Phase difference torque control of USM for walking assist system

Abstract:
Ultrasonic motor (USM) has unique merits such as high torque density without reduction gear and silent drive. While these characteristics are useful to realize a walking assist system comfortable for wearers, driving control of USM is relatively difficult comparing with electromagnetic motor. In this research, we studied output characteristics of USM by analytical simulation considering elasticity of rotor and confirmed its result by actual measurement. According to the results of output characteristics simulation and measurement, we found that keeping large vibration amplitude of stator improve the accuracy of torque control. To confirm this hypothesis, several control methods of torque control was examined experimentally, and the result was consistent with our hypothesis. From these results, we conclude that the large vibration amplitude is essential for torque control accuracy and elasticity of rotor has a great effect on output characteristics and controllability of the torque output.