| The high-end CNC(Computerized Numerical Control)system is an effective tool for manufacturing aeronautics and astronautics core components,such as impeller and blade.As the control core of CNC system,the motion trajectory control function directly affects the machining efficiency and quality of machine tools,and is an important index to evaluate the performance of CNC system.However,for the import of high-end CNC systems,western developed countries have strict restrictions on China.Therefore,in order to achieve the goal of the "Made in China 2025" and form the core competitiveness of the "China Intelligent Manufacturing",the "High-end CNC machine tools and basic manufacturing equipment" is listed as one of 16 major projects in the “National Mid-and Long-term Science and Technology Development Plan”,and in the "Several Opinions of the State Council on Accelerating the Revitalization of Equipment Manufacturing Industry",it is pointed out in particular that "to develop large-scale,high-accuracy and high-speed CNC equipments,CNC systems and functional components for changing the current situation that large-scale and high-accuracy CNC machine tools mostly rely on imports and meeting the needs of the development of machinery,aerospace and other industries".Based on the current research and development trend of domestic and international high-end CNC systems,this dissertation focuses on two key technologies of the motion trajectory control for the high-speed and high-accuracy machining: trajectory smoothing method and velocity planning method.Relying on national projects undertaken by the Shenyang Institute of Computing Technology,Chinese Academy of Sciences,the following research work has been carried out:1.Trajectory smoothing algorithm for typical patterns.To implement the high-speed and high-accuracy machining of the circle arc and elliptic arc in the micro-line form,the smooth compression interpolation algorithm for typical patterns is proposed.Based on the double-chord error limit,the machining path could be divided into two region types.For those regions comprised of discontinuous small-line segments,the linear interpolation was performed between the adjacent command points.For those continuous regions,firstly,the shape-defining point was selected by the curvature extremum and bend direction of the machining path;Secondly,these points were fitted into quadratic rational Bézier curves to compress segments and smooth contours;Thirdly,the circle arc and elliptic arc were recognized by the curve feature,then transformed to geometric form and merged with adjacent one;Lastly,the circle arc or elliptic arc interpolation was performed on the curve in the geometric form.The results of experiments reveal that the proposed algorithm can reduce the velocity fluctuation and implement the high-quality CNC processing of the circle arc and elliptic arc.2.G2 continuous Bézier trajectory smoothing algorithm based on specified tolerance band.The traditional global trajectory smoothing algorithm can only guarantee that the shortest distance from the discrete command point to the spline curve satisfies the specified contour error,but it can not guarantee that the largest distance from the small line segment composed of adjacent discrete command points to the spline curve does not exceed the specified contour error.In addition,because the traditional global trajectory smoothing algorithm needs to iterate or solve complex non-linear equations,the computational complexity will grow with the increase of the number of command points,and the real-time and locality will decrease significantly.Therefore,the G2 continuous Bézier trajectory smoothing algorithm based on specified tolerance band is proposed.According to the double-chord error limit,regions comprised of continuous small-line segments were recognized.For these continuous regions,firstly,small-line segments were locally interpolated into G1 quadratic rational Bézier curves.Secondly,the G2 continuous trajectories were obtained by scaling the weights of every two adjacent curves and adjusting the tangent direction at the joint point.Lastly,undesired curves were detected and corresponding adjustments were conducted in the specified tolerance band.The results of experiments reveal that the algorithm is real-time well and can realize the online trajectory smoothing.The synthetic trajectories satisfy the G2 continuity and contour error limit strictly,which can improve the machining efficiency greatly.3.Axial jerk-smooth corner transition smoothing algorithm.The traditional corner transition smoothing algorithm needs two computational steps.In the first step,the transition geometry is constructed by inserting parametric curves at the corner.In the second step,according to synthesized trajectories,the motion profiles are scheduled.This method is,therefore,not very efficient.To reduce the time spent by the smoothing process and velocity planning and improve the smoothness and accuracy at the junction,a real-time and look-ahead interpolation algorithm with an axial jerk-smooth transition scheme is proposed.In one step,the algorithm finished the transition scheme construction and velocity planning by using the Trigonometric Velocity Planning Method(TVPM).The TVPM could utilize the maximal acceleration and/or jerk capabilities of drive axis to achieve smooth axial kinematic profiles while satisfying the user-specified chord error.In addition,a real-time look-ahead method was developed to plan the global feedrate profile and adjust the transition schemes without intersections constantly.The simulation results demonstrate that the proposed algorithm could realize the smooth axial velocity,acceleration and jerk control and improve the smoothing process and velocity planning efficiency and the machining accuracy.4.Real-time flexible acceleration and deceleration algorithm based on adaptive look-ahead and predictor-corrector method.Traditional velocity planning algorithms,such as S-type velocity planning algorithm and trigonometric velocity planning algorithm,can achieve the smooth speed,acceleration and jerk control,but these algorithms are complex and require a large number of numerical calculations.So,most of them are off-line.In addition,during the machining,the CNC system with these algorithms can not respond to machining parameters changes.Therefore,a novel real-time flexible acceleration and deceleration algorithm is proposed,which contains three parts: adaptive look-ahead process,predictor-corrector real-time velocity planning and dynamic override.The acceleration-deceleration feasibility and real-time forward-looking function were guaranteed in the look-ahead process with acceleration-deceleration feasibility conditions.The velocity planning module combined with the predictor-corrector method calculated the next feedrate and ensured the smooth velocity and acceleration curves,which reduced vibrations in the high-speed CNC machining.In order to respond to the changes of machining parameters immediately,adjusting look-ahead results and current velocity planning results,which satisfied the dynamic performance of machine tools,was carried out in the dynamic override module.The results of experiments reveal that the proposed algorithm could realize the real-time flexible acceleration/deceleration control,support the dynamic override and implement the high-quality processing. |
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