Robust Speed Control of Ultrasonic Motor Based on H.INF. Control with Repetitive Compensator.

Abstract

A constant speed control system of an ultrasonic motor(USM)is designed based on H∞ control with a repetitive compensator. Generally driving mechanism of USM uses frictional force. It comes the following disadvantages of the motors : (1)the dynamic response shows non-linear natures and depends on the operating temperature ; (2)the average rotational speed gradually decreases with temperature rise ; (3)the speed fluctuates periodically synchronizing with rotation. To tackle the problems, multiplicative uncertainties are introduced into the plant model to represent the non-linearity and the temperature dependency. The decrease and the fluctuation in speed are treated as a step and a periodic disturbance respectively. The control system is designed as follows. Firstly, and integral type H∞ controller is designed which guarantees robust stability against the uncertainties. Secondly, a repetitive compensator is attached to the system without loosing stability to attenuate the periodic disturbance. Experiments confirm the validity of the proposed control system.