Research Of Trajectory Optimization Modeling And Interpolation Techniques With Look-ahead For High Speed Machining Of Micro Line Segments

High speed machining technology, which is widely used in the aerospace, mold and automotive industry, is an important means to improve the machining quality and efficiency. High speed machining technology is identified as one important direction of the manufacturing technology development of the 21 st century by the International Academy for Production Engineering(CIRP). Micro line segment is still the widespread tool path representation form in machining impeller, blade, mold and other complex curve surface parts. High speed machining of micro line segments becomes one of the core technology of high speed machining complex curved surface. The inherent shortcoming of continuous short segments is first-order discontinuity at the corner, which leads to the frequent acceleration and deceleration, and is an obstacle to achieve smooth and high-speed machining.Speed smoothing at the segment junctions and feedrate scheduling for crossing different segments are the biggest problems of continuous micro line interpolation. These two techniques are still the hot spots used to improve the machining efficiency and quality by advanced numerical control system manufacturers. This paper proposes a G~2 continuous B-spline global smoothing algorithm and interpolation technology, and an interpolation method with corner blending algorithm and speed optimization algorithm based on corner error distribution. An adaptive feedrate scheduling method based on the real-time path smoothing, corner speed optimization, flexible acceleration/deceleration and look-ahead function is proposed. The above methods are integrated into an open CNC system to achieve efficient machining for micro line segments, to confirm the validity and practicability of the presented method. The main research contents and innovative results are as follows:(1) A G~2 continuous cubic B-spline global fitting method is proposed to smooth the linear segments tool path, and the method mixed the linear interpolation with spline interpolation is used to interpolate the new tool path. A cubic B-spline is used to compress the corresponding data of consecutive linear segments and achieve global G~2 continuity. The S-curve acceleration and deceleration strategy based on the reduction of characteristic equation is developed to implement the adaptive speed planning and mixed linear/spline interpolation for the newly generated cubic B-spline path. The S-curve acceleration and deceleration strategy is utilized to schedule the velocity of the generated tool path with the constraint of acceleration and deceleration characteristics of the machine tool as well as the feedrate and accelerate constraint of the geometric characteristics of the tool path trajectory. This method has great advantages on dealing with tinny and numerous intensive linear tool path and smooth path with small corners, which can realize the data compression, significantly improve the feedrate, reduce the fluctuation of the acceleration, and shorten the machining time. Finally, the experiment of machining a scoop mold is conducted to valid the developed algorithm whose results indicate the surface quality has improved a lot and the machining efficiency is improved 4 times compared with the process without using the developed algorithm.(2)The corner error distribution model for 3-axis linear tool path is established to get the optimal corner transition speed. The setting accuracy of CNC is assigned to smoothing approximation error and interpolation chord error. And on this basis, the linear tool paths at corners are smoothed with the Bézier transition curve pair(BTP). Then, error distribution model is used to optimize the corner speed while considering the machine tool’s axis servo capacity constraints. Look-ahead function is utilized to achieve real-time interpolation of micro line segments tool path. In the experiment of machining a mask, compared to the traditional methods, the proposed method reduces the machining time by 25%.(3) The corner error distribution model for five-axis linear tool path is established, and a real-time G~2 continuous fitting method is proposed to smooth the linear tool path in WCS. The system setting accuracy is assigned to the smoothing approximation error of tool tip point and tool axis point and chord error. On this basis, by inserting two cubic Bézier curves at the line segment connecting, the linear tool path in workpiece coordinate system is fitted to the double G~2 continuous spline trajectory in real-time. The algorithm takes the conditions of approximation error constraint, the parameterized synchronization constraint and continuous curvature constraint into consideration, hence ensures that the new double spline trajectorymeets the accuracy and the tangent/curvature continuity. At the same time, to obtain the optimal transition speed while satisfying the interpolation contour error allowance, the curve transition error and interpolation error are taken into consideration. The adaptive speed scheduling is realized with the constraints of maximum tangential acceleration/jerk and machine tool servo capacity. Simulation and experiment are performed to verify the effectiveness of the proposed method in five axis tool path smoothing, rotation speed smoothing and angle tracking accuracy controlling.(4) The servo feedrate and acceleration envelopes of machine tool axes are obtained by exciting the servo motor with positive and negative direction method. And the optimal maximum velocity and maximum acceleration are obtained. Then the optimal constraint values are set to NC system for interpolation.(5) To meet the demand for complex parts machining, an open CNC system is built based on multi-thread concurrent execution management, and the algorithm in this paper was integrated into the system. Cubic B-spline global smoothing algorithm and Bezier transition algorithm based on the corner error distribution model are integrated into Guangzhou CNC GSK27 digital bus high-grade CNC system. Five-axis smoothing algorithm in WCS based on corner error model is integrated into Guangzhou CNC GSK25 i five-axis linkage machining center CNC system.