Precise speed control of ultrasonic motors with repetitive control

Ultrasonic motors (USMs) have many excellent performance and features such as high torque, low speed, compactness in size, no electromagnetic interferences and so on, and have been expected to be of practical use. The driving principle of ultrasonic motors is, however, different from that of electromagnetic type motors and its mathematical model has not been developed yet. Moreover, the speed characteristics of ultrasonic motors vary with time. Therefore, it is difficult to drive ultrasonic motors with precise speed. A traveling-wave type ultrasonic motor is suitable for variable-speed drive. However, this type of ultrasonic motor has speed ripple. This speed ripple of ultrasonic motors arises from inherent natures caused by its driving principle and structure. Since this speed ripple is larger than that of other small motors, it is a significant problem from a practical point of view. This paper presents a repetitive control scheme in order to reduce the speed ripple. Experimental results in this paper show the effectiveness and usefulness of the repetitive controller.