A mode conversion rotary traveling-wave ultrasonic motor (TWUSM) has potential applications in miniature robotics. However, its electrical drive circuit presents unique challenges, particularly in producing a high frequency (about 40 kHz), high voltage (400 V peak-to-peak) signal input to a low impedance capacitive motor, while achieving high efficiency. A novel driving circuit is proposed to provide two-phase balance voltage for a TWUSM. This paper describes the design of TWUSM drive circuit, intended for simultaneously employs both the driving frequency and phase modulation control scheme. The operating principles and a detailed analysis of the proposed driving circuit, consisting of voltage-controlled oscillator (VCO), voltage-controlled phase-shifter circuit and power amplifier circuit, are introduced. To drive the USM effectively a two-phase power amplifier converter using non-resonant technique was designed to provide a balanced two-phase voltage source, and the two-phase output driving voltages could be maintained at the same peak voltage value at the driving frequency under varying phase-modulation processes. Detailed experimental results are provided to demonstrate the effectiveness of the proposed driving circuit. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.