Research On NURBS Interpolation And Linear Motor Robust Control Technology For High Speed CNC Machining

Computer numerical control (CNC) is a key technology in modern machining industry. It integrates the technology of micro-electronics, computer, signal processing, automatic measurement, automatic control and etc. With the advantages of high precision and high efficiency, CNC machining plays an important role for machining industry. High speed CNC machining requests for excellent data communication mechanism, advanced interpolation and servo control strategy, fast decoding and cutter offset algorithm, and etc. Among these functions, interpolation and servo control are two core modules of CNC machining, which have great impacts on machining precision and efficiency. Research on interpolation and servo control technology for high speed CNC machining is very important to enhance the performance of high speed machining. In this thesis, high speed CNC machining oriented NURBS interpolation technology and linear motor robust control technology are investigated, which include:1 A smooth adaptive NURBS (Non-Uniform Rational B-Spline) interpolation algorithm with limited acceleration and jerk constraints is proposed, considering the relationship between feedrate and interpolation error, and flexible acceleration/deceleartion mechanism. The 2nd order Taylor expansion is employed for next point parametric iteration. The feedrate is adaptively changed according to the demand of interpolation precision. In the adaptive area, an acceleration/deceleration and jerk limited S curve is used to replan the feedrate, to get smooth feedrate profile. With this interpolator, steady and fast feedrate, confined interpolation error, and a smooth and shock-free machining process is obtained, which enhances the machining efficiency and quality.2. Feedrate and feed acceleration are important machining parameters. If not well designed, it will either reduce machining efficiency, or have bad impact on machine tools' life span. In this thesis, from machine tool's acc/dec capability and interpolation precision point of view, constrains of feed motion for high speed machining is investigated. The feed motion is projected into each feed axis, and based on its mechanical model(motor driving torque/force, Inertial force and cutting force), the constraint relationship between feedrate, feed acceleration, and feed drive capabilities and toolpath geometric characteristics is derived. With this constraint model, the acceleration and deceleration of each axis can be limited within its capability. 3. An adaptive interpolation algorithm with segmented constant feedrate satisfying both constraints of interpolation error and machine tool's acceleration/deceleration capabilities is proposed as an improvement against the traditional "worse condition machining" method. It identifies "worse condition points" along the path according to the curve information, machine tool's capability and machining requirement. Integrated with adaptive interpolation, segmented high speed machining is adopted. Different constant speed segments and speed transition segments are obtained according to the feed motion constraint model, and the corresponding feed information are stored in a penta-element lookup chain table. By referring to the lookup table, the interpolator can automatically determine the feed motion. Compared with "worst case machining", it can take more potentials of machine tools and reduce machining time, under the precondition of satisfying interpolation accuracy and machine tool's acc/dec capabilities, and thus, enhance the machining efficiency.4. As a typical high speed feed drive, linear motor has become a trend in high speed machine tools. Due to its direct drive characteristics, system parameters perturbation (load mass, vicious coefficient and etc) and load disturbance will have a direct impact on its servo performances. In this thesis, a novel controller "MMC(model match control) +H_∞+MFC(Model following control)", integrating 2 degree-of-freedom (2DOF) control philosophy and H_∞method is proposed. It designs controllers for input and output respectively, which ensures precise tracking capability, and strong robustness to uncertain disturbances.Based on the above research work, a 2-axis linkage prototype system, consisting an XY cross linear motor table and an industrial PC platform, is set up to test the algorithms investigated in the thesis. Interpolator experiments show that the interpolator algorithm proposed in this thesis ensures a high speed feedrate and meets the demand of interpolation precision. What's more, the whole running process is smooth and shock free with high contour precision. Besides, the step response experiment of the proposed PMLSM controller shows strong robustness to system parameter changes, verifies its feasibility and good performances.