Applications of integrated motion controllers for precise CNC machines

The error resources of precise motion control systems are basically categorized into linear and nonlinear effects. To realize the precise motion of industrial computer numerical control (CNC) machines, this paper presents an integrated motion control structure with modular algorithms, including both the linear control and the nonlinear compensation. In the linear control design, this study applies three algorithms: (1) feedforward control to address the tracking errors, (2) cross-coupled control to reduce the contouring errors, and (3) digital disturbance observer to lessen the effects of modeling errors and disturbances in real applications. The results indicate that the linear motion controller achieves greatly improved accuracy in both tracking and contouring by reducing the servo lags and mismatched dynamics of the different axes. However, the adverse effect due to friction still exists and cannot be eliminated by applying the linear motion controller only. This study further integrates the nonlinear compensator and develops friction estimation and compensation rules for CNC machines. The digital signal processors are suitable to implement all the developed linear and nonlinear algorithms, and the present controllers have been successfully applied to industrial CNC machines. Experimental results on a vertical machining center indicate that, under different feed rates, the CNC machine with the integrated motion controller significantly reduces the maximum contouring error by 135% on average.     

MODELING AND COMPENSATION TECHNIQUE FOR THE GEOMETRIC ERRORS OF FIVE－AXIS CNC MACHINE TOOLS

One of the important trends in precision machining is the development of real-time error compensation technique. The error compensation for multi-axis CNC machine tools is very difficult and attractive. The modeling for the geometric error of five-axis CNC machine tools based on multi-body systems is proposed. And the key technique of the compensation-identifying geometric error parameters-is developed. The simulation of cutting workpiece to verify the modeling based on the multi-body systems is also considered.     

[可靠性建模及试验技术]基于元动作模块的多轴数控机床精度分析

为充分发挥多轴数控机床大数据的价值,降低运动精度的预测难度,提出了基于元动作模块的精度分析方法。采用多体模型描述机床运动系统的结构和运动关系,利用旋量理论和微分方法推导了用于运动精度评价的坐标误差模型;规划了多轴数控机床大数据驱动的精度分析的结构框架,并重点论述了以元动作模块为基本组成单元的分布式元动作数据库的构建方法,该方法充分发挥历史大数据和实时动态数据的价值,保证了机床运动系统的仿真和精度预测的稳定性与准确性。通过对五轴联动加工中心的刀具运动系统的实例分析,验证了精度分析方法的简便性和适用性。     

Identification of and compensation for backlash on the contouring accuracy of CNC machining centres

The paper describes a technique for the identification of and compensation for backlash on the contouring accuracy of computer numerically controlled (CNC) machining centres. A circular test that has been developed for rapidly checking the contouring accuracy of CNC machine tools is used to measure the contouring error due to backlash. Backlash error on the contouring profile is found as the direction of motion is reversed. A compensation strategy based on a simulated annealing (SA) optimisation algorithm is then developed to reduce the backlash error. Computational simulations and experimental tests have shown that the contouring error due to backlash can be greatly reduced by using this new approach.     

A Fuzzy Integral Sliding Mode Control Algorithm for High-Speed Laser Beam Focus Tracking Control

In this paper, a fuzzy integral SMC (sliding mode control) strategy is developed for the intelligent focus control module of an intelligent multi-axis laser machining CNC system and some tracking experiments with this control strategy are performed. The results of the intelligent positioning experiment with fuzzy integral SMC strategy show that the intelligent multi-axis laser CNC machining with this intelligent focus positioning controller has high robustness and global stability, high tracking speed and precision.     

Error modelling and measurement for the rotary table of five-axis machine tools

Rotary tables are widely used with multi-axis machine tools as a means for providing rotational motions for the cutting tools on the three-axis machine tools used for five-axis machining operations. In this paper, we present a comprehensive procedure for the calibration of the rotary table including: geometric error model; error compensation method for the CNC controller; error measurement method; and verification of the error model and compensation algorithm with experimental apparatus. The methods developed were verified by various experiments, showing the validity and effectiveness of the presented methods, indicating they can be used for multi-axis machine tools as a means of calibration and precision enhancement of the rotary table.     

