Mechanical and electrical characteristics of traveling wave ultrasonic motors during the whole life of friction material

It is difficult to evaluate the characteristics of traveling wave ultrasonic motors (TWUSMs) during the whole life of friction material (FM) because the wear of FM is quite slow. To solve this problem, an accelerated life test method was proposed and the mechanical and electrical characteristics of TWUSMs in the whole life of FM were analyzed systematically. The results show that the mechanical characteristics including speed and efficiency have a decline trend after an ascent with the increase of FM thickness, especially when the load torque is great. When the load torque is greater than 0.4 Nm, the optimum range of FM thickness is 0.10-0.20mm. When the residual FM thickness is less than 0.17mm, holding torque is greater than stall torque. Electrical characteristics of TWUSM show monotonicity in the whole life of FM. Therefore, the variations of electrical characteristics could be the wear indicator of FM. Based on experiment analysis, mechanism of interactions and couplings in the energy transfer process of the TWUSM system was explored. The energy transfer process of TWUSM was divided into four subprocesses. The wear of FM is in the intermediate subprocess, which has influence on both output performance and drive circuit of TWUSM. The four subprocesses interact and couple with each other closely. Consequently, the mechanical part and electrical part of TWUSM should be not isolated, and not only the forward action, but also the backward action in energy transfer process should be considered for a more realistic theoretical model of TWUSM.