Compliance control of circular traveling wave motor

Traveling wave ultrasonic motors (TWUSMs) were used as actuators for a prosthetic arm, because of their light weight, compact size, silent motion, high-speed response, and low-speed rotation. The prosthetic arm has to move in compliance with the environmental force around every joint, as in the human arm. In order to realize a compliant prosthetic arm, it is desired that the actuator have a compliant characteristic in itself, but TWUSMs have no such characteristic. The TWUSM is driven by two-phase sine-wave electric signals. Bending traveling waves which generate rotational torque on the rotor are propagated in its stator. Usually the phase difference of the driving signals is adjusted to 90° to get the maximum output torque, but can be adjusted in range from 90° to -90° to regulate rotational torque. The authors were able to develop an adjustable compliant motion to the TWUSM using a phase difference control method of the driving signals. This controlled system acts much like a compliance adjustable spring with a minor damping characteristic