Joël Agnus

Asynchronous Event-Based Visual Shape Tracking for Stable Haptic Feedback in Microrobotics

Micromanipulation systems have recently been receiving increased attention. Teleoperated or automated micromanipulation is a challenging task due to the need for high-frequency position or force feedback to guarantee stability. In addition, the integration of sensors within micromanipulation platforms is complex. Vision is a commonly used solution for sensing; unfortunately, the update rate of the frame-based acquisition process of current available cameras cannot ensure-at reasonable costs-stable automated or teleoperated control at the microscale level, where low inertia produces highly unreachable dynamic phenomena. This paper presents a novel vision-based microrobotic system combining both an asynchronous address event representation silicon retina and a conventional frame-based camera. Unlike frame-based cameras, recent artificial retinas transmit their outputs as a continuous stream of asynchronous temporal events in a manner similar to the output cells of a biological retina, enabling high update rates. This paper introduces an event-based iterative closest point algorithm to track a microgrippers position at a frequency of 4 kHz. The temporal precision of the asynchronous silicon retina is used to provide a haptic feedback to assist users during manipulation tasks, whereas the frame-based camera is used to retrieve the position of the object that must be manipulated. This paper presents the results of an experiment on teleoperating a sphere of diameter around 50 μm using a piezoelectric gripper in a pick-and-place task.

Overview of Microgrippers and Design of a Micromanipulation Station Based on a MMOC Microgripper

This paper deals with an overview of recent microgrippers. As the end-effectors of micromanipulation systems, microgrippers are crucial point of such systems for their efficiency and their reliability. The performances of current microgrippers are presented and offer a stroke extending from 50mum to approximately 2 mm and a maximum forces varying from 0.1 mN to 600 mN. Then, micromanipulation system based on a piezoelectric microgripper and a SCARA robot is presented

Robust Feedforward-Feedback Control of a Nonlinear and Oscillating 2-DOF Piezocantilever

Many tasks related to the micro/nanoworld require, not only high performances like submicrometric accuracy, but also a high dexterity. Such performances are only obtained using micromanipulators and microrobots with multiple degrees of freedom (DOF). Unfortunately, these multi-DOF systems usually present a strong coupling between the different axis making them very hard to control. The aim of this work is the modeling and control of a 2-DOF piezoelectric cantilever dedicated to microassembly/micromanipulation tasks. In addition to the coupling between the two axis, the piezocantilever is very oscillating and strongly nonlinear (hysteresis and creep). In the proposed approach, the nonlinearity and vibration are first compensated thanks to the feedforward technique. Afterwards, we derive a decoupled model in order to synthesize a linear robust H∞ controller. The experimental results show the efficiency of the proposed approach and their convenience to the micromanipulation/microassembly requirements. Note to Practitioners-The main motivation of this paper is the need of both high performances and high dexterity in micromanipulation and microassembly tasks. In such a case, not only a sub micrometric accuracy and stability are needed, but also numerous degrees of freedom. For that, in the literature, there exist piezoelectric based structures with two or more DOF. Unfortunately, the coupling between its axis, the nonlinearities (hysteresis and creep) and the structure vibration make them very difficult to control and therefore make performances lost. A classical feedback con troller can be employed but when the nonlinearities and vibration become strong, it is impossible to synthesize a linear controller. In this paper, we show that the combination of feedforward techniques, to minimize the nonlinearities and vibration, and feedback techniques makes possible to reach the high performances required in micromanipulation/microassembly. We notice that the proposed approach can also be applied to other nonlinear, oscillating and multi-DOF systems, such as piezotubes.

A micromanipulation cell including a tool changer

This paper deals with the design, fabrication and characterization of a tool changer for micromanipulation cells. This tool changer is part of a manipulation cell including a three linear axes robot and a piezoelectric microgripper. All these parts are designed to perform micromanipulation tasks in confined spaces such as a microfactory or in the chamber of a scanning electron microscope (SEM). The tool changer principle is to fix a pair of tools (i.e. the gripper tips) either on the tips of the microgripper actuator (piezoceramic bulk) or on a tool magazine. The temperature control of a thermal glue enables one to fix or release this pair of tools. Liquefaction and solidification are generated by surface mounted device (SMD) resistances fixed on the surface of the actuator or magazine. Based on this principle, the tool changer can be adapted to other kinds of micromanipulation cells. Hundreds of automatic tool exchanges were performed with a maximum positioning error between two consecutive tool exchanges of 3.2 µm, 2.3 µm and 2.8 µm on the X, Y and Z axes respectively (Z refers to the vertical axis). Finally, temperature measurements achieved under atmospheric pressure and in a vacuum environment and pressure measurements confirm the possibility of using this device in the air as well as in a SEM.

