Research On Path Optimization And Error Control Of Free-Form Curves In CNC Systems

Along with the growing of industrial manufacturing technology,computer-ized numerical control(CNC)systems are commonly used in various automation fields for their high level of automation,good reliability and other advantages.Also,their performance is increasingly developing towards high precision,high speed and high efficiency.The machining precision of CNC systems on free-form curve path is a key indicator to measure their performance,among which chord errors and contour errors of the toolpath are important research topics to improve the machining precision.Finding an error control method,which can effectively control chord errors to meet the system limit,i.e.realizing curve path optimization,and achieve great contour error control effect,is of great significance for CNC systems to realize high precision machining.Among all curve models,the non-uniform rational B-spline(NURBS)curve has a powerful ability to describe free-form curves.Therefore,this thesis considers NURBS curve as the object,and conducts a research on path optimization and error control during CNC machining on the basis of NURBS curve interpolation technique.The main work of this thesis is as follows:(1)In order to solve the problem that the precision of chord error evaluation through approximating the desired curve path by osculating circle will deteriorates when the desired curve path changes sharply,a chord error iterative algorithm is proposed based on the both-direction Hausdorff distance definition of chord error.This iterative algorithm can upgrade the evaluation precision of system chord error limit ?max in scenarios with arbitrary curve curvature changes,thereby obtaining the maximum feed step length and the maximum allowed feedrate within the in-terpolation period.Based on this algorithm,feedrate constraints can be improved,and the effective limitation of chord errors can be realized in the subsequent ma-chining process to achieve curve path optimization.(2)In order to solve the problems of previous contour error control theories,that the contour error control performance can be affected by contour error estimation precision,and the effective control of contour errors cannot be achieved under multiple disturbance scenarios,an indirect compensation algorithm for con-tour error is proposed.In some of the feature zones on the curve path,the tracking error is reduced by the compensation of prescheduled toolpath length,and then the contour error within this zone can also be limited to a lower level due to the mag-nitude relationship of these two errors,i.e.realizing its indirect compensation.With the continuous reduction of tracking errors in multiple selected feature zones,the effective control of contour error of the toolpath will be realized.This algo-rithm can achieve great contour error control effect under multiple disturbance scenarios without relying on contour error estimation.In addition,a three-stage jerk rescheduling model is designed,which is used to refine the prescheduled feedrate profile in each selected feature zone,to realize the compensation of pre-scheduled toolpath length in this algorithm.(3)MATLAB codes of the curve path optimization algorithm based on chord error iteration and the toolpath contour error control algorithm based on tracking error compensation are implemented.Moreover,the software and hardware of CNC host computer system and central control system are designed based on the embedded hardware control scheme of dual STM32 and the software compilation on the ?C/OS-? operating system.Then,with the help of a testing three-axis CNC machine,a three-axis CNC experiment platform is constructed based on the de-signed host computer system and central control system.Through simulation verifications of the proposed algorithms in MATLAB and CNC machining experiments on the three-axis CNC experiment platform,the results validate that,the proposed curve path optimization algorithm can effec-tively control chord errors of the interpolation trajectory to meet the system chord error limit ?max during the curve path implementation,and achieve the purpose of curve path optimization;the proposed contour error control algorithm can improve the contour precision of the toolpath under multiple disturbance scenarios,and achieve great contour error control effect.Moreover,both two algorithms are fea-sible and practical in actual CNC machining process.