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

A constant speed control system of an ultrasonic motor (USM) is designed based on H∞ control with a repetitive compensator. In spite of the excellent features, USM has the following problems because of the driving mechanism using frictional force: (1) the dynamic response is non-linear and depends on the temperature. (2) the averaged rotational speed gradually decreases as the temperature rises. (3) the speed fluctuates periodically depending on the rotational angle. In this study, the input and output of the USM are driving frequency and rotational speed respectively. The multiplicative uncertainty is introduced into the plant model to represent the non-linearity and the dependency on the temperature. The decreasing and fluctuating speed are considered as a step and a periodical speed disturbance respectively. The control system is designed as follows: Firstly, an integral type H∞ controller is designed which guarantees robust stability against the uncertainty. Secondly, a repetitive compensator is attached to the system without loosing stability to attenuate the periodical disturbance. Experiments confirms the validity of the proposed control system.