Actionneurs "stick and slip" pour micro-manipulateurs

This thesis is a contribution to the development of simple micromanipulators, having high resolution and several degrees-of-freedom, dedicated to the manipulation of miniature objects, the manipulation of biological specimens or to the assembly of microsystems. The actuators for these micromanipulators must have a submicronic resolution over displacements of a few centimeters at a speed of several millimeters per second. They must also be compact and rigid in order ease their integration and to have a high rejection of the external perturbations (vibrations, temperature changes, etc.). Stick and slip actuators studied in this work fulfil very well these requirements. Their main features are: a resolution better than 5 nm over strokes of a few centimeters at a speed of several millimeters per second (2-5 mm/s); a high rigidity (6.5 N/μm) giving an excellent rejection of external perturbations; an extreme simplicity obtained by the combination of the guiding and actuating functions and by using an innovative concept of monolithic flexible structures. This report presents all the stages of our research work: the state of the art: originally, stick and slip actuators have been developed for the scanning probe microscopy (STM or AFM). We have adapted this concept to our purposes. Several innovative solutions allow us to simplify and improve significantly these actuators; the modeling: it allows us to understand in details the behavior of stick and slip actuators and to optimize them; the study of scaling down: it demonstrates that stick and slip actuators can be miniaturized. New applications in the field of microsystems are therefore promising; the experiment: the characterization of a one-degree-of-freedom actuator demonstrates its performances and validates our modeling; the-implementation-and-tests-of-severa1micro-mani.pulators-actuated-by-stick and slip confirm the pertinence of our approach; The results of this thesis will help engineers to design and implement efficient micromanipulators using stick and slip actuators

An in‐situ indentation system for high dynamic nanomechanical measurements

Nanoindentation is typically confined to quasi-static strain rates of testing. This poster presents the development of an in-situ indenter designed to measure the response of materials at high strain rates and high oscillation frequencies at the nanoscale. This builds up on the previous work that was the first to report on in-situ nanoindentation in a SEM in 2004 which eventually resulted in the founding of the company Alemnis AG, today one of the key players in in-situ high temperature and high dynamic nanoindentation. The motivation for variable strain rate studies is that this allows analysis of activation parameters of the physical deformation processes. Once the activation parameters are known, the deformation mechanism(s) can be determined and materials science approaches to improve materials performance can be developed. Ultra-high frequency nanoindentation enables high strain rate studies and high cycle fatigue tests that can be performed within reasonably short timespan. Compared to other actuation principles, piezo actuators offers very fast response time and high force density and are compatible with vacuum environments. At the technological heart of this innovation is a transducer called “SmarTip” consisting of a diamond tip mounted on miniaturized and embedded three-axis piezo-actuators and sensors. The SmarTip allows a full range displacement of 1μm along the three axes and to measure forces up to 1N. The theoretical bandwidths are up to 10kHz and 40kHz for lateral and axial displacements respectively. We aim to reach strain rates as high as 105s-1 meaning that the speed of displacement must reach 60mm/s for a displacement of 600nm. With such high ambitions, several parameters have to be taken into consideration such as resonant frequencies of the indenter, self-heating and cabling inducing spurious capacitance. This poster will report on these design aspects, instrumentation and technique development in addition to presenting initial data on high strain rate and high cycle fatigue tests at the micron scale. It is hoped that the multi-axis capabilities of the SmarTip will result in additional breakthroughs for applications on nano-tribology, fretting and more generally on the translation of dynamic mechanical analysis (DMA) to the micro/nanoscale. Acknowledgments Research work partially co-funded by the Commission for Technology and Innovation (CTI), the State Secretariat for Education, Research and the Innovation Eurostars program and project UHV

Unified mechanical approach to piezoelectric bender modeling

Anewanalytical modeling approach for piezoelectric bending elements is described. The approach is based on the beam theory under quasi-static equilibrium condition. It uses the theory of superposition of piezoelectric action in the bender and external moments and forces acting on the bender. Due to the differential approach, this model is applicable to any geometrical design for which the beam theory holds. The distinction between the piezoelectric action and the external loads makes the model applicable for any boundary conditions. The bottom-up approach from the electrically induced strain in the piezoelectric part enables the determination of stresses and strains at every point in the bender. The effects on internal strain distribution by the different kinds of actuation where demonstrated in an experiment. The resulting model from this approach will be well suited to design and optimize piezoelectric bending actuators for any purpose. The implied calculation of the strain and stress in the modeling allows the dimensioning of actuators that are equipped with strain gauges as well as the calculation of the electrical properties for capacitive transducers

