Study On The Control Strategy Of XBPM Beam Spot Stabilization System Based On Hammerstein Model

XBPM(X-ray Beam Position Monitor)is the key to judge the stabilization of X-ray beam focus,which directly determines the quality of high-precision experiment results.In this thesis,the piezoelectric system used by XBPM is modeled and the control strategy is studied to overcome the errors in the positioning accuracy due to the hysteresis of piezoelectric actuator,external vibration and other factors.Through high-precision positioning control of the micro-nano precision platform,the XBPM central detection area is driven to coincide with the X-ray beam center,ensuring the stability and effectiveness of XBPM detection beam spot size,and meeting the requirements of the fourth generation synchrotron radiation source for X-ray beam stabilization.The main contents are as follows:(1)Firstly,the working principle and the model and parameters of each component of XBPM beam spot stabilization system were introduced,and then two main components,the micro-nano precision platform and piezoelectric actuator were selected.The structure of the platform was introduced from two aspects of parallel guide mechanism and displacement amplification mechanism,which provided the basis for the open loop transfer function and dynamics model.Using the domain theory,the causes of inverse piezoelectric effect of PZT and nonlinear characteristics such as hysteresis and creep were analyzed at the microscopic level,and these characteristics and their effects on output displacement were also introduced,so as to carry out hysteresis modeling of PZT.(2)In the modeling research of system,firstly,a Hammerstein model consisting of static nonlinear module and dynamic linear module in series was selected to describe the hysteresis of PZT.P-I model was selected in this thesis for static module and polynomial was introduced to improve the hysteresis to asymmetry,and ARX model was selected for dynamic module.In the displacement range of 5-30μm,the maximum prediction error of Hammerstein model is less than 0.90μm,and the root mean square error is less than 0.50μm.Then a comprehensive electromechanical coupling model was established,which consisted of the piezoelectric actuators and the platform open-loop transfer functions.Finally,the input hysteresis of the system was characterized by Hammerstein model,which was combined with the electromechanical coupling model to obtain the dynamic model of the system with hysteretic nonlinear input.It is proved that the output error of the model is less than 2.5% of the maximum output,which can effectively predict the output of the micro-nano precision platform.(3)In the control strategy research of system,firstly,based on the inverse PZT model of Hammerstein structure and the uniaxial displacement amplification ratio predicted by the dynamics model,the feedforward control strategy of the micro-nano precision platform was established.Then,based on feedforward control,PID feedback controller was introduced and the feedforward PID compound control strategy was proposed.Among them,by changing the number of sampling points in feedforward PID compound control,it was found that it could effectively solve the problem of the large range of system error caused by the change of input signal frequency.This thesis analyzed it,and considering the variable sampling control could not automatically adjust the number of sampling points according to the actual situation,and had the problem of slow operation caused by long sampling data,so the feedforward voltage data with the minimum error under each waveform and frequency was collected,and then the reference voltage was synthesized.Next,the LMS algorithm was introduced in this thesis to design an improved adaptive control strategy that could pre-adjust and real-time adjust the parameters of Hammerstein inverse model of PZT based on the reference voltage.This strategy inherits the advantage of small error of variable sampling control and overcomes the problem of slow operation caused by too many sampling points.Ensure the stability of platform control.Finally,according to the decoupling principle of micro-nano precision platform,the control method of platform coupling displacement was designed.(4)In the experimental test and evaluation of system,the LABVIEW programs of control strategies above were written firstly.Then the micro-nano precision platform was used to track the sinusoidal,triangular and multi-frequency signal of different frequencies.By comparing the error of three strategies,the experiment results show that the improved adaptive control is superior to others in improving the precision and stability of platform.Finally,the decoupling control strategy of platform was carried out and more than 65% of the coupling displacement could be offset.It has a good decoupling effect.This thesis has 63 figures,14 tables and 86 references.