A Single Piezoelectric Trimorph Driven Bi-Directional Linear Ultrasonic Motor

The design of using a single piezoelectric (PZT) trimorph actuator ultrasonically to drive a slider for two directional motions is reported. The working principle is the use of the resonant vibration induced force provided by the PZT trimorph driven at the specific frequency. In contact with a slider, the trimorph’s force overcomes the static friction of the slider and pushes the slider moving in one direction. The backward motion of the slider is achieved similarly by the different vibration mode of the PZT trimorph operated at the different resonant frequency. The merit of this work is the design of the rectangular PZT trimorph and its modes of vibration. The 3 mm by 9 mm rectangular trimorph was made of a 50 microns thick copper layer laminated with a 175 microns thick PZT layer on each face. As for ultrasonic motor operation, only one long edge of the trimorph was clamped. When the PZT layers were driven electrically at its resonant frequency and at the right phase, the PZT’s in-plane extensional vibration will turn into bending vibration. With extensive modal analysis in the Finite Element modeling, the 3:1 ratio of the length to width of the trimorph was found for the best performances. Both resonant frequencies and associated vibration modes were all identified. The performance of the prototype was experimentally evaluated by using laser interferometer and spectrum analyzer. Under 10 Vpp and 0.5 N preload, the measurement results show that the motor achieved the velocity of 200 mm/s and generated force of 0.1 N. If we divided the generated force and the slider velocity by the motor volume, the ultrasonic motor achieved the specific force of 3,282,447 3N/m³ and the specific velocity of 4,106,280 1/m²s, respectively. Comparing with the published data, the specific velocity in this study is 100 times larger. The results in this work are suitable for applications in auto-focusing and zooming lens in cellular phone camera.