Reducing static friction to improve backdrivability in torque control of traveling wave ultrasonic motor

Abstract In this study, we aimed to improve the backdrivability and torque control of a traveling wave ultrasonic motor (TWUSM). Because of its operating principle, a TWUSM has strong holding torque. While this feature is useful for maintaining the rotor in a fixed position in many applications, it can be problematic for applications that require backdrivability, such as assistive exoskeletons or haptic devices. We used the theoretical contact model, which considers tangential elasticity, to calculate the torque–speed characteristics of a TWUSM. To evaluate backdrivability, we simulated not only the case in which the TWUSM drives the output shaft actively, but also the case in which the output torque opposes the rotation direction and the TWUSM is driven by an external torque passively. According to the analysis of the contact force distribution, the holding torque was reduced because the sticking contact area was decreased by stator vibration. To verify the simulation results, we built an experimental setup to measure the actual torque characteristics, and the obtained results agreed with the simulation. Furthermore, using the same experimental setup, we compared two torque control methods, driving frequency control and phase difference control. As the theoretical model predicted, the latter method resulted in higher control accuracy.