Drive circuit for a mode conversion rotary ultrasonic motor

A mode conversion rotary ultrasonic motor (USM) has potential applications in miniature robotics. However, its electrical drive circuit presents some unique challenges, particularly in producing a high frequency (similar to 40kHz), high voltage (similar to 200V peak-to-peak) signal into a low impedance (similar to 100 Omega) capacitive motor, while achieving high efficiency. This paper describes the design of such a drive circuit, intended for use with a 12V battery. The drive circuit consists of a switch-mode power converter driving the USM via a step-up planar transformer. Compensation and resonant elements are added to improve the power efficiency. While the peak efficiency of this circuit is 45%, in practice the equivalent impedance of the USM changes with mechanical load and temperature, resulting in an average efficiency of 16%. The admittance vs. frequency characteristic and the equivalent electrical model for a USM prototype are also presented in this paper. The circuit simulations and loaded testing of a full-bridge DC-AC resonant converter with DC-offset module were performed. A load-adapted frequency tracking method has also been proposed to improve the efficiency and stability of the drive circuit.