[可靠性建模及试验技术]面向数控机床可靠性试验的多维力随动加载装置

面向五轴联动数控机床的可靠性试验提出多维力随动加载方法,对五轴联动进给主轴施加多维力载荷,形成复杂进给抗力,部分模拟主轴复杂切削力环境。基于6-PUS并联机构研制多维力随动加载装置,采用模糊PID控制器建立显式力控制系统,比例和积分增益可随加载误差自适应调节。在五轴联动数控机床上开展多维力随动加载实验,结果表明,加载装置能够跟随机床主轴的单轴、三轴联动和五轴联动进给运动,根据期望值对主轴施加三维力,加载误差小于3.2%,在机床执行多种加工轨迹时形成有效进给抗力,为后续引入动态加载模拟复杂切削力提供理论和装备支撑。研究成果可为数控机床可靠性试验提供低成本、可循环的加载方式,有利于测试的规模化和标准化发展,也可为精度保持性、超载试验、跑合试验等机床性能测试提供新的负载模拟思路。     

一种变速箱加工的数控系统设计与实验研究

针对传统的多轴数控机床较难满足变速箱加工过程中精度要求,提出了一种基于“ARM+DSP”硬件结构的参数可重构嵌入式数控系统的设计。所设计的数控系统通过参数化自动编程模块,实现了根据变速箱齿轮的参数、加工刀具参数和加工过程参数自动生成数控加工程序,还能够通过专家数据库优化数控程序中周期性参数的设置,同时在所设计的数控系统中应用了一套新型的软件电子变速器,以达到提高多轴同步运动控制精度的目的。实验结果表明,将所设计的嵌入式数控系统应用于六轴滚齿机,能够有效提高变速箱齿轮的加工精度。     

Development of 2-axis hybrid positioning system for precision contouring on micro-milling operation

A 2-axis hybrid positioning system was developed for precision contouring on micro-milling operation. The system was developed to overcome the micro-positioning limitations of conventional linear stage positioning system on machine tools. A 2-axis flexure hinge type piezoelectric stage was added on a standard milling machine to obtain better machining results. The control method used for the hybrid system was active error compensation type, where errors from linear stages are cancelled by the piezoelectric stage motion. Positioning experiments showed an improvement of machine accuracy which was confirmed by the machining results. A micro-pillar was fabricated for the validation of long-range and high-precision contouring capability. The system was successfully implemented on micro-milling machining to achieve high-precision machining results.     

An optimisation approach to the contour error control of CNC machine tools using genetic algorithms

A contour control strategy has been studied in this paper to improve the contour error of CNC machine tools. First, a single axis controller is analysed and then a velocity feedforward controller is added in the velocity loop. To further reduce the contour error, a cross-coupled controller is adopted and an algorithm for an on-line estimation of the contour error in arbitrary curved contouring is proposed. These controller parameters are all optimised by an efficient robust optimisation technique using genetic algorithms. Experimental results are provided to illustrate the proposed methods.     

Three-dimensional tool radius compensation for multi-axis peripheral milling

Few function about 3D tool radius compensation is applied to generating executable motion control commands in the existing computer numerical control (CNC) systems. Once the tool radius is changed, especially in the case of tool size changing with tool wear in machining, a new NC program has to be recreated. A generic 3D tool radius compensation method for multi-axis peripheral milling in CNC systems is presented. The offset path is calculated by offsetting the tool path along the direction of the offset vector with a given distance. The offset vector is perpendicular to both the tangent vector of the tool path and the orientation vector of the tool axis relative to the workpiece. The orientation vector equations of the tool axis relative to the workpiece are obtained through homogeneous coordinate transformation matrix and forward kinematics of generalized kinematics model of multi-axis machine tools. To avoid cutting into the corner formed by the two adjacent tool paths, the coordinates of offset path at the intersection point have been calculated according to the transition type that is determined by the angle between the two tool path tangent vectors at the corner. Through the verification by the solid cutting simulation software VERICUTwith different tool radiuses on a table-tilting type five-axis machine tool, and by the real machining experiment of machining a soup spoon on a five-axis machine tool with the developed CNC system, the effectiveness of the proposed 3D tool radius compensation method is confirmed. The proposed compensation method can be suitable for all kinds of threeto five-axis machine tools as a general form.     

