Transient response of wedge-wave rotary ultrasonic motors

This paper presents an effort to improve performance of the wedge-wave ultrasonic motors (WW-USM) driven by circumferentially traveling flexural waves. In general, USM is operated in the vicinity of the resonant frequency of a specific vibration mode, which needs to be well separated away from the resonant frequencies of other modes. In the current prototype, transient response to the designed flexural mode is accompanied by longitudinal modes because of poling of the PZT tube and electrode layout around the tube. Uncertain disturbance induced at the instant of switching on/off or changing direction of the motor were found through both simulation and measurement. The fluctuations are attributed to unexpected vibration modes occurring around the operating frequency. Simulated results indicate that the performance has been improved if the electrodes are placed over the entire wavelength around the hoop of PZT tube and/or the tube is periodically poled through the thickness in alternate directions. Transient response becomes smooth and has twice the amplitude of displacement when the longitudinal modes are suppressed from excitation of WW-USM.