A 2-DOF H<inf>∞</inf> control strategy for a 3 axes robotic system operating at the nanometer scale

Abstract

The capability of doing dexterous robotic tasks at the nanometer scale inside a Scanning Electron Microscope (SEM) is a critical issue for nanotechnologies. SEM-integrated nano-robotic systems have consequently emerged in many robotics laboratories. They can be composed of one or more actuators assembled into nano-robotic platforms with one or several effectors. When driven by Piezoelectric Stick-Slip (PSS) actuators, SEM-integrated nano-robotic systems are able to do a coarse positioning with a millimeter displacement range and a fine positioning with a travel range of few micrometers. Control is challenging because of several characteristics related to PSS actuators, namely the friction, the hysteresis and the undamped vibrations which degrade their performance in terms of precision and speed. This paper deals with the design of a robust control for a 3-axes SEM-integrated nano-robotic system operating in a fine positioning mode. The objective is to provide a high-resolution positioning with a closed-loop bandwidth close to the fundamental resonance frequency. Such performances are fundamental in several micro/nano-robotic tasks such as fast and accurate nano-assembly and nano-material characterization. The controller is based on a 2-DOF H ∞ strategy for which the hysteresis is modeled through a multi-linear approximation. Experimental results show precise nano-positioning with static error lower than 0.5% and a response time lower than 1 ms. Such performance have never been achieved in the case of PSS actuators and allow new perspectives for automated nano-robotic tasks inside a SEM.