| The complex curved parts with high accuracy and high surface quality are widely used in the fields of aerospace,national defense and military industries,etc.However,due to the limitation of the servo system structure of machine tools as well as the restriction of the movement coordination and friction disturbance of each axis of machine tools,these will inevitably lead to the issues of out of tolerance,high scrap rate and difficult to meet the performance requirements.Thus,it is urgent to systematically study on formation mechanism and effective control of contour error during the precision machining.Therefore,to solve the problem of precision machining of thin-walled curved surface parts in a precision physics experiment,this paper depends on the open machining platform of precision turning&grinding integrated platform,and researches on the following aspects:tracking error prediction based on servo system structure,feed-forward control of LuGre friction,suppression of thrust fluctuation of permanent magnet synchronous linear motor,modeling of global spatial contour error and multi-axis cross-coupled pre-compensation control.The details are shown as follows:For the modeling of tracking error,the influences of servo delay,friction disturbance,and thrust fluctuation on tracking error are considered,and the error equations of each part are established.Firstly,according to the signal transmission characteristics of the linear servo system,the complex frequency domain transfer function is established.The differential equation expression of the servo delay tracking error is given,and the calculation of transfer function can be simplified through parameter substitution,which improves the solving efficiency.Secondly,the LuGre model is used to analyze the dynamic friction during the precision machining.At the same time,the improved Drosophila Swarm Algorithm based on cross mutation is used to identify the dynamic and static parameters in the friction model efficiently and accurately,and the expression of friction tracking error is obtained by the friction state space.Finally,the thrust fluctuation of the U-type permanent magnet linear synchronous motor is restrained to improve the stability of the platform.The equivalent magnetization method and the equivalent current method are used to establish the air gap magnetic field strength model of the permanent magnet magnetic field and the armature permanent magnet magnetic field respectively,and the mathematical model of the linear motor is obtained by Clark and Park coordinate transformation.At the same time,considering the influence of cutting force and combining with the electromagnetic thrust,positioning force,and friction force,the dynamic equation of linear motor is derived,and the error expressions caused by thrust fluctuation are given.For the controller design of tracking error,some control strategies are designed to compensate for the tracking error caused by the friction and the thrust fluctuation.Feedforward compensation control is designed to compensate for the tracking error caused by friction,which avoids signal lag in the traditional friction controller.To restrain the thrust fluctuation and improve the accuracy of the linear motor,the low-frequency thrust fluctuation is compensated by a linear thrust fluctuation observer,and the compensation current of thrust fluctuation is compensated to the servo system by using the new sliding mode controller.Besides,considering the occasional resonance of system at the specific frequency,the double T-Notch filter is designed to eliminate the interference signal to ensure the stability of the system output.For the modeling of contour error,the estimation model based on the spatial geometric relationship between the tracking error and feed rate is established.By calculating the servo delay and delay displacement,the closest point on the design curve to the actual point is carried out,and then the estimated contour error can be obtained.Compared with the traditional models,for the proposed model,the complex iteration can be omitted and the calculation speed model is faster than the traditional method when the accuracy is similar.To compensate for the estimated contour error,the actual point is compensated to the design point by twice displacement compensation according to the trigonometric vector.For multi-axis control,the pre-compensation strategy of contour error based on adaptive cross-coupled control is proposed.The mismatching factors of multi-axis parameters which impact on the machining profile accuracy are analyzed.This strategy integrates the friction feedforward compensation control and thrust fluctuation compensation control,which can reduce both the tracking error and the contour error.Besides,this strategy can also solve the mismatch of the compensated value of contour error caused by the inertia difference of each axis.The central controller of cross-coupled control consists of the fuzzy PID adaptive controller.In order to further improve accuracy,the improved particle swarm optimization algorithm is used to optimize the bottom tolerance of membership function in fuzzy control.A series of comparative experiments are designed to prove the effectiveness of proposed method,and comparative examples are introduced to analyze the influences of friction disturbance,thrust fluctuation and cross-coupled control on tracking accuracy and contour accuracy,which verifies practicability and effectiveness of the proposed method for precision machining. |
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