Hemispherical resonators made of soft and hard piezo material for planar ultrasonic motors

Monolithic piezoelectric hemispheres used as ultrasonic resonators are a distinguishing feature when used in a planar XYPhi motor. They are relatively easy to manufacture compared to composite resonators. This paper focuses on the optimization of motion trajectories of half-side excited hemispheres by using hard and soft piezo material. The trajectory can be reconstructed using its radial and tangential displacement and the phase between them. A simulative approach to predict and test the mode shapes of soft piezo hemispheres in linear small signal range is shown. High deflection amplitudes are associated with higher mechanical quality due to less damping. Hard piezo material offers higher values regarding the mechanical quality factor than soft material, which offers broadband excitability. Therefore, resonator prototypes were built from both material types in order to measure deflections in radial and tangential directions by using an out-of-plane and in-plane laser vibrometer setup. When half side was operated with a 15 V amplitude at resonance frequency, hard piezo material shows a significantly higher displacement, especially in the tangential direction. However, larger deviations occur with the different directional excitations in the tested hard piezo material prototype.