Research On Low Speed Servo Control Of Ring-type Travelling Wave Ultrasonic Motor

Different from the traditional electromagnetic micro-motor,ultrasonic motor is a new type of direct drive motor,which has the advantages of fast action response,self-locking,simple structure and so on.Because of special working principle,ultrasonic motor does not need a specific speed reduction mechanism to get low speed.It does not generate magnetic field under working condition and is free from magnetic interference and radiation source,which make it suitable for precision low-speed control fields like aerospace.In this thesis,the control of 60 mm ring-type traveling wave ultrasonic motor(RTWUSM)at low speed is studied.The main contents are as follows:Based on the analysis of the operating mechanism of RTWUSM,the three speed control methods are analyzed and compared,and the methods used in this thesis is determined as the frequency modulation method.In research of control,a reasonable mathematical model is the basis of controller design.Considering the complexity and undirect controllability of the analytical model of USMs,this thesis uses nonparametric model identification method to establish an approximate linear mathematical model of RTWUSM.A second-order transfer function is used to characterize the motor speed characteristics when frequency takes a step change,and the parameter value is obtained by the approximation method.The measured points are taken in the working frequency range to get the parameter valued of each point.The mathematical model of the motor is obtained by using the least squares method.Accurate control model is the key to accurate low-speed control.According to the mathematical model of RTWUSM,this thesis uses the linear sliding surface and the exponential approach law to design the low-speed dual-input sliding mode controller and uses the frequency modulation to obtain the predetermined speed control target.The designed synovial controller controls the balance of the system by using the frequency difference and the driving frequency of the previous state as the state quantity,in this case,the state variable of the control system gradually converges to zero.The hard ware control platform with DSP as the core controller is established.The motor is controlled by this experimental platform,and the low speed control target with the speed of 1.5r/min and fluctuation of 2% is initially achieved.