Operating frequency tracking of single phase driving type piezoelectric motors

We developed a multi (or mixed) mode excitation type piezoelectric motor that uses a rectangular bar type piezoelectric multilayer stator as the vibrating element. The stator has a first longitudinal vibration mode in a direction of longest length and second flexural mode in a direction of thickness. During operation of the motor, these two modes are coupled so at the two end surfaces, elliptical motions are generated. The elliptical motion at one end of the stator is transferred to a moving element through frictional contact. This paper introduces structure and operating principal of the piezoelectric multilayer stator. An equivalent circuitry model for the free stator and a frequency tracking method to operate the motor at its optimum operating frequency are also proposed. The frequency tracking method not only finds the best operating frequency, it also tracks the frequency variations from sample to sample and operating frequency shift due to temperature change.