Speed control of cylindrical ultrasonic motor

The cylindrical ultrasonic motor (CUSM) is a novel type of piezoelectric actuator, which utilizes a monolithic PZT (lead zirconate titanate) tube as its stator. CUSM, like other types of rotary USMs, has many attractive properties that conventional electromagnetic motors do not possess, and therefore has received much attention. In this paper, three different control inputs, namely voltage amplitude, driving frequency and phase difference, are evaluated, and the voltage amplitude is finally chosen for speed control purpose. A PI speed controller incorporating linearization component is then designed based on the CUSM's frequency response. However, the steady state responses of both the open-loop and closed-loop systems show periodic speed ripples. Thus, a repetitive control (RC) compensator is proposed to work in conjunction with the speed controller to achieve speed ripple minimization. The dynamic as well as steady state performances of the speed controllers and RC compensator are verified experimentally using a 10 mm diameter CUSM.