Design and Control of a Piezo-based Fast Tool Servo System for Precision Diamond Turning

A novel fast tool servo driven by piezoelectric actuator for precision diamond turning is designed in this paper. To overcome the inherent hysteresis and drift nonlinearity effect of the piezoelectric actuator, a closed-loop control system is established using strain gauge integrated in the actuator for position feedback, which has compact structure and can avoid interference in the machining. Furthermore, a fuzzy PI control algorithm is presented. It has not only the advantages of agility and adaptability of fuzzy control, but the characteristics of high accuracy of PI arithmetic. At last, experiments are carried out and the results show that the fuzzy PI control provides significantly better tracking accuracy and robustness against hysteresis and drift effects. Introduction Recently, fast tool servo has become an indispensable component in precision diamond turning machines to obtain higher positioning accuracy and bandwidth [1]. Various fast tool servo concepts have been presented in the past. Hara et al. presented a piezo driven fast tool servo to detect the initial contact between the tool and the workpiece [2]. Li et al. presented a piezoelectric fine positioning system for compensation of spindle errors in precision diamond turning [3]. Shamoto and Moriwaki designed various piezoelectric actuators to deliver long and continuous strokes in precision positioning as well as an actuator to deliver elliptical vibrations to diamond turning tool to reduce the chip-tool friction [4–6]. Kim developed a piezoelectric actuator to mount on a conventional lathe in order to control depth of cut precisely and compensate the waviness on the surface of the workpiece [7]. Most of these applications were for diamond turning where the cutting force disturbances were neglected, and the sensors were laser interferometer, grating ruler or capacitance micrometer, which are complex, costly, environment dependent and difficult to control. In this paper a fast tool servo driven by a piezoelectric actuator is presented. To correct the inherent hysteresis of piezoelectric actuators, fuzzy PI feedback control is implemented, which has not only the advantages of agility and adaptability of Fuzzy Control, but the characteristics of high accuracy of PI arithmetic [8]. The sensor used in this closed loop system is strain gauge integrated in the piezoelectric actuator, which has compact structure and can avoid interference in the machining. Design of fast tool servo The details of the FTS designed in this investigation are shown in Fig. 1. The FTS is composed of a main body base, a tool holder, a micro motion stage with two piezoelectric actuators, and a height adjusting mechanism with wedge block. A single diamond tool is mounted on the tool holder, which is bolted to the micro motion stage. The structure of the FTS makes for higher bandwidth because the mass of the moving body is small. The most important part of the FTS is the twodimensional micro motion stage, driven by two piezoelectric actuators. The configuration of the stage is shown in Fig. 2. The stage has the structure of a monolithic nested-loop type moving plates, in which each moving plate is guided by two four-link mechanisms. The four hinges of the four-link mechanism are implemented by four round-notched flexure hinges. Due to the nested-loop type structure, the moving plates are actuated independently each other by piezoelectric actuators so that Key Engineering Materials Vol. 339 (2007) pp 195-199 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland Online available since 2007/May/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.33-16/04/08,09:45:38) the stage can avoid coupled interference motions. To increase the motion range, a lever mechanism is applied. By experiment, the travel range of the stage is 25×20μm, and the first and second resonant frequency without the piezoelectric actuator is respectively 1375 Hz and 1500Hz. Tool holder Micro motion stage Height adjusting mechanism Fig.1 Composing structure of fast tool servor Fig.2 Structure of two-dimensional micro motion stage Inner plate Lever Outer plate Fixed frame