Research On Direct Contour Control Method For Complex Machining Path

With the increasing demand of parts quality in aerospace,automotive engine,precision mold and other fields,higher requirements are put forward for high-speed and high-precision NC machining of complex curved parts.An important criterion of NC machining quality is contour error,which is defined as the shortest distance from the actual machining position to the programming curve position.For the problem of contour error control,the main research at present are all kinds of contour control methods based on position tracking servo system,and the control method directly aiming at contour control has not been realized.This paper proposed a new type of direct contour control method for the complex machining tool path,which completely eliminates the position tracking link in the traditional control,and can directly control the contour error,and the new solution for the contour control problem of complex machining tool path has been put forwarded.This paper studies the real-time exact contour error calculation and the direct contour error control method,two-axis and three-axis direct contour control(DCC)are designed,the two DCC methods are verified through simulation and experiment.The main research was concluded as follows:(1)The implementation of DCC scheme first needs to realize the real-time and accurate monitoring of complex curve contour error.The problem that the approximate estimation methods are mostly used to approximate the contour error instead of the real value in contour control,this paper proposed the real-time accurate algorithm for calculating the contour error value of NURBS parameter curve.According to the definition of foot point,the foot point solution model is established,and the selection of multiple candidate foot points under the condition of large curvature is discussed.For the requirement of real-time contour control,the definition of control contour error(CCE)is proposed.Brent method is used to calculate the exact value of the foot point,which can avoid the problem of poor convergence of Newton method,and the foot point interval search algorithm is also studied.Finally,the feasibility of the algorithm is verified by NURBS parameter curve.(2)For contour control,a Coordinate Transformation-DCC(CT-DCC)method is proposed in this paper.The scheme is composed of the upper controller in the contour control coordinate system and the lower controller in the world coordinate system,and the contour error control can be decoupled from the feed direction speed control,which can control the contour motion directly,and the position tracking link is eliminated.The error dynamic characteristics of the scheme are analyzed through the mathematical model.NURBS curve is taken as an example,the feed speed command planning in parameter domain and real-time monitoring of contour error are realized,and the CTDCC scheme is verified by simulation and experiment.(3)With the problems of large amount of calculation of multi-axis coordinate transformation,complex parameter matching of two-layer controller and poor robustness in CT-DCC method,a Dual Sliding Mode DCC(DSM-DCC)method without coordinate transformation is explored.With the principle of DCC control,the contour error sliding mode surface,feed speed sliding mode surface and dual sliding mode surface are designed,and the dual sliding mode control law is designed according to Lyapunov method.Experiments show the effectiveness of DSM-DCC scheme.(4)Based on the two DCC methods proposed in this paper,the three-axis NC numerical control system is designed and verified on the machining center.The experimental results show that compared with the traditional contour error estimation method and contour controller scheme,the DCC methods proposed in this paper can effectively improve the contour control accuracy.The Stable experimental process shows that,DCC methods can be well compatible with the existing CNC system hardware and satisfied with the demand of the real-timeThe research of this paper shows that DCC as a new contour control scheme,can take the contour error as the control target directly,with the benefits of the intuitive control parameters,the convenient adjustment is,and the simple implementation.The complex coupling problem of position tracking and contour control in the previous control scheme could be avoided,and the higher contour machining accuracy can be obtained.It is helpful to improve machining accuracy of complex surface parts and promote theoretical and engineering innovation of high-grade CNC system.