Design of a Linear High Precision Ultrasonic Piezoelectric Motor

Abstract

To understand the operating principles of linear ultrasonic piezoelectric motors, a motor made by Nanomotion Ltd. was examined and a model of the driving process was developed. A new motor has been designed that uses the same driving process but improves resolution, speed, efficiency and especially controllability. All designs involve at least two independently driven piezoelectric elements, one generating the normal load at the interface and the second generating the tangential driving force. The greatest challenges in developing this motor are 1) the actuator needs to have two different mode shapes at nearly the same frequency and 2) each mode shape must be exclusively excited by one actuator and not by the other. The quality of the operation of the motor directly depends on how well the excitation of both vibrations can be separated.Finite element analysis (FEA) has been used to model the actuator and predict the dynamic properties of a future prototype. The model includes all significant features that have to be considered such as the anisotropy of the piezoelectric material, the exact properties and the dimensions of the actuators (including all joints). Several prototypes have been built, and the resulting mode shapes and natural frequencies have been measured and compared to the computer models. The design concepts as well as the modeling techniques have been iteratively improved. Open loop testing has shown that the motor generates slideway motion such that the steady state slideway velocity is proportional to the excitation voltage. To fully characterize the motor and to demonstrate its full potential for positioning tasks, the motor has been tested in a closed loop control system. Despite saturation of the control input and nonlinearities in dynamics of the motor-slideway system, it was shown that a simple feedback control system using proportional gain or proportional-integrating control algorithms can be used to achieve a stable responsive positioning system.