Ultrasonic motor with the capability of adjustable self-locking torque

This paper discusses a new ultrasonic motor (USM) with the adjustable mechanism of self-locking torque. A USM has the capability of generating a resistant torque against an external one even under power-off, which is well known as a self-locking characteristic and conveniently utilized to save energy while maintaining rotor position. The goal of the paper is to widely change the self-locking torque between lock and free states. In order to achieve our goal, we propose a new type of USM having a pressure adjustable mechanism enabling us to control the contact force between the rotor and stator. Two d.o.f. PWM control, in which OFF command is included in addition to CW (combined clockwise) and CCW (counter clockwise) commands, is a good tool for changing the self-locking torque from the viewpoint of control. By combining the new USM with two d.o.f. PWM control, we show that the self-locking torque can be largely changed from free to firmly locked states. We also show several experimental results to verify the effectiveness of the idea proposed.