Analysis, Measurement and Improvement of the Contouring Error of CNC Machine Tools

• Main Authors: Ten Wang cheng, 程添王
• Other Authors: Jeng Shyong Chen
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/06858494713686568530

碩士 === 國立中正大學 === 機械系 === 87 === Accuracy of a produced workpiece is determined by two factors: the static geometric errors and dynamic contouring error of a machine tool. There are two ways to improve the machining accuracy. One is to build a highly accurate machine through design and manufacturing. The other is to identify the machine error sources, and then compensate for these errors using software method. In this thesis, we will adopt the second method to improve the machine accuracy. We will use measurement devices to identify the contouring error sources of a CNC machine tool equipped with a FANUC O-MF controller. We then adjust the servo parameters of the FANUC CNC controller to eliminate the contouring errors. In this paper, we will use measurement device called KGM 101 to calibrate the dynamic contouring errors of the CNC machine tool. Using the KGM 101, we are able to realize the dynamic characteristics of the feed-drive loop when the servo parameters of the feed-drive loop are modified. In order to get a deep understanding of the contouring error sources and their error patterns, we derive the mathematical model of servo feed-drive loop, study the characteristics by computer simulation of a circular motion. Through the systematic approach and computer simulation, we are able to realize the contouring error sources, and their physical meanings. Finally, we then decide proper servo parameters of the FANUC CNC controller, which will give a better match to the mechanical characteristics of the real machine axis, in order to reduce the contouring errors. First, we study the servo error characteristics under the feed-forward, and acceleration/deceleration control. We found that the servo errors were at minimum when the feed-forward control coefficient was set to 0.5. To study friction force effect, we formulated and incorporated a classical friction model into our mathematical model. Through computer simulation, we proposed a modification on the FANUC backlash acceleration compensation algorithm. We also found that the spike error that occurred at a circular test could be reduced if low position gain and feed-rate were used. We also found that spike error could not be totally eliminated by tuning the FANUC servo parameters. Through the experiment, we also found than the static and kinetic friction forces of the Leadwell MCV-OP CNC machining center were 8kgf-cm and 7kgf-cm respectively. In conclusion, in this thesis, we focus on the error source analysis especially on the servo error and friction force. We also do extensive computer simulation to understand the effect of the servo parameters on the contouring errors. Through computer simulation, we are also able to propose proper servo parameters that will give the machine tool better dynamic contouring accuracy. We also propose a modification on the FANUC backlash acceleration compensation.