【作者】 刘宜；

【导师】 丛爽；

【作者基本信息】 中国科学技术大学 ， 系统工程， 2008， 博士

【摘要】 多轴轮廓运动系统是运动控制技术的一个重要分支,其主要用途是实现高性能轮廓加工,高性能轮廓加工是现代制造业中的一项核心技术,广泛地应用于众多工业领域,具有重要的战略意义与研究价值。为改善多轴轮廓运动系统的加工质量与加工效率,需要采用先进的控制策略对运动轨迹的规划与控制进行性能优化。本论文结合一个典型的多轴轮廓运动系统—基于PC+运动控制卡的开放式三轴数控雕铣机控制系统的开发,围绕“多轴轮廓运动系统的轨迹生成与性能优化”这个主题,针对高性能轮廓加工与多轴轮廓控制中存在的一些关键问题,着重从轨迹逼近、实时插补、轨迹优化与协调控制等方面进行了系统深入的研究,主要研究内容可归纳如下:(1)针对光滑曲线的逼近问题,采用直线/圆弧逼近方法进行了设计与对比的研究。以一个平面四分之一椭圆弧为例,根据期望的精度要求分别采用直线和圆弧来逼近该椭圆弧,其中直线逼近采用基于曲率圆模型的等误差法和等参数增量法;圆弧逼近采用基于minimax逼近的最优圆弧逼近和以多边形为中介的双圆弧逼近。通过不同逼近方法的逼近方式与误差控制模型,分别从逼近点数的多少及轨迹的连续性两方面来进行具体设计,并根据仿真实验结果来分析不同参数对逼近误差的影响,以及不同逼近方法对运动控制精度的影响。(2)针对空间曲面加工中存在的进给速度波动问题,提出了基于时间分割法与数字积分法混合实现的空间直线插补方法及其连续进给运动的实现方法。该插补方法是采用时间分割法中的时间分割原理,对数字积分法中的累加溢出过程按照进给速度的要求采用可控的插补周期进行时间分割,以消除零头距离并实现平滑的进给速度。为配合上述插补方法实现连续进给运动,提出了一种离线分段速度规划方法对由大量微小直线段组成的加工代码进行前瞻预处理,确定每段直线段代码在梯形加减速方法下可以实现的终点速度,以提高加工效率,避免在转角处的冲击。然后,在插补过程中,提出了一种非对称梯形加减速控制方法,采用逐点判断的方式按照非对称梯形速度曲线进行加减速控制。最后,通过仿真实验与实际加工实验对该方法的有效性与优越性进行了研究,其中实际加工实验在三轴数控雕铣机上进行。(3)针对空间曲面加工中可能出现表面凹坑、光洁度较差等问题,提出了一种基于网格均匀化的刀位轨迹优化方法。该方法在Cimatron软件生成的加工刀位轨迹基础上,分横向光顺与纵向平滑两步进行,首先进行横向光顺,通过建立主导轮廓线并对不规则的点进行修正,以降低相邻刀路倾斜度的变化程度,然后进行纵向平滑,对同一条刀路上的刀位点进行光滑连续直线段的识别,再采用分段三次样条进行拟合,以提高同一刀路上的曲线光滑程度。最后,通过仿真实验与实际加工实验对该方法的有效性与优越性进行了研究,其中实际加工实验在三轴数控雕铣机上进行。(4)针对在多轴轮廓运动中轮廓误差难以直接计算与控制的问题,提出了一种基于工作坐标系的最优轮廓控制方法。该方法通过在期望轨迹上建立活动的Frenet坐标系作为工作坐标系,用跟踪误差在工作坐标系中的法向分量来近似轮廓误差,将系统动力学方程由全局坐标系变换到工作坐标系,采用最优线性二次型方法来设计最优轮廓控制器,通过提高法向误差分量的权值来改善轮廓精度。最后,通过仿真实验对该方法的效果进行了验证。更多还原

