超声波电机原理

Ultrasonic motors (USMs) belong to the class of piezoelectric motors. In this work the term USM will be used for the motor only (power electronics and closed loop control are not included). The system, composed of the motor, power electronics and the closed loop control will be called ultrasonic actuator or piezoelectric actuator. The working principle of these motors has been well known for at least 50 years WALLASCHEK (1995, [59]). However, they gained widespread interest first with the influencing work of SASHIDA (1982, [43]). Before, piezoceramic materials with high conversion efficiency and fast electronic power control of the ultrasonic vibrations were not available. Through their specific advantages compared to conventional electro-magnetic motors they fill a gap in certain actuator applications. Advantages of USM over electromagnetic motors are their compactness, i.e. their high stall torque-mass ratio and their high torque at low rotational speed, often making speed reducing gears superfluous. Additionally, with no voltage applied, an inherent holding torque is present due to the frictional driving mechanism. It is also worthwhile to mention that their compactness and the high frequency electrical excitation make quick responses possible. But besides that, USM also offer a high potential for miniaturization. These actuators produce no magnetic field since the excitation is quasi-electrostatic. On the other hand, some drawbacks can be easily pointed out. Due to the frictional driving mechanism, the lifetime is limited and the motor may heat up drastically within minutes of operation. The complicated stator-rotor contact behavior and the temperature drift call for special control strategies. Last but not least, the electrical excitation frequencies in the ultrasonic range, the capacitive or inductive behavior of the motor out of resonance and the temperature dependence of the motor materials require sophisticated power electronics and closed loop controls. Due to the frictional driving mechanism between stator and rotor, USMs are not intended to be used as permanently driven actuators. The friction between stator and rotor causes wear and abrasion effect as well as temperature rise of the motor,