Pseudo-elastic Flexure-Hinges in Robots for Micro Assembly

The increasing tendency of products towards miniaturization makes the substitution of conventional hinges to flexure hinges necessary, since they can be manufactured almost arbitrarily small. On account of their multiple advantages like no backlash, no slip-stick-effects and no friction, their application is especially reasonable in high-precision robots for micro assembly. Particular pseudo-elastic shape memory alloys offer themselves as material for flexure hinges. Since flexible joints gain their mobility exclusively via the elastic deformation of matter, the attainable angle of rotation is strongly limited when using conventional metallic materials with approximately 0.4% maximal elastic strain. Using pseudo-elastic materials, with up to 15% elastic strain, this serious disadvantage of flexure hinges can be avoided. A further problem of flexible joints is their kinematic behavior since they do not behave exactly like conventional rotational joints. In order to examine the kinematics of the hinges an experimental set-up was developed whereby good compliance with theoretical computed values could be achieved. A three (+1) degree of freedom parallel robot with integrated flexure hinges is investigated showing its kinematic deviations to its rigid body model. The data of the kinematic model of the flexible joint can then be implemented into the control of this compliant mechanism in order to gain not only a higher repeatability but also a good absolute accuracy over the entire working space

Compliant parallel robot with 6 DOF

In this paper a patented parallel structure1 will be presented in which conventional bearings are replaced by flexure hinges made of pseudo-elastic shape memory alloy. The robot has six degrees of freedom and was developed for micro assembly tasks. Laboratory tests made with the robot using conventional bearings have shown that the repeatability was only a couple of 1/100 mm instead of the theoretical resolution of the platform of < 1 pm. Especially the slip-stick effects of the bearings decreased the positional accuracy. Because flexure hinges gam their mobility only by a deformation of matter, no backlash, friction and slip-stick-effects exist in flexure hinges. For this reason the repeatability of robots can be increased by using flexure hinges. Joints with different degrees of freedom had to be replaced in the structure. This has been done by a combination of flexure hinges with one rotational degree of freedom. FEM simulations for different designs of the hinges have been made to calculate the possible maximal angular deflections. The assumed maximal deflection of 20° of the hinges restricts the workspace of the robot to 28x28 mm with no additional rotation of the working platform. The deviations between the kinematic behavior of the compliant parallel mechanism and its rigid body model can be simulated with the FEM

High-temperature nano-impact testing of a hard-coating system

Forging and cutting tools for high-temperature applications are often protected using hard nanostructured ceramic coatings. While a moderate amount of knowledge exists for material properties at room temperatures, significantly less is known about the system constituents at the elevated temperatures generated during service. For rational engineering design of such systems, it is therefore important to have methodologies for testing these materials to understand their properties under such conditions (i.e. high strain rate, temperature, or impact). In this work, we present our first results using a newly developed Alemnis piezo actuated nanoindenter device which utilizes dynamic indentation testing at frequencies approaching 10 kHz. A sinusoidal displacement amplitude input is provided, while a stage heater allows for sample temperatures exceeding 500 °C. Thermal drift can be minimized through high frequency, and therefore low contact time, impacts. We investigated a thin (4.65 μm) physical vapor deposited chromium nitride (CrN) ceramic coating, which had been deposited onto plasma nitrided tool steel. Forces of approximately 400 mN were applied sinusoidally at 500 Hz using a 5 μm diameter diamond flat-punch at room temperature, 200°C, 300°C, 400°C and 500°C. It was found that increasing the number of impacts led to plastic deformation and fatiguing of the hard ceramic coating. At 300°C a transition to increased material flow and consequently larger crater size, and crack initiation and propagation in the ceramic, was observed. These ceramic deformation results are understood using high-resolution scanning electron microscopy (HR-SEM), elastic simulations, and large scale batch processing of force-deformation data which are generated during high-frequency measurement and collected at a sampling rate of 50 kHz. The results are further put into context by understanding recently measured small-scale high-temperature fracture toughness and yield strength properties of thin CrN films. The presented results are the first for in situ high-temperature nano-impact testing, and will be useful for hard coatings industries involving high service temperatures and high impact strain rates, such as for forging processes

High strain rate plasticity in microscale glass

Understanding the materials behavior at high strain rates is critical for the design of structures subjected to accidental overloads such as crash testing of vehicles and impact resistance of surface coatings. From a scientific perspective, experimental determination of high strain rate properties at the micro- and nano-scale will allow the bridging of time scales between atomistic simulations and experiments, leading to a direct comparison between the two methods. Despite many efforts to expand the range of micro and nanomechanical testing in terms of forces, temperatures and loading conditions, the achievable strain rates are still around 10-5 s-1 to 10-2 s-1. This limited range of strain rates is primarily due to lack of testing platforms capable of simultaneous high-speed actuation and high-speed sensing of microscale displacements and millinewton loads. This presentation will report, a piezo-based experimental methodology for conducting high strain rate in situ micropillar compression testing at rates upto ~2000/s inside a scanning electron microscope (SEM), including a brief overview of the advantages and challenges of microscale high strain rate testing compared to traditional macroscale, Kolsky bar based, high strain rate testing. Please click Additional Files below to see the full abstract

