Study on rotation speed control of coiled stator ultrasonic motor using pulse width modulation

We have developed a traveling-wave-type miniature ultrasonic motor using a helical coiled waveguide as a stator. In this motor, the elliptical motion of the surface particle due to the flexural ultrasonic waves rotates the rotor, which can be placed adjacently inside or outside the stator, via the frictional force. In this motor, there is no pre-pressure between a stator and a rotor, there is a gap of constant size between them. Hence, it is difficult to rotate the rotor by low voltage, because the frictional force cannot work properly when the amplitude of the flexural waves is small. To avoid this problem, in this study, we apply the pulse width modulation (PWM) to this motor as a driving scheme.