Dynamic preload control of traveling wave rotary ultrasonic motors for energy efficient operation

Ultrasonic motors (USM) have great potential to become the actuators of the next generation. However, the current control schemes of USM are still suffering from some shortcomings including limitation of output speed and torque range, and excessive heat generation. Driving schemes based solely on frequency control suffer from reduced efficiencies across most of its operation band due to having a single optimum driving frequency around resonance. In this paper, the effect of preload on USM driving characteristics was demonstrated and it was clarified that preload can affect the driving efficiency, resonance frequency, and output power range. Dynamic preload control has, thus, been proposed as a supplementary controller besides frequency control to enhance the USM performance. The results show that compared to static preload, dynamic preload can expand the output power range of USM (up to 5 W) while keeping a maximum driving efficiency (up to 22%).