Active control of self-locking characteristic of ultrasonic motor

This paper discusses a new control method for an ultrasonic motor (USM). An USM has the capability of generating a resistant torque against an external one even under power-off which is well known as self-locking characteristic and conveniently utilized to save energy while maintaining rotor position. The goal of the paper is to relax the self-locking characteristic and to widely change it between lock and free states. In order to achieve this, we newly propose the two d.o.f. PWM control, in which OFF command is included in addition to CW and CCW commands. By applying this, we can also control the torque of USM in open loop without using any complicated circuit. In this paper, we first demonstrate the basic structure of the two d.o.f PWM control, and then show several experimental results to verify the effectiveness of the scheme proposed.