Modeling And Experimental Research On Electro-mechanical And Contact Friction Coupling Of Sandwich Type Linear Ultrasonic Motor

With the development of smart materials and modern control theory,ultrasonic motor(USM)has been quickly developed in the past decades.And USM has become a brand new way of driving,which has replaced the traditional electromagnetical motor in areas such as special condition and high accuracy condition.Ultrasonic motor's superiorities against the traditional electromagnetical motor mainly reflect on nonelectromagnetic interference,low speed,large output thrust,high power density,high accuracy,quick response and selflocked when power off.And these merits have made it of great value to research and to apply in areas such as aerospace,fiber optical communication,scientific instruments,precise manufacture,MEMS,etc..The working process of ultrasonic motor consists of two parts: electro-mechanical coupling stage and contact friction coupling stage.During the electro-mechanical coupling stage,excited by an alternating voltage with certain frequency,the stator of the ultrasonic motor comes into microscopic vibrating.During the contact friction coupling stage,the microstate vibrating of stator transfers into the macroscopic motion of rotor.Thus,the working performance of the ultrosonics motor is directly depended on these two stages.So far,researchers have mainly commited on structure innovations and most of these new structures have not achieved the expected high electro-mechanical coupling efficiency.In addition,in the area of contact friction coupling,most of the existed researches are about the continuous contact of travelling wave ultrasonic motor,the impact contact of standing wave ultrasonic motor has been seldom reported.Considering the low electro-mechanical coupling efficiency of the existed ultrasonic motors,sandwich type ultrasonic motor(STUSM)was chosen,electro-mechanical process of longitudinal and bending vibration modes of STUSM is studied.Mathematical model of electro-mechanical process of the two vibration modes are built.Based on the mathematical model,excitation position of PZT ceramics are investigated,and most suitable excitation position was obtained.Five prototypes of STUSM were fabricated,and their vibration characteristics and electro-mechanical couping coefficient were tested.The comparison of mathematical results and testing results provides a validation to the mathematical model.Instructed by the mathematical model of electro-mechanical process,corresponding to longitudinal and bending vibration modes,two STUSMs,whose PZT ceramics were placed at the most suitable position,were proposed.Finite element method(FEM)was used to extract the vibration shapes and trajectories of driving feet of the proposed motors,which provide an explanation for the working principle of the proposed motors.Prototypes of the proposed STUSMs were fabricated,and their vibration characteristics,impedance characteristics,and mechanical output characteristics were tested and evaluated.For longitudal vibration,a frog shaped linear ultrasonic motor was proposed.The motor used a longitudinal ultrasonic transducer and four PZT ceramics(volume of PZT is 4197 mm3)vibration to generate linear motion,and it achieves a maximum output thrust of 11.8 N with a mass of 0.255 kg,and the output thrust density is 46.27 N/kg.The effiective electromechanical coupling coeffiect is 21.29%.This motor only uses one longitudinal vibration to work,which successfully abbreviate the verbose frequency degeneration.And this abbreviation makes the motor simple to drive,easy to control and miniaturization.For bending vibration,a bending hybrid linear ultrasonic motor was proposed.The motor used two orthogonal third bending vibrations and four PZT ceramics(volume of PZT is 5596 mm3)vibration to generate linear motion,and it achieved a maximum output thrust of 23 N with a mass of 0.185 kg,and the output thrust density was 124.3 N/kg.The effiective electromechanical coupling coeffiect of the two bending vibration were 35.04% and 34.69%,respectively.The motor adopted a sectional excitation method,which made the PZT ceramics of two orthogonal bending vibrations at the most suitable position.In addition,the motor does not need frequency degeneration and is easy to miniaturization.Aimmed at the impact contact of ultrasonic motor,a dynamic contact friction model of stator and rotor of USM was proposed.The proposed model was built based on the Greenwood-Williamson toeory which takes the contact surface of stator and rotor as random roughness surface.Gererally considered the influence of materials of rotor and stator,vibrating amplitude of driving foot,and the microscopic characteristics of contact surface etc,the model obtained the simulated results of dynamic normal contact force,dynamic friction coefficient,dynamic friction force,and steady state output characteristics.Experimental tests of steady state output characteristics under different conditions were carried out using the proposed bending hybrid linear ultrasonic motor.These tests of steady state output characteristics were the test of different material matches,the test of different roughnesses,and the test of different normal preloads.These test results were compared with the simulated ones,which provided a solidation to the dynamic contact friction model.