Precise Speed Control of a Travelling Wave Ultrasonic Motor for Prosthetic Hands

Abstract

A repetitive speed controller with zero phase error tracking compensation (ZPETC) is proposed to reduce the intrinsic speed ripples of travelling wave ultrasonic motors (TWUMs). The proposed control scheme is also intended to compensate the speed variations due to load and temperature changes. This controller is expected to improve the performance of the impedance control system of a prosthetic hand, which is partially powered by a TWUM. Repetitive controllers are capable of improving the response of dynamical systems under periodic disturbances by learning from the stored error values of preceding periods. Since it is desirable for the stabilizing compensator of repetitive controllers to be zero phase and a perfect error tracker, the ZPET controller is a natural choice when the servo’s closed-loop transfer function has non-minimum phase zeros. The performance of the proposed controller was compared against that of a Pi-repetitive controller and its effectiveness experimentally demonstrated.