Microcompression high cycle fatigue tests up to 10 million cycles

Nanomechanical tests are moving beyond hardness and modulus to encompass host of different mechanical properties like strain rate sensitivity [1, 2], stress relaxation [3], creep, and fracture toughness [4] by taking advantage of focused ion beam milled geometries. Adding high cycle fatigue to this list will be useful to extend the gamut of properties studied at the micro/nanoscale. However, this presents inherent challenges like low oscillation frequencies, long duration of tests and large thermal drift when attempted with standard indenters. This presentation will report, for the first time, the development of micropillar compression-compression high cycle fatigue tests going up to 10 million cycles. This has been made possible by the development of a novel piezo-based nanoindentation technique that allows accessing extremely high strain rates (\u3e104 s-1) and high oscillation frequencies (up to 10 kHz). The associated instrumentation and technique development, design of the fatigue tests at the micron scale, data analysis methodology, experimental protocol and challenges will be discussed. Validation data on single crystal silicon, a reference material, will be presented to demonstrate the reliability of the designed high cycle fatigue tests. Finally, case studies of compression-compression high cycle micropillar fatigue on nanostructured materials will be presented and their results will be discussed in light of existing literature data, particularly the operative deformation mechanism(s). The convolution of time dependent plasticity in such tests will also be addressed. It is hoped that this study will pave way for routine high cycle fatigue tests of metals at the micron scale and provide clues for designing a similar indentation fatigue test. [1] Mohanty G, Wheeler JM, Raghavan R, Wehrs J, Hasegawa M, Mischler S, et al. Elevated temperature, strain rate jump microcompression of nanocrystalline nickel. Philosophical Magazine 2015;95:1878-95. [2] Wehrs J, Mohanty G, Guillonneau G, Taylor AA, Maeder X, Frey D, et al. Comparison of In Situ Micromechanical Strain-Rate Sensitivity Measurement Techniques. Jom 2015;67:1684-93. [3] Mohanty G, Wehrs J, Boyce BL, Taylor AA, Hasegawa M, Philippe L, et al. Room temperature stress relaxation in nanocrystalline Ni measured by micropillar compression and miniature tension. Journal of Materials Research 2016;In press. [4] Sebastiani M, Johanns K, Herbert EG, Carassiti F, Pharr G. A novel pillar indentation splitting test for measuring fracture toughness of thin ceramic coatings. Philosophical Magazine 2015;95:1928-44

A review on actuation principls for few cubic millimeter sized mobile micro-robots

Actuation systems for few cubic millimeter sized mobile autonomous robots are subject to severe constraints in terms of e.g. size, fabrication or power consumption. Also the onboard electronics has limited performance due to both size and power restrictions, so actuation voltages, currents and frequency should be minimized. Various principles of electrical to mechanical energy conversion will be presented (piezoelectric, polymer, electrostatic) and their performances compared considering the above mentioned constraints. For propulsion, a further mechanical to mechanical conversion is necessary to allow long strokes. We will compare four principles for this conversion: inertial drives, walking, inch-worm and propulsion based on asymmetrical friction forces. Solutions where the energy is not onboard but rather scavenged in the environment are also reviewed. These solutions try to circumvent the energy limitations but present some inconveniences, especially when several micro-robots have to be simultaneously steered and/or propelled

Nanoindentation cracking in gallium arsenide: Part I. In situ SEM nanoindentation

The nanoindentation fracture behavior of gallium arsenide (GaAs) is examined from two perspectives in two parent papers. The first paper (part I) focuses on in situ nanoindentation within a scanning electron microscope (SEM) and on fractographic observations of cleaved cross-sections of indented regions to investigate the crack field under various indenter geometries. In the second parent paper (part II), cathodoluminescence and transmission electron microscopy are used to investigate the relationship between dislocation and crack fields. The combination of instrumented in situ scanning electron microscopy nanoindentations and cleavage cross-sectioning allows us to establish a detailed map of cracking in the indented region and cracking kinetics for conical and wedge indenter shapes. For wedge nanoindentations, the evolution of the half-penny crack size with the indentation load is interpreted using a simple linear elastic fracture model based on weight functions. Fracture toughness estimates obtained by this technique fall within the range of usual values quoted for GaA