Multi-degree-of-freedom ultrasonic motors using rotation-symmetric piezoelectric vault geometries

The multi-degree-of-freedom (multi-DOF) ultrasonic motors described in this paper use one rotation-symmetric vault geometry (e.g. a hemispherical shell) made of piezoelectric material to provide three dimensional (3-D) trajectories on the top of this vault shaped vibrator. There with spherical, planar or a combination of linear and rotatory drives can be driven in several directions by a friction contact. The 3-D trajectories are generated by superposition of one horizontal and one vertical eigenmode. The horizontal eigenmode causes the vibrators top to move in several horizontal directions and the vertical eigenmode causes a movement in the vertical direction. So the superposition of these movements results in 3-D trajectories on the top of the vault vibrator. The coincidence of the resonance frequencies of the used eigenmodes and the fixing of the vibrator are key issues that have to be incorporated by the development of such motors. Furthermore, the excitation structure - specified by the electrode division that can be ordered circular segmented on the vibrator - looms large, too. There are different types of vault shapes (e.g. bell shells) and excitation techniques such multi-DOF ultrasonic motors can be build from. Out of this arises a multiplicity of opportunities to realize several variants of this type of motor. A prototype vibrator, composed of a hemispherical shell glued on a metal disk, will be described at the end of this paper that proves to be promising to use vault shaped vibrators for the development of multi-DOF ultrasonic motors.