Research On Feedrate Scheduling Method Based On Contour Error Control

Feedrate scheduling is the key step of CNC, which has a significant influence on the machining efficiency and quality. As an important performance index of high speed and high precision CNC system, contour error and its control method is of great significance too. Therefore scheduling feedrate based on contour error control is advantageous to achieve high speed and high precision machining.First, an undetermined coefficient system identification method is proposed to identify the transfer function model of servo control system. In this method, the transfer function of servo system is expressed by a standard form. Then least square method is used to deal with the frequency sweep test data to obtain the value of angular frequency sequence and the corresponding amplitude frequency sequence. Finally generalized inverse matrix is solved to obtain the undetermined parameters. This method is proved to have high accuracy by comparing with the frequency domain identification method.Then, a contour error model based on the dynamic characteristic of servo system is established. In this method, contour error is defined as the distance from the actual processing point to the close circle of desired machining point. The contour error model, which combined the transfer function model of servo system with the approximate estimation model of contour error, is not only considering the influence of dynamic characteristics of the drive system on the accuracy of contour machining, but also deriving the analytical relation between feedrate and contour error.After that, a feedrate scheduling method based on contour error control is proposed. First, describe the tool path by NURBS curve, from which geometric information such as curvature and vector can be easy obtained. Then, derive the relation formulas between feedrate and chord error constraint, velocity constraint of machine's axes and acceleration constraint of machine's axes by combining the geometric information and the model of each constraint. Finally, the linear programming algorithm is used to solve the feedrate to meet the constraint conditions.Finally, the system identification experiment is carried out using three axis experiment platform, and the system transfer function is obtained by the method of undetermined coefficient system identification. In order to further verify the effectiveness of the feedrate customization algorithm in contour error control, the typical "Dove" and "crown" shape of three axis machining path are choose to schedule the feedrate and carried out experiment. The experiment results show that proposed method has a significant influence on reducing contour error and satisfying all the given constraints.