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

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.

Prototyping of a highly performant and integrated piezoresistive force sensor for microscale applications

In this paper, the prototyping of a new piezoresistive microforce sensor is presented. An original design taking advantage of both the mechanical and bulk piezoresistive properties of silicon is presented, which enables the easy fabrication of a very small, large-range, high-sensitivity with high integration potential sensor. The sensor is made of two silicon strain gauges for which widespread and known microfabrication processes are used. The strain gauges present a high gauge factor which allows a good sensitivity of this force sensor. The dimensions of this sensor are 700 μm in length, 100 μm in width and 12 μm in thickness. These dimensions make its use convenient with many microscale applications, notably its integration in a microgripper. The fabricated sensor is calibrated using an industrial force sensor. The design, microfabrication process and performances of the fabricated piezoresistive force sensor are innovative thanks to its resolution of 100 nN and its measurement range of 2 mN. This force sensor also presents a high signal-to-noise ratio, typically 50 dB when a 2 mN force is applied at the tip of the force sensor.

Description and performances of a four-degrees-of-freedom piezoelectric gripper

The aim of the research conducting to this paper was to design a simple, efficient and cheap microrobot, with a view to mass production. The devised microprehensile microrobot on chip (MMOC) is able to grip, hold and release submillimetric-sized objects. It is a two-fingered micro-gripper, with fingers able to move independently one from each other in two orthogonal directions. The gripper thus has four degrees of freedom. The paper describes the functioning principle of the MMOC and details its elementary piezoelectric actuator, which is compared to existing piezoelectric tubes. The realised prototype is then presented and its performances are commented.

Feedforward and IMC-feedback control of a nonlinear 2-DOF piezoactuator dedicated to automated micropositioning tasks

This paper presents the characterization, modeling and precise control of a 2-dof piezoactuator dedicated to precise and automated micropositioning tasks. The piezoactuator is characterized by a strong hysteresis and a high coupling between the two axes making the synthesis of a controller very difficult. We therefore propose to compensate first the hysteresis (feedforward control) in order to obtain an approximate linear system. Afterwards, an internal model control (IMC) structure is applied (feedback control) to enhance the performances of the piezoactuator. The main advantage of the proposed approach is its simplicity both for computation and for implementation making it very convenient for real-time embedded systems. Finally, the experimental results demonstrate its efficiency and conveniency for precise positioning.

A four‐degree‐of‐freedom microprehensile microrobot on chip

This paper presents a cheap and easy‐to‐produce microprehensile microrobot on chip (MMOC). This four‐degree‐of‐freedom (DOFs) microprehensor is able to grip, hold and release submillimetric‐sized objects. The research conducted relied heavily on the design of a simple and efficient monolithic piezoelectric two‐DOF actuator, requiring no further motion transformation system and asking for no supplementary guiding system. The integration of all these functions in a single part eliminates nearly all assembly concerns. Each finger of the gripper is an actuator, called a duo‐bimorph, which provides higher deflections than piezoelectric tubes. The paper presents the developed MMOC prototype, comments its performances and details the functioning of the duo‐bimorph.

A mechanical de-tethering technique for silicon MEMS etched with a DRIE process

Getting micro-electro-mechanical systems (MEMS) out of a wafer after fabrication processes is of great interest in testing, packaging or simply using these devices. Actual solutions require special machines like wafer dicing machines, increasing time and cost of de-tethering MEMS. This paper deals with a new solution for manufacturing mechanical de-tetherable silicon MEMS. The presented solution could be done with a DRIE process, already used in silicon MEMS fabrication, without additional time or cost. We are proposing a new way to create a notch on tethers linking both wafer and millimetric MEMS, especially designed to break with a specified mechanical force. A theoretical silicon fracture study, the experimental results and dimensional rules to design the tethers are presented in this paper. This new technique is particularly useful for microscopic MEMS parts, and will find applications in the field of the MEMS components microassembly.