Research On Hybrid Ultrasonic Motors Using Longitudinal And Torsional Vibration Modes

Hybrid ultrasonic motor using longitudinal-torsional vibration modes (LTUSM), which has become a hot spot of technical research, has larger ratio of torque to volume than other types of ultrasonic motor(USM). However, conventional LTUSM has some deficiencies. Firstly, utilization ratio of longitudinal vibration is not high; friction loss at the contact surface between the stator and rotor is large. Secondly, design of the motor becomes complex and cost increases greatly as torsional PZTs are used. Thirdly, the LTUSM is difficult to miniaturize. In view of these three questions, stress-type LTUSM, LTUSM using mode conversion and surface mount type LTUSM based on d31 effect were presented. The stress-type LTUSM has longitudinal PZTs at the contact surface, which can control contact pressure effectively and improve output torque of the motor. The LTUSM using mode conversion whose torsional vibration is converted from longitudinal vibration uses only longitudinal PZT, thus, motor structure will be simplified and design will be easy to implement and the cost will be reduced. The piezoelectric ceramics with d31 effect are used in the surface mount type LTUSM based on d31 effect, whose longitudinal and torsional modes are excited by mount ceramics on the surface of the motor, this type of motor is easy to miniature.The main contributions of the dissertation were summarized as follows:(1) The working mechanism which converts electrical to mechanical energy by strain of PZT was analyzed. Elliptic trajectory formation mechanism of particle on the stator surface and driving mechanism for the LTUSM during the working were studied. A new classification method for the LTUSM was proposed.(2) Two kinds of stress-type LTUSM were developed. Structure designs were carried out to the stress-type LTUSM with one rotor and the stress-type LTUSM with two rotors respectively. It was put forward that the lonigitudinal PZT should be placed on the surface of the stator. The FEA software ANSYS is used to anlysis the structure parameters which affect the 1st torsional vibration mode and the 2nd longitudinal vibration mode and the method to realize the frequency coherence between the 1st torsional vibration mode and the 2nd longitudinal vibration mode was put forward. The experiments for the stress-type LTUSM with two rotors shows that the stalling torque of the motor with diameter in 20mm has reached 1.28 N·m, however the world's top level is 0.8 N·m.(3) Two kinds of LTUSM using mode conversion were developed. The LTUSM using mode conversion with holes was put forward based on the research of LTUSM using mode conversion with slots. Based on ANSYS software, the effects that parameters of LTUSM using mode conversion had on mode frequency and amplitudes of longitudinal and torsional vibrations were studied. Aiming at large torque, the stators of the two kinds of LTUSM using mode conversion were optimized. According to contact situation between the stator and the rotor, force transmitting model between the stator and rotor was proposed by using theorem of impulse; energy loss and force transmitting efficiency were studied. Simulation results brought out the effects that when the motor operates steadily, the amplitudes of longitudinal and torsional vibrations, pre-pressure and friction material had on output mechanical characteristics.(4) Two kinds of the surface mount LTUSM based on d31 effect were developed. Stress states of the pipe when the longitudinal wave and torsional wave travel in the pipe are studied, and the unidirectional and bidirectional surface mount LTUSM based on d31 effect were put forward.A new exicting mechanism for longitudinal and torsional vibration mode was put forward. The polarization and arrangement of the PZT are studied, and the exciting method and the method to realize the frequency coherence between the 1st torsional vibration mode and the 2nd longitudinal vibration mode are presented. Two prototypes were developed and realized unidirectional and bidirectional rotation respectively.(5) Testing of the LTUSM using mode conversion was carried out, best friction pair and best pre-pressure spring for the LTUSM was put forward. Contact situation between the stator and the rotor under operating conditions and output mechanical characteristics under various friction pairs and pre-pressure springs were studied. Conclusions can be gotten: Using hard/soft friction pairs and pre-pressure spring with strong nonlinearity can help improve the output mechanical characteristics. No-load speed of the prototype with 15mm in diameter can reach 1082 r/min, stalling torque can reach 0.32 N·m, and the maximal efficiency can reach 32.3%, however, the world's top level for the same motor are 1100 r/min, 0.4N·m and 22%.