization (i.e., from characterization onto the silicon wafer) point out the relevance of the bottom-up design methodology proposed in this paper. For the first time, visible effects up to the human scale are conceivable from silicon-based electrostatic motors that will allow innumerable actuation applications to be performed in the near future.

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.

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.

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.