Bi-dimensional finite element analysis for modal separation of a circular cylindrical WW-USM

This paper describes a modal separation model that improves the performance of wedge wave ultrasonic motors (WW-USMs) using the bi-dimensional finite element analysis (Bi-d FEA) method. Most USMs operate near the resonant frequency of a specific vibration mode, which must be well separated from the resonant frequencies of other modes. In the current prototype, unexpected disturbance induced at the instant of switching on/off or changing direction of the motor appeared in simulations and actual measurements. These fluctuations are due to vibration modes other than the driving mode occurring around the operating frequency. Bi-d FEA numerical simulation results indicate that selecting an appropriate height for the upper base under the piezoelectric tube achieves excellent modal separation. Measurements of the traveling wave taken by modal sensors show that the proposed motor performs better than traditional motors, offering a faster transient response in revolution speed and a more stable design. Suppressing the unexpected modes in excitation of the WW-USM increased the maximum output torque by 50% and the efficiency of mechanical/electrical transformation by 50%.