A ZPET-repetitive speed controller for ultrasonic motors

The application of a repetitive controller with zero phase error tracking compensation (ZPETC) to the speed control problem of travelling wave ultrasonic motors is investigated. The proposed control strategy is aimed at reducing the typical speed ripples of ultrasonic motors as well as compensating the load and temperature changes. Repetitive controllers are capable of improving the response of dynamical systems under periodic disturbances by learning from the stored error valves of preceding periods. Since it is desirable for the stabilizing compensator of repetitive controllers to be zero phase and perfect error tracker, the ZPET controller is a natural choice when the servo's closed-loop transfer function has non-minimum zeros. The performance of the proposed controller is compared against that of a PI-repetitive controller and its effectiveness experimentally demonstrated.