Research On Comprehensive Control Strategy Of Cross-coupling Contour Error Compensation For Two-axis CNC Machining System

Multi-axis computer numerical control precision machining technology has been widely used in the field of mechanical manufacturing and processing,and is increasingly used in the high-speed and high-precision machining of complex curved parts.Aiming at the problem that it is difficult to satisfy various condition elements at the same time in the high-speed and highprecision machining of complex curved parts,a comprehensive control strategy of crosscoupling contour error compensation for two-axis CNC machining systems is proposed.And,the strategy consists of four modules: real-time contour error estimation model,NURBS curve interpolator,cross-coupling controller and improved position error compensator.In order to estimate the contour error of the target reference path in real time,a real-time contour error estimation model of three machining trajectories of straight line,circular arc and free curve is established.At the same time,aiming at the problem that the generation of target command position needs to consider various conditions in the process of servo motion control,a NURBS curve interpolator is proposed,which can satisfy the constraints of speed,acceleration,jerk and chord error at the same time,and a series of discrete interpolation points are generated for the servo drive.In order to reduce the contour error of the dual-axis servo system,a PI-type cross-coupling controller is proposed based on the cross-coupling structure,and its stability is analyzed by using the contour error transfer function.At the same time,the cut-off frequency of the contour error transfer function is used to determine the ranges of gain parameters of the cross-coupling controller.Aiming at the problem of optimizing gain parameters in cross-coupled controllers,a gain parameters optimization strategy of cross-coupled controller based on deep reinforcement learning is further proposed.And,a neural network structure suitable for contour error compensation is established,which can continuously adjust the ranges of gain parameters according to the contour error in the process of servo motion control,thus finally helping to obtain the optimal values of gain parameters.In order to solve the problem that the traditional cross-coupling control method cannot meet the chord error requirement in large curvature positions,this paper takes the NURBS curve as the object and proposes a feed rate adjustment method based on the chord error constraint.The combination of this method and the cross-coupling control strategy can not only satisfy the requirement of large curvature positions for the chord error,but also further improve the contour control accuracy of servo motion control system.At the same time,in order to further improve the tracking accuracy and contour control accuracy of the dual-axis servo motion control system,an improved position error compensation method is further presented by combining the feed rate regulator with the position error compensator,which improves the tracking accuracy via the advance compensation of tracking error,and further reduces the contour error of large curvature positions.Dual-axis servo motion control system and CNC lathe motion control system are taken as the research objects respectively,and biaxial contour control experiments are carried out with NURBS curves.The experimental results indicate that the off-line learning based on the transfer function model of the X-Y servo motion control experimental platform is of good reliability,and that the experiments using the optimal gain parameters as the conditions can significantly improve the contour control accuracy in biaxial contour tracking tasks.Furthermore,the proposed integrated control strategy not only satisfies the requirements for the chord error in the machining process,ensures the continuity of acceleration and jerk,but also further reduces the tracking error and the contour error in large curvature positions.That is to say,the proposed comprehensive control strategy of cross-coupling contour error compensation for two-axis CNC machining systems can be applied to the high-speed and high-precision machining of complex curved parts,which has important engineering significance.