Patent ID: 7576472

Claim:
An ultrasonic motor comprising: a plurality of ultrasonic transducers including alternately stacked first piezoelectric elements and second piezoelectric elements; and a control unit for applying the same two-phase driving AC voltages to the plurality of ultrasonic transducers, wherein the ultrasonic motor simultaneously generates longitudinal vibration and flexural vibration at the ultrasonic transducers to generate substantially elliptic vibration at output ends of the ultrasonic transducers and relatively moves the ultrasonic transducers and a driven body in contact with the ultrasonic transducers, and wherein the control unit includes vibration detecting part for detecting the vibrations of the ultrasonic transducers and outputting an average-vibration detection signal, phase comparing part for detecting a phase difference between one of the two-phase driving AC voltages applied to the ultrasonic transducers and the average-vibration detection signal, and frequency controlling part for controlling the frequency of the driving AC voltages such that the phase difference detected by the phase comparing part is a predetermined value.