The Research Of NURBS Direct Interpolation Technology For Embedded CNC System

With the increase of requirements for high speed and high precision, NURBS direct interpolationfunction will play a more and more important role in the interpolation function of the CNC system,and have profound implications for high speed and high precision CNC system. A small number offoreign high-end CNC systems such as FANUC, SIEMENS, etc., have owned NURBS directinterpolation function. Part of the domestic CNC system based on PC architecture also increases thisfunction, but the embedded CNC system does not yet have the function. There are many publishedstudies about NURBS direct interpolation technology, but most of these studies only stay in thesimulation stage and are implemented for the PC hardware platform. Therefore, research of theNURBS direct interpolation algorithms applied to the actual process of NURBS for embedded CNCsystem has a very important significance.Firstly, based on the analysis of the NURBS parametric direct interpolation algorithmscomprehensively, a predictor-corrector NURBS parametric direct interpolation algorithm based onpredicted error compensation is presented. The simulation experiments of five-pointed star NURBScurve demonstrate that the proposed algorithm has advanced characterization in predicted accuracyand convergence of correction aspects, and is more suitable for high speed and high precisionembedded CNC system platform. Through analyzing the flexibility and computational complexity ofthe acceleration and deceleration algorithms, the traditional S-shape acceleration and decelerationalgorithm is improved. Retaining the good flexibility of acceleration and deceleration, the accelerationand deceleration process is divided into two stages of acceleration and deceleration. This effectivelysimplifies classifications of the models for acceleration and deceleration, and reduces the amount ofcomputation for acceleration and deceleration planning. Secondly, piecewise NURBS parametricdirect interpolation algorithm based on improved S-shape acceleration and deceleration is presented.The algorithm is divided into two stages of pre-interpolation and real-time interpolation. In thepre-interpolation stage, the NURBS curve is divided into the acceleration or deceleration curvesegment, and then the speed is planned for curve segments respectively according to the improvedS-shape acceleration and deceleration to obtain a feed speed curve which satisfies chord height error,acceleration and jerk constraints. In real-time interpolation stage, the ideal feed step in currentinterpolation cycle is gained according the planned speed curve, and then the proposed parametricinterpolation algorithm is used to obtain the corresponding parameter value achieving the task ofNURBS direct interpolation. Finally, the interpolation algorithm proposed in this paper is transplanted to embedded CNCsystem interpolator, and the performance of interpolation is tested. The proposed parametricinterpolation algorithm has significant improvements to control the feed rate fluctuations andtime-consuming aspects of calculating. And the integration test of piecewise NURBS parametricdirect interpolation algorithm based on improved S-shape acceleration and deceleration is carried outin this paper. The results show that the algorithm can achieve the required technical indicators intime-consuming of calculating, precision control in the interpolation, the feed speed control andacceleration and deceleration control. Then the embedded CNC system is used to control five-axisdouble turntable CNC milling machine, and in the three-dimensional space coordinate systemfive-pointed star NURBS curves on the semicircular work blank are processed practically. The resultsverify the feasibility and validity of this algorithm.