Author:

Abstract:

Piezoelectric ultrasonic motors are characterized by a multitude of attractive properties. However, despite continuous improvements since the 70's, their limited ruggedness still hampers broad application. Two inherent phenomena are responsible: the sensitivity of the eigenmodes used to amplify the short stroke of the piezo elements, and the friction interface that converts this amplified mesoscopic stroke to the envisioned macroscopic system motion. The friction interface introduces nonlinear behavior, and also wear of the tribopair that constitutes the interface. This dissertation focuses on the second phenomenon from a tribological point of view. The aim is dual: gaining a better physical understanding of ultrasonic motor behavior, and improving ultrasonic motor durability. To gain a better physical understanding of ultrasonic motor behavior, generic models have been developed that allow to investigate the influence of control parameters and tribopair characteristics on motor performance. The most advanced version employs a transient implementation of Kalker's rolling contact theory, and is able to predict microslip effects in the ultrasonic motor contact zone. By further extension of the models with wear equations, the missing link between the contact mechanism and the resulting wearing-out of the tribopair, has been theoretically underpinned. Given the complexity of the considered system, a satisfying correspondence between simulation and reality has been achieved. To improve ultrasonic motor durability and to validate the models discussed above, extensive wear tests on an actual ultrasonic motor setup have been performed. Preceding these experiments, a novel strategy based on a combined-property failure mechanics approach has been developed to facilitate tribopair preselection. During the experiments, online measurements were performed to determine wear rates of the selected tribopairs. Extensive post mortem analysis has allowed to gain a solid understanding of the active wear mechanisms.