High speed servo control of multi-axis machine tools

Advances in high-speed machining technology, including those in spindle speeds and cutters, are out-pacing advances in the servo control performance of machine tools. To close this gap, new machine tools and improved controls must be developed. Improvements to machine tools under development include special-purpose machine tools, the use of advanced materials, the replacement of ball screws and ways with linear motors and roller guides, and the use of parallel link actuators. This paper focuses on the control issues that will become increasingly important as these high-speed machining and high-speed machine tool advances are realized.The main issues in high-speed servo control are feed rate planning, and servo loop control laws. A method is developed in this paper which takes advantage of the full performance envelope of each axis in an arbitrary path. This near-complete usage of the servo capabilities of a machine tool results in reduced cycle time or reduced path error. A servo loop control law is then developed that uses the axis performance envelope as well as instantaneous position, velocity, and acceleration information of the target path and machine axis to improve servo performance in the presence of disturbances.     

电火花成形机床双轴同步模型预测控制研究

为减少龙门式电火花成形机床双轴同步误差,以永磁交流同步伺服电机及滚珠丝杠为例建立了X轴的双轴进给系统模型;提出了动态矩阵控制算法,分别设计了与该算法相结合的并行同步控制策略、交叉耦合控制策略双轴同步控制器,通过与传统PID算法控制方案比较及Matlab仿真,最后选择同步误差更小的交叉耦合控制策略双轴同步控制器,并将该控...     

Thermal behavior of a machine tool equipped with linear motors

Development of a feed drive system with high speed and accuracy has been a major issue in the machine tool industry. Linear motors can be used as efficient tool to achieve the high speed and accuracy. However, a high speed feed drive system with linear motors, in turn, can generate heat problems. Also, frictional heat is produced at the ball or roller bearing of LM block when driven at high speed. It can affect the thermal deformation of the linear scale as well as that of the machine tool structure.In this paper, important heat sources and resulting thermal errors in a machine tool equipped with linear motors were investigated when it was operated at high speed. The thermal deformation characteristics were identified through measuring the thermal error caused from thermal deformation of the linear scale and the machine tool structure. The dominant thermal error components were identified from the thermal error analysis using finite element method. It was shown that the proposed analysis scheme is efficient in identifying the dominant thermal error components and its magnitudes such as the thermal expansion and movement of the linear scale, thermal deformation of the machine tool slide.     

基于模糊推理自校正控制的直接驱动进给系统稳定性分析

直线电机直接驱动进给系统是实现数控机床进给高速化的主要形式，但加工过程中切削力甚至运动部件质量的变化都是系统的干扰，易引起系统不稳定和定位精度下降。离线模糊推理、在线对控制系统实现自校正的控制方法能提高系统的抗干扰能力，并具有良好的实时性。为了研究系统的稳定性，本文介绍了这种基于模糊推理自校正控制的直接驱动进给系统稳定性分析方法，为同类系统设计提供理论参考。     

Kinematic and dynamic synthesis of a parallel kinematic high speed drilling