Repetitive controller for speed ripple reduction of ultrasonic motors

Ultrasonic motors are driven by the friction force between the rotor and the vibrator, which often induces speed fluctuations synchronizing with the rotor position periodically, namely, speed ripples. The mathematical description of an ultrasonic motor for speed ripples is introduced using Fourier series in this paper, and then the speed controller is designed with a plug-in type repetitive controller to enhance speed control accuracy. Such design allows the repetitive controller to be separated from the designed servo controller. The plug-in type repetitive controller can be added subsequently without disturbing the stabilizing control system. Experiments confirm the validity of the proposed ultrasonic motor model which significantly reduces speed ripples.