Trajectory Generation And Performance Optimization Of Multi-Axis Contouring Motion System

Multi-axis contouring motion system is an important branch of motion control technology. Its main use is to accomplish high performance contouring machining. High performance contouring machining is a core technology in modern manufacturing. It is widely applied in many industries and has important strategic sense and high research value. To improve machining quality and efficiency of multi-axis contouring motion system, it is necessary to adopt advanced control strategy to optimize the plan and control of motion trajectory. In this dissertation, a typical contouring motion system - open three-axis CNC milling and engraving control system was developed. The topic of the dissertation is trajectory generation and performance optimization of multi-axis contouring motion system. To solve some key problems in high performance contouring machining and multi-axis contouring control, research was developed systematically and deeply from four aspects, including trajectory approximation, real-time interpolation, trajectory optimization and coordinated control. The main work is as following.Firstly, to approximate smooth curves, design and comparative research were developed with straight line/circular arc approximation methods. Taking a quarter of a planar ellipse for an example, straight line segments and circular arc segments were adopted respectively to approximate the planar ellipse according to the desired accuracy. In the process of straight line approximation, constant error approach and constant parameter increment approach, which are both based on the model of curvature circle, were applied. In the process of circular arc approximation, optimal circular arc approach, based on the principal of minimax approximation, and bi-arc approach with a polygon approximation were applied. Through the approximation manner and error control model of different approximation methods, detailed design was developed from two aspects including the number of approximation segments and the continuity of the trajectory. According to the result of simulation experiment, the effect of different parameters on approximation error and the effect of different methods on motion control accuracy were analyzed.Secondly, to solve the feedrate fluctuation in machining three-dimension sculptured surface, a new hybrid space linear interpolation method, based on the principles of Time-division and Digital Differential Analyzer (DDA), and implementation method of continuous feed motion were proposed. This interpolation method applied the principle of time-division to the procedure of accumulation in DDA by means of controlling interpolation cycle time. Therefore, it could eliminate the odd distance and maintains smooth feedrate. To accomplish continuous feed motion in company with this interpolation method, an off-line segmented speed planning method was proposed to preprocess NC codes composed of a large number of short straight line segments in advance and determine a realizable end speed for each straight line segment by means of trapezoid speed curves. Therefore, machining efficiency was improved and jerk at corners was avoided. Then, a non-symmetrical trapezoid acceleration/deceleration control method in the process of interpolation was proposed, which adopted a point-by-point arbitration manner to control acceleration and deceleration according to non-symmetrical trapezoid speed curves. The validity and superiority of the proposed methods were studied through simulation and actual machining experiment implemented on a three-axis CNC milling and engraving machine.Thirdly, to solve the disadvantages such as surface dents, poor roughness in machining three-dimension sculptured surface, a tool path optimization method based on mesh uniformization was proposed. Based on the original tool path generated by Cimatron software, this method was implemented with two steps including transverse fairing and longitudinal smoothing. In the process of transverse fairing, irregular points were modified by means of constructing the dominant contour to reduce the variety of the inclination degree between the succeeding paths. Then, in the process of longitudinal smoothing, smooth continuous line segments along the same path were identified and fitted by means of using piecewise 3-order spline curve to improve the smoothness of curve along the same path. The validity and superiority of the proposed method were studied through simulation and actual machining experiment implemented on a three-axis CNC milling and engraving machine.Fourthly, to solve the difficult computation and control of contouring error in multi-axis contouring motion, an optimal contouring control method based on task coordinate frame was proposed. By establishing a movable Frenet coordinate frame on a desired trajectory as the task coordinate frame, contouring error was approximated by the normal component of tracking error in the task coordinate frame. Then by transforming the system dynamics from the world coordinate frame to the task coordinate frame, the optimal Linear Quadratic Regulator (LQR) approach was used to design the optimal contouring controller. To improve the contouring accuracy, the weight of normal error component was enhanced. The effect of the proposed method was validated through simulation experiment.