Research On The Driving Characteristics And High Precision Measurement And Control Method Of Ultrasonic Motor

Ultrasonic motors,which use piezoelectric ceramics as the transducer and friction interface as the transmission media,present many significant merits,such as rapid response,high torque at low-speed,self-lock when power off,and no electromagnetic radiation.They have obtained preliminary applications in the area of advanced manufacturing,aerospace,biomedical and other fields.However,there are still some difficulties in the development of ultrasonic motor,which restricts motors’ application in precision equipments.For example,performance test of multi-parameter nonlinear charcteristics is inefficient and unclear,and there is a lack of high-stability speed control methods and high-resolution position control ones.In order to solve the above problems,this paper aims to improve the ultrasonic motor’s speed stability and position resolution.The contents cover electromechanical coupling model of driving characteristic,high-precision speed control method and high-precision position control method,which lays a foundation for the application and development of ultrasonic motor in the future precision assemblies.The main contents include the following parts:1.To overcome the problems of traditional measurement system,such as inflexible parameter regulation,incomplete performance analysis,and low-efficiency control algorithm,an experimental approach with multi-parameter rapid measurement and control function is designed.In the test system,the problems which include generation of pre-pressure,acquisition of interface temperature,and high-frequency signals are well solved.Also,a standardized test flowchart is formed.Experiments show that the developed test system realizes precise online adjustment of 8 control parameters containing pre-pressure and rapid measurement of 13 process parameters including voltages.It can precisely describe the startup-shutdown process’ s transient properties and realize the precise adjustment of multi-parameters.The platform has excellent control-integrated ability,which provides a complete experimental platform for measurement and control of ultrasonic motors.2.In order to characterize the nonlinear mapping relationship between the driving parameters and performances accurately,a complete electromechanical coupling model is proposed.The model includes modules of multi-parameter driving circuit and motor body,.The stator teeth-discretized model is adopted to solve the unsophisticated problem of traditional contact models.Experimental results show that response speed and steady-state accuracy of the proposed teeth-discretized contact model have been improved compared with those without teeth-discretized,and the fitness is increased from 55% to 85%.Besides,the model’s effectiveness is verified by the experimental results of voltages,currents,power and mechanical characteristics.The proposed electromechanical coupling model can analyze the changing attributes of displacement,torque and energy comprehensively,which provides the theoretical support for further control.3.In order to grasp the macro/micro rules with different working states,the driving characteristics of the ultrasonic motor during the startup-shutdown process,reversing process and steady-state operation process are studied.Experimental results show that the amplitude-decaying stage and the self-locking stage with underdamped vibration have a marked effect on the stepwise resolution.In the reversing process,the transformation is in sequence of voltages,currents,stator amplitude,output torque and speed.And higher driving frequency cause the shorter switching time.During the steady-state process,speed ripple originates from unaveraged contact pressure caused by the shaft misalignment.Therefore,the roughness of teeth surface and the radial runout must be reduced.As for the multi-parameter driving characteristics,the mapping relationship between speed and driving parameters is studied.And the sensitivities of pre-pressure on the key characteristics are researched,such as interface contact characteristics,speed fluctuation characteristics,long-time temperature rise and torque properties,which helps to form the rules for selecting pre-pressure according to different application.4.The ultrasonic motor has many control parameters and its speed control performances are limited because of several nonlinearities,including speed fluctuations and transient jump when reversing.Therefore,the control precision of constant value control and reversing tracking control is limited.In order to describe the multi-parameter nonlinear speed characteristics of the ultrasonic motor effectively,a Hammerstein speed model including steady-state speed regulation equation,dynamic transfer function and speed fluctuation equation,is established.The fitness value between theoretical results and experimental ones are up to 87.5%,which indicates the model availability.In order to effectively suppress the speed ripple,a double-loop combined control strategy combined with position closedloop and velocity closedloop is proposed,which makes the speed stability in the range from 5°/s～72°/s reaches better than 0.5%,and the control precision is doubled when comparing with the single velocity closedloop control.In order to relieve speed jump at the reversing moment,a multi-parameter combined control method based on harmonic feedforward is designed.Finally,the tracking error is reduced by 3.5 times compared with amplitude control,and reduced by 4.4 times compared with frequency control,which effectively improves the ultrasonic motor’s speed control accuracy.5.The precise positioning of USM is affected by encoder’s resolution and transient driving characteristics.There are still some deficiencies,like unclear recoginition on factors affecting the micro-stepping resolution and low positioning accuracy.To analyze the micro-stepping resolution’s influencing factors,an underdamped function is adopted to discribe the vibration behavior of the stator and the rotor.Experimental results show that the damping ratio and natural frequency of the shutdown process are closely related to the transient properties at the shutdown moment.A prediction model of stepwise resolution can effectively draw the micro-stepping characteristics.The sensitivity of control parameters on the stepwise resolution is studied.A piecewise approximation strategy which is combined continuous motion with stepwise motion is designed to improve te stepwise precision.Finally,the control results show that the ultrasonic motor’s angular resolution can be up to 0.375μrad,and the positioning error is up to 1.7μrad,which effectively enhanced the precise positioning capability of the ultrasonic motor.