Theoretical Research On Bidirectional Driving Longitudinal And Torsional Hybrid Ultrasonic Motor

Ultrasonic motor is a new type of actuator using the inverse effect of piezoceramic and ultrasonic vibration to transform material micro deformation into macro movement via resonant enlargement and frictional coupling. In comparison with the traditional electric and magnetic motor, this motor has many excellent features such as small inertia, quick response, low speed and high torque, large power density, perfect control character and no magnetic disturbance etc. It has found a wide application in the fields of aeronautics and aerospace, robotics, medical instruments, accurate apparatus, automobile industries and so on.Longitudinal and torsional type ultrasonic motor is a type of standing motor developed since 1990's, which consists of longitudinal and torsional piezoelectric vibrators. Excited by two voltages which have an identical frequency but different phase shift, these two piezoelectric vibrators will generate two orthogonal standing waves and form an elliptical locus on the stator surface. The rotor is driven by the frictional force between the stator and the rotor. In contrast to the traveling wave ultrasonic motor, longitudinal and torsional type ultrasonic motor has much larger output torque and higher control precision. However, the theoretical research on this motor is not perfect. On one hand, the previous models for vibration analysis of this motor are all based on one-dimension theory, whereas the diameter of this motor is usually great. Obviously, the transverse deformation of the motor should not be neglected and the one-dimensional theory is not suitable for vibration analysis any longer. On the other hand, the key and difficult issue to enhance the output torque and efficiency of the motor is to make the longitudinal vibrator and torsional vibrator operate at an identical resonant frequency. Nevertheless, the previous work lack of further study on matching the eigenfrequencies of these two vibrations.Considering the difficulties and problems in research on thelongitudinal and torsional type ultrasonic motor, a new technical prototype of symmetrical structure hybrid ultrasonic motor with two stators was developed according to S.Ueha' prototype and the present work in this paper were completed as follows:1. A symmetrical longitudinal and torsional type ultrasonic motor with one rotor and two stators was proposed. The contact area of rotor and stator is double that of the traditional longitudinal and torsional type ultrasonic motor, which is beneficial to enhance the output torque of the motor. The symmetrical structure changes the mechanical boundary condition of this motor. This feature makes the first-order eigenfrequencies of longitudinal and torsional vibration close to each other and provides a favorable factor for matching the eigenfrequencies of longitudinal and torsional vibrations.2. A new idea was proposed to realize the eigenfrequencies degeneration of longitudinal and torsional vibrations by adding two adjusting rings to the stators. These two adjusting rings change the mechanical boundary condition and have different influence on the eigenfrequencies of longitudinal and torsional vibrations. By means of changing the mass and the position of the rings, the first-order eigenfrequencies of longitudinal and torsional vibrations were coincide and the motor operated at the resonant frequencies of both vibrations.3. An accurate theoretical model for longitudinal and torsional type ultrasonic motor with two stators was established. Through introducing kinetic energy of longitudinal and transverse vibrations concurrently into Hamilton's equation, the differential equations of stator vibration and the dynamical boundary conditions were deduced by taking into account of the transverse vibration effect of the stator. The frequency equation was obtained by means of solving these differential equations. The influence mechanism originated from the adjusting rings on the eigenfrequencies of longitudinal and torsional vibrations was analyzed.4. The theoretical model and FE model were utilized to optimize thegeometry of longitudinal and torsional type ultrasonic motor with two stators. The eigenfrequencies of longitudinal and torsional vibrations were calculated by ANSYS software and verified the theoretical results. The geometry parameters of the motor were optimized by these two models and the prototype of longitudinal and torsional type ultrasonic motor was fabricated.5. The eigenfrequency and eigenform of the present motor were measured and analyzed in the laboratory. This motor ran successfully at the first times. The experimental results showed that the working principle of the present motor was verified to be correct. The experimental results agreed well with theoretical results. The design theory and method presented in this paper help develop all kinds of hybrid ultrasonic motor.