Abstract
In piezoelectric ultrasonic motors, high-frequency mechanical oscillations are used to create frictional driving forces. The surface points of the cuboid-shaped ultrasonic oscillator presented perform an elliptical motion which is obtained by the superposition of longitudinal and bending vibrations. Both vibration modes can be obtained as solutions of the wave equation and the Timoshenko-Bresse beam equation, respectively, if linear piezoelectric constitutive equations are assumed. Damping and nonlinear material behaviour, which can be observed experimentally, must be taken into account. For this purpose, generalised constitutive equations for piezoceramics were investigated and identified experimentally for SONOX P88 ceramics. The effects of damping and nonlinear material behaviour were introduced in the modelling of the oscillator. The results of the analytical calculation allow an accurate definition of the geometry of the oscillator, so that the two vibration modes have the same resonance frequency. Thus, good controllability of the motor can be obtained. A prototype motor was designed and investigated experimentally. We discuss the material modelling as well as some other important design aspects of this type of motor (e.g. the design of the support, force flow at the stator/rotor interface, and tolerances).
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