An Ultrasonic Motor Driven by Traveling Cylindrical Wedge Waves

Abstract

A novel ultrasonic motor driven by flexural waves traveling circumferentially along a circular cylindrical wedgelike acoustic waveguide is presented. A rotary motion can be induced via friction forces if a rotor normally contacts the internal inclined surface of the cylindrical wedge near the tips. The rotary motor stator has a simple structure constructed by a metal cylindrical wedge adhered on a piezoelectric tube whose polarization is in the radial direction. The wedge acoustic waves are excited by two sets of comb electrodes painted on the outer surface of the piezoelectric tube. Both comb transducers with an angular space apart are excited by AC signal in +/-90 degrees phase difference to achieve bidirectional operation for the motor. There exist a number of axial modes and circumferential modes in the stator in a broad frequency range below 100 kHz. Only the fundamental modes of the latter are considered to drive this rotary motor. Along with commercial codes of finite element method (FEM), a bi-dimensional FEM has been developed to determine the resonance frequencies of circumferential modes and their corresponding modal displacements. The angular speed and direction are controllable by altering the driving voltage and the phase difference between two comb transducers.