The rotational speed of an ultrasonic motor is related to the vibration speed of its stator. Because drift of the piezostator characteristics will cause variation of the motor speed, a controller is thus necessary in order to produce a constant output speed. Variations of the motor output characteristics will also make it very difficult to obtain the efficiency of the motor. This paper points out the origins of possible drifts in a new disc-type piezoelectric ultrasonic motor which has three nonequal-triangular (120°-90°-150°) fixed points near the edge. The frequency behavior of the proposed motor and its consequences on speed control scheme are discussed. The speed control scheme is implemented by using current modulation, so the rotational speed will be kept constant
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