| Free-form surface optical elements are widely applied in many fields for its excellent optical characteristics,and diamond turning technology is considered as one of the most promising free-form surface machining technologies with the fast tool servo system(FTS).As for linear motor of FTS in free-form surface processing,high precision tracking is required for the periodic reference signals of high frequency response,whereas some factors such as linear motor inherent disturbance and processing of cutting force may affect the FTS processing accuracy.Based on this,the control method of the system under the high frequency periodic reference signal is studied.The main researches of this thesis are as follows:Numerous domestic and foreign corresponding references on linear motor served as the actuator of FTS are researched and referenced at first,on the basis of which,the mathematical model of ironless permanent magnet linear synchronous motor,employed as the actuator of FTS is constructed,meanwhile,the control algorithm for FTS is analyzed.Considering the existence of strong cutting force disturbance during the process of the workpiece,the sliding mode control with strong robustness is conceived as a more appropriate control strategy.However,the control precision of the system is influenced by the chattering presented in the traditional sliding mode control,accordingly,an incremental sliding mode control is proposed.The solver step and the maximum acceleration of the actuator are put into the control law and the inherent chattering of the sliding mode control is weaken through superimposing the control of the previous moment in the current control.At the same time,due to the incremental effect of the superposition,the control accuracy of the system is improved,compared with the traditional sliding mode control.Then the Matlab/Simulink is implemented to model and simulate the designed control system,the effectiveness of the proposed control strategy is verified subsequently.Besides mentioned aperiodic disturbance,such as cutting force,in FTS,there exists periodic disturbance in the process of high frequency cutting,a method of super twisting improved plug-in repetitive control is proposed consequently.On the basis of the principle of the integral sliding mode control,that is,the equivalent control and the nonlinear control are designed respectively and independently,meanwhile,the plug-in repetitive controller can be inserted directly with the original controller unchangeable,thus,the super twisting control and the repetitive control are combined into the proposed method.The the super twisting control of strong robustness is applied to eliminate aperiodic interference and the repetitive control is employed to suppress periodic interference.Moreover,the proposed method improves the traditional plug-in repetitive control algorithm,which advances the control system for higher control precision.Finally,on the basis of theoretical analysis,Matlab/Simulink is implemented to model and simulate the proposed control system,the simulation results of the traditional repetitive control and the designed control system are compared to prove the feasibility and validity of the proposed method. |
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