Contouring Control And Trajectory Planning For Robotic Systems

With the extensive application of robotic systems in the processing and manufacturing,there are increasingly high requirements for motion accuracy.The conventional controller of the robotic system usually aims to reduce the tracking error,which is the distance between the actual position and the desired position.However,in the process of contour machining,the contouring error characterizes the product quality more effectively.As a significant part of the motion control,the contouring accuracy can be improved by the contouring control technique.Thus,it's proposed that the contouring control technique should be expanded to apply in the robotic systems.The cross-coupled control is an effective contouring control method,but it's hard to utilize that method directly because of the nonlinear kinematics of robotic systems.So the position loop-based cross-coupled control is proposed to realize the contouring control of the robotic systems.In this method,the contouring error for free-form path is estimated as the minimum distance from the actual position to the approximated circle,and the position-reference input are modified to implement the contouring error compensation.In addition,to strengthen the coordination of the two actual axes further,we proposed to add the synchronization controller to the position loop-based cross-coupled control structure.For the planar motion,the synchronization error of workspace is defined as the difference value between the tracking error of different coordinate axis.Through the synchronization controller,the synchronization error is compensated in the joint space instead of the workspace,which is designed to improve the motion accuracy more directly.A number of experiments are carried out for several kinds of contour at different speed.The results show the proposed method really leads to improved contouring accuracy.In the practical application,there is the contouring error restriction for the whole machining process,which means the contouring error would vary below the given value all the time.To improve the machining efficiency,it's essential to adjust the feedrate by appropriate velocity planning method.Therefore,a novel velocity planning method is proposed,which combines the current value with the predicted value of contouring error.Also,the predicted value of contouring error can be obtained by the contouring error information of last machining cycle.By the adjustment of the threshold for acceleration and deceleration,the contouring accuracy can be improved further under the restriction of contouring error.The effectiveness of the proposed velocity planning method also is verified by some experiments.