A Method to Realize Low Velocity Movability and Eliminate Friction Induced Noise in Piezoelectric Ultrasonic Motors

In a piezoelectric ultrasonic motor (USM) or resonance drive type piezoelectric motor (RPM), movement is generated between a vibrator (stator) and a slider (rotor). Since the microscopic vibrations on a stator are transferred to a slider through friction interaction, the movement of a slider has a nonlinear characteristic due to the nature of the friction force. This nonlinear behavior causes large position errors due to the occurrence of discontinuous stick-slip movements and unpleasant audible noise, especially at a low velocity drive. This friction induced acoustic sound is magnified at low velocities as the natural frequency of the mechanical system of a piezoelectric motor with mass and the holding and prestress spring forces are dependent on the closed loop motion controller. This article addresses the abovementioned issues. First, a mechanical model, which considers the nature of movements in a resonance drive type piezoelectric motor, was established. The model could suitably define the friction induced forced vibration and noise source. Second, a new driving method for resonance drive type piezoelectric motors was proposed, in which the piezoelectric vibrator was excited using two driving sources at two different frequencies. The difference between the two excitation frequencies was synchronized to the servo sampling frequency of the digital control unit. Finally, the performance of the proposed driving method was compared with those of the conventional driving methods. It was noted that in addition to the realization of silent and smooth low velocity movements, the positioning error for the linear movements between the desired and actual positions decreased to less than 10 nm for velocities ranging from 1 to 0.001 mm/s.