Quick and precise position control of ultrasonic motors with two control inputs

The ultrasonic motor (USM) has an excellent performance and many useful features. Since this motor is superior in holding torque and high response characteristic, it has been expected to be used as a position servomotor for small motors. The driving principle of an ultrasonic motor is, however, different from those of electromagnetic type motors and its mathematical model has not been developed yet. Moreover, its speed characteristics vary with driving conditions. Therefore, it is difficult to control ultrasonic motor with high performance. This paper presents a new position control scheme of an ultrasonic motor, which controls both driving frequency and phase difference, to achieve a quick and precise position control. This control scheme consists of a PI controller and an adaptive controller, which compensates the speed characteristic variations. The effectiveness of this control scheme is demonstrated by experiments.>