To improve positioning precision of the traveling wave ultrasonic motor (USM), a flexible shaft was used, and the structure was redesigned to reduce the fluctuations of step angles and speed. The start and stop response performances of the motor were analyzed theoretically and tested by a high-resolution test rig. The step characteristics were calculated, which showed the minimum step angle is below 0.20″ under different torques. The positioning process included two modes: continuous mode and step mode. In continuous mode, the speed of the motor was controlled by PID control based on the tests of frequency-speed characteristic of the motor. In step mode, the step characteristics of the motor was tested under different load torques and applied different lengths of driving voltage to find the minimum of step angle. From the tests, the step angle increases rapidly with the periods of driving voltage and the single-step angle fluctuations of the motor with flexible shaft is much less than that of motor with normal shaft. Finally, the positioning performances of USM were tested. The results showed that the minimum positioning error of the motor with flexible shaft could achieve 0.37″ (1.79 μrad) and the time costed was below 1 s.

为了提高行波超声电机（USM）的定位精度，使用了挠性轴，并对结构进行了重新设计，以减少步距角和速度的波动。理论上分析了电动机的启动和停止响应性能，并通过高分辨率测试仪进行了测试。计算了步进特性，其显示了在不同转矩下最小步进角在0.20英寸以下。定位过程包括两种模式：连续模式和步进模式。在连续模式下，基于电动机的频率-速度特性测试，通过PID控制来控制电动机的速度。在步进模式下，在不同的负载转矩下测试了电机的步进特性，并施加了不同长度的驱动电压，以求出最小步进角。从测试中 步进角随驱动电压周期的增加而迅速增大，柔性轴电动机的单步角波动远小于普通轴电动机的单步角波动。最后，对USM的定位性能进行了测试。结果表明，带有挠性轴的电动机的最小定位误差可以达到0.37英寸（1.79μrad），所花费的时间不到1 s。