【Abstract】 Multi-axis contouring motion system is an important branch of motion control technology. Its main use is to accomplish high performance contouring machining. High performance contouring machining is a core technology in modern manufacturing. It is widely applied in many industries and has important strategic sense and high research value. To improve machining quality and efficiency of multi-axis contouring motion system, it is necessary to adopt advanced control strategy to optimize the plan and control of motion trajectory. In this dissertation, a typical contouring motion system - open three-axis CNC milling and engraving control system was developed. The topic of the dissertation is trajectory generation and performance optimization of multi-axis contouring motion system. To solve some key problems in high performance contouring machining and multi-axis contouring control, research was developed systematically and deeply from four aspects, including trajectory approximation, real-time interpolation, trajectory optimization and coordinated control. The main work is as following.Firstly, to approximate smooth curves, design and comparative research were developed with straight line/circular arc approximation methods. Taking a quarter of a planar ellipse for an example, straight line segments and circular arc segments were adopted respectively to approximate the planar ellipse according to the desired accuracy. In the process of straight line approximation, constant error approach and constant parameter increment approach, which are both based on the model of curvature circle, were applied. In the process of circular arc approximation, optimal circular arc approach, based on the principal of minimax approximation, and bi-arc approach with a polygon approximation were applied. Through the approximation manner and error control model of different approximation methods, detailed design was developed from two aspects including the number of approximation segments and the continuity of the trajectory. According to the result of simulation experiment, the effect of different parameters on approximation error and the effect of different methods on motion control accuracy were analyzed.Secondly, to solve the feedrate fluctuation in machining three-dimension sculptured surface, a new hybrid space linear interpolation method, based on the principles of Time-division and Digital Differential Analyzer (DDA), and implementation method of continuous feed motion were proposed. This interpolation method applied the principle of time-division to the procedure of accumulation in DDA by means of controlling interpolation cycle time. Therefore, it could eliminate the odd distance and maintains smooth feedrate. To accomplish continuous feed motion in company with this interpolation method, an off-line segmented speed planning method was proposed to preprocess NC codes composed of a large number of short straight line segments in advance and determine a realizable end speed for each straight line segment by means of trapezoid speed curves. Therefore, machining efficiency was improved and jerk at corners was avoided. Then, a non-symmetrical trapezoid acceleration/deceleration control method in the process of interpolation was proposed, which adopted a point-by-point arbitration manner to control acceleration and deceleration according to non-symmetrical trapezoid speed curves. The validity and superiority of the proposed methods were studied through simulation and actual machining experiment implemented on a three-axis CNC milling and engraving machine.Thirdly, to solve the disadvantages such as surface dents, poor roughness in machining three-dimension sculptured surface, a tool path optimization method based on mesh uniformization was proposed. Based on the original tool path generated by Cimatron software, this method was implemented with two steps including transverse fairing and longitudinal smoothing. In the process of transverse fairing, irregular points were modified by means of constructing the dominant contour to reduce the variety of the inclination degree between the succeeding paths. Then, in the process of longitudinal smoothing, smooth continuous line segments along the same path were identified and fitted by means of using piecewise 3-order spline curve to improve the smoothness of curve along the same path. The validity and superiority of the proposed method were studied through simulation and actual machining experiment implemented on a three-axis CNC milling and engraving machine.Fourthly, to solve the difficult computation and control of contouring error in multi-axis contouring motion, an optimal contouring control method based on task coordinate frame was proposed. By establishing a movable Frenet coordinate frame on a desired trajectory as the task coordinate frame, contouring error was approximated by the normal component of tracking error in the task coordinate frame. Then by transforming the system dynamics from the world coordinate frame to the task coordinate frame, the optimal Linear Quadratic Regulator (LQR) approach was used to design the optimal contouring controller. To improve the contouring accuracy, the weight of normal error component was enhanced. The effect of the proposed method was validated through simulation experiment.更多还原