Velocity Control of Ultrasonic Motors for Robot Arms by Pulse Width Modulation.

A travelling wave-type ultrasonic motor is one of the promising motors for robot arms since it has a simple construction, a high response, and large torque at low speed. The rotational direction of the rotor can be changed when the direction of the travelling wave is reversed. This paper proposes a rotational speed control method by changing the duty ratio of the reverse-direction duration using PWM(Pulse Width Modulation). The basic characteristics of the ultrasonic motor using this velocity control were examined experimentally. It was confirmed that the rotational speed of the ultrasonic motor decreased with the increase of the duty ratio of the reverse-direction duration and the linearity between two variables was almost satisfactory, even in an open loop control.