Excitation Method And Experimental Research Of Ultrasonic Motor Using Longitudinal-bending Hybrid Modes

Ultrasonic motors(USMs) transfer electrical energy directly into mechanical energy via the converse piezoelectric effect of the piezoelectric element, which is entirely different from conventional motor of obtaining rotation speed and torque by way of electromagnetic effect. Because of new principle and structure, ultrasonic motors exhibit merits of large torque in the low speed range, high torque/weight ratio, self-locking without applied electric power, good dynamic response, fine positional accuracy, no magnetic field generation and absence of magnetic interference, silent operation. Compared with conventional electromagnetic motors, ultrasonic motors are better candidates distinctly in certain applications, such as aerospace equipments, medical apparatus and instruments, precise manufacture, automotive industry, MEMS, semiconductor manufacture, and so on.Ultrasonic motors usually work in resonance mode. Generally, different resonance modes are generated by the excitation of different piezoelectric ceramic elements, among which there are inevitable vibration coupling effects. Thus, the number of resonance modes adopted by USM is in direct relation to the number of ceramic groups. As mentioned above, the conventional excitation of USMs certainly results in structure complexity of the stator, high processing and assembly precision requirements, large mass of the transducer, and so on. This thesis carries out studies aiming at the longitudinal-bending(L-B) modes composition and sandwich type orthogonal-bending(O-B) modes composition. Two excitation, hybrid excitation of longitudinal-bending vibrations and modified hybrid excitation of sandwich type orthogonal-bending vibrations, are proposed to improve the performance and efficiency of ultrasonic motors.Firstly, the basic characteristics of longitudinal and bending vibration modes of rectangle beam were presented. The structure of conventional excitation of the longitudinal-bending modes composition was given, while the vibration mechanism was analyzed. Based on the above analysis, hybrid excitation of sandwich type longitudinal-bending vibrations was proposed, which achieved structural simplification of the composite beam, as well as the longitudinal-bending modes composition. Subsequently, hybrid excitation of bonded-type longitudinal-bending vibrations was proposed. Simplified model of every excitation was developed, based on which the vibration equations of points on the end face of composite beam were established. Finally, the structure of conventional excitation of the sandwich type orthogonal-bending modes composition was presented, and a modified hybrid excitation of orthogonal-bending vibrations was proposed. Based on the same structure size, the modified hybrid excitation could realize high amplitude of orthogonal-bending modes composition.Based on the hybrid excitation of sandwich type longitudinal-bending vibrations, a sandwich type longitudinal-bending ultrasonic motor with hybrid excitation was proposed. Firstly, the motor structure was presented, and the working principle was analyzed. Modal analysis of finite element method was adopted to accomplish the degeneration between the resonance frequencies of two vibration modes. And the simulation of the vibration characteristics was carried out after the optimization of the motor structure. Lastly, the prototype was fabricated, the vibration characteristics and impedance characteristics of which were tested. Typical output of the prototype is maximum no-load speed of 728.85mm/s, maximum thrust force of 16 N and maximum power of 3.13 W.Based on the hybrid excitation of bonded-type longitudinal-bending vibrations, the miniaturization of sandwich type longitudinal-bending ultrasonic motor was studied. The bonded-type longitudinal-bending ultrasonic motor with hybrid excitation employed the d31 working mode of piezoelectric ceramics, the working principle of which is similar to the sandwich type one. The degeneration between the longitudinal and bending modes of transducer was accomplished by adjusting the structure parameters with finite element method. The elliptical motion trajectories of the feet were obtained by simulation. Finally, the prototype was tested about the vibration characteristics and impedance characteristics. Typical output of the prototype is maximum no-load speed of 467.84mm/s, maximum thrust force of 1.8N and maximum power of 0.27 W.Based on the hybrid excitation of sandwich type longitudinal-bending vibrations and modified hybrid excitation of sandwich type orthogonal-bending vibrations, a sandwich type multi-degree-of-freedom(multi-DOF) ultrasonic motor with hybrid excitation was proposed. The motor adopted pair combination of three resonance modes to drive the sphere rotations about three coordinate axes. When the sphere rotates around any axis, all piezoelectric ceramics of the proposed motor were excited for the vibration of the motor. The final structure parameters of the motor were confirmed by way of finite element method, and the elliptical motion trajectories of driving areas were verified. At last, the prototype was fabricated and tested. The prototype achieved no-load speeds of 109.8r/min, 107.9r/min and 290.8r/min under the YOZ, XOZ and XOY driving modes, respectively.