可配置型五坐标B样条插补控制器的研制

针对采用五轴联动数控机床的线性插补功能进行数控加工存在的不足,提出了一种B样条插补控制策略用于五轴联动数控机床以实现复杂曲面零部件的高速高精数控加工。参考开放式、模块化体系结构控制器(OMAC)标准,开发了具有B样条插补功能的五轴联动数控机床运动控制器。该控制器将控制任务按照实时性要求进行划分。人机交互、代码解析及参数映射关系构造等过程离线完成,插补运算、离散逻辑控制及逆运动学变换等过程由实时线程执行,保证了数控系统的硬实时性。为简化NC程序的编制过程,控制器设计为接收工件坐标系下的加工信息。通过开发适应各种形式数控机床的逆运动学变换模块,并将机床参数设计为可用户定制,使得控制器具有良好的通用性。在控制器内部建立NC程序文件中位置曲线和方位曲线间的参数映射关系,使得机床平动轴与转动轴间的运动规划符合实际加工要求,并可保证加工精度。实际加工实验中,在采用B样条插补算法的NC程序量降低为线性插补NC程序量15%倍时,其插补误差为线性插补误差的45%,控制器插补精度为0.68,表明该B样条插补控制器可以满足五坐标数控加工的要求。     

Anisotropic force ellipsoid based multi-axis motion optimization of machine tools

The existing research of the motion optimization of multi-axis machine tools is mainly based on geometric and kinematic constraints, which aim at obtaining minimum-time trajectories and finding obstacle-free paths. In motion optimization, the stiffness characteristics of the whole machining system, including machine tool and cutter, are not considered. The paper presents a new method to establish a general stiffness model of multi-axis machining system. An analytical stiffness model is established by Jacobi and point transformation matrix method. Based on the stiffness model, feed-direction stiffness index is calculated by the intersection of force ellipsoid and the cutting feed direction at the cutter tip. The stiffness index can help analyze the stiffness performance of the whole machining system in the available workspace. Based on the analysis of the stiffness performance, multi-axis motion optimization along tool paths is accomplished by mixed programming using Matlab and Visual C++. The effectiveness of the motion optimization method is verified by the experimental research about the machining performance of a 7-axis 5-linkage machine tool. The proposed research showed that machining stability and production efficiency can be improved by multi-axis motion optimization based on the anisotropic force ellipsoid of the whole machining system.     

用于五轴联动数控机床的曲线插补控制策略

为实现复杂曲面零部件的高速高精数控加工,提出了一种用于五轴联动数控机床的曲线插补控制策略.实现该控制策略的数控机床控制器将控制任务分为非实时任务和实时任务两部分.非实时任务包括刀具路径信息中位置矢量的曲线插值计算和刀位矢量的曲线插值计算;实时任务包括位置矢量插值曲线和刀位矢量插值曲线的实时插补计算,以及插补点坐标的逆机床运动变换.加工实验表明,该插补加工方法町以用于五坐标数控机床的运动控制,具有良好的应用前景.     

A Cutter Orientation Modification Method for the Reduction of Non-linearity Errors in Five-Axis CNC Machining

In the machining of sculptured surfaces, five-axis CNC machine tools provide more flexibility to realize the cutter position as its axis orientation spatially changes. Conventional five-axis machining uses straight line segments to connect consecutive machining data points, and uses linear interpolation to generate command signals for positions between end points. Due to five-axis simultaneous and coupled rotary and linear movements, the actual machining motion trajectory is a non-linear path. The non-linear curve segments deviate from the linearly interpolated straight line segments, resulting in a non-linearity machining error in each machining step. These non-linearity errors, in addition to the linearity error, commonly create obstacles to the assurance of high machining precision. In this paper, a novel methodology for solving the non-linearity errors problem in five-axis CNC machining is presented. The proposed method is based on the machine type-specific kinematics and the machining motion trajectory. Non-linearity errors are reduced by modifying the cutter orientations without inserting additional machining data points. An off-line processing of a set of tool path data for machining a sculptured surface illustrates that the proposed method increases machining precision.     

