Design Of A Practical Real-Time Mixed-Path Interpolator For 3-Axis CNC Machining

Non-Uniform Rational B-Spline (NURBS) curve has long been favored in computer-aided design (CAD) systems for geometric representation of many shapes. But if the NURBS curve to be machined is very short or discontinuous, or the tool-paths contain both NURBS curves and small continuous line segments, the only chioce is approximated these kinds of tool-paths by many short linear and circular segments and applied with traditional linear and circular interpolator. Such traditional approach may result in several problems such as large data size, velocity and acceleration discontinuity, shocks or variations in mechanical systems and low machining efficiency. So it is neccesery to develop an effcient and practical real-time mixed-path interpolator, which can dispose the mixed-path composed of all kinds of NURBS curve and the continuous linear segements.Two processing modules, velocity planning module, and curve interpolation module, are used simultaneously to realize real-time algorithm in this paper. In one module, a long NURBS curve will be split into small NURBS curves according to the minimum splitting length first, and then the velocity planning of one small curve will be done. Another module will do interpolation according to the velocity planning results of every small curve at the same time, the whole procedure won't finish until the end of the tool-path. This algorithm can be extended to discontinuous NURBS curve by splitting curve at cusp. If one NURBS curve or the curve between to adjacent cusps is too short, it will be written as small continuous line segments within the limitation of chord-error, and a look-ahead velocity planning method will be used for small continuous line segments, so this algorithm is extended to the mixed-path naturally.All kinds of tool-path are given in this thesis, and the real-time mixed-path interpolator has a higher performance than linear interpolator from both the simulation and experiment results. And the compute time is compared with machining time, which is in the tolerance range for the real-time machining.