PWM Driving Characteristics with Its Self-Locking of Ultrasonic Motor

A traveling wave-type ultrasonic motor (USM) has a self-locking characteristic due to a static frictional force present when its power supply is turned off, because its rotor is pressed strongly to the stator in the USM. We propose a pulse width modulation (PWM) method in consideration of this self-locking characteristic in order to control the motion of USM. The vibration waveform and the driving characteristics of USM were examined experimentally at different switching frequencies under PWM. When the period of PWM switching pulse is longer than the time constant of the piezoelectric element in USM, the rest duration appears in the vibration waveform of USM. It was confirmed that the rotor speed of USM was approximately proportional to the duty ratio of the applied voltage under the condition of the low switching frequency of PWM, and the large torque was obtained even when the duty ratio was small.