立式加工中心VDL-1000数控机床增加四轴的研究

本文阐述了立式加工中心VDL-1000数控机床改造,设计了改造过程中的改造方案,研究件的安装、参数调试、机床精度检测等方面内容,实现了机床多轴联动,产生很好的经济效益,对今后数控机床的改造具有指导作用。     

弧齿线圆柱齿轮全修形齿面的CNC修形加工方法

针对弧齿线圆柱齿轮全修形齿面的修形控制问题,提出一种在展成加工的同时对齿廓和齿线两个方向进行修形控制的加工方法。通过调整直刃刀盘的展成运动回转半径,实现齿面齿廓方向的修形量控制;通过调整刀倾角,实现齿面齿线方向的修形量控制。基于空间运动学原理,利用矢量旋转公式,在具有倾角的数控机床上与普通多轴联动数控机床上展成加工齿面时,保持刀具和工件的相对位置和相对运动相同,进行两类机床之间的运动转换,建立机床运动控制方程,从而实现弧齿线圆柱齿轮在普通多轴联动数控机床上的修形加工。并通过试件的切削试验和齿面接触分析试验,证明所提出的修形加工方法和分析的加工运动方程的正确性。与弧齿线圆柱齿轮传统的加工方法相比,这种加工方法效率高,运动关系简单,便于推广应用。     

高质量加工中四次多项式速度规划算法研究

通过分析数控机床加工时常用的速度规划算法,针对加工过程中由于加速/减速时速度不平滑、加速度及加加速度突变导致数控机床振动的问题,提出一种四次多项式加减速控制方法,并给出了四次速度方程的详细推导步骤,然后介绍了基于四次多项速度规划的参数曲线插补方法。该方法能保证数控机床在高速加工过程中的速度、加速度及加加速度实现连续变化,缓解高速加工产生的过冲。仿真实验表明,四次多项式速度规划算法能够使机床实现柔性加减速控制,从而满足高质量加工的要求。     

A novel curvature circle iterative algorithm for contour error control of multi-axis CNC machine tools

High-precision real-time estimation of contouring errors is a prerequisite for contouring errors control of multi-axis CNC machine tools. This paper focuses on developing a nearest point projection curvature circle iterative (NPP–CCI) algorithm to achieve real-time estimation of multi-axis contouring errors. It is found that the traditional curvature circle iterative (CCI) method has two major shortcomings. The first is that the iterative process may terminate incorrectly at the local contour position, and the other is that the actual tool position and local curvature circle are not necessarily coplanar in three-dimensional space, which would lead to inaccurate calculation of the delay time parameter and eventually affect the estimation accuracy. In order to address the problem of false termination, an index method is used to find the closest reference position with respect to the actual position. At the same time, the projection technology is proposed to overcome the problem met in extending the planar curvature circle iterative method to the spatial applications. The proposed NPP-CCI algorithm is more suitable for spatial contouring errors estimation in tracking complex trajectories and has higher estimation accuracy than the traditional CCI algorithm. Various experiments with different tool paths are conducted on an in-house developed multi-axis experimental platform to verify the effectiveness of the proposed algorithm. The experimental results show that the NPP-CCI algorithm can estimate the contouring errors with higher accuracy than the traditional CCI algorithm, and with the help of real-time computation and compensation, the contouring errors are reduced by more than 44% in terms of the MAX and RMS values.     

基于前馈及自适应滤波的零跟踪误差伺服控制器

针对常规进给数控伺服系统中由于存在跟踪误差从而导致加工几何精度下降的现象,利用数控加工轨迹已知的特点,提出了基于自适应滤波的前馈控制方案。该方案通过速度精插补器产生速度前馈给定信号;同时该位置给定通过对称滤波器施加位置反馈,消除位置环重复给定的影响;并采用广义最小二乘法进行对象辨识以保证该滤波器的参数匹配。通过分析证明了该方案能在理论上实现无跟踪误差的伺服控制,并通过Matlab仿真验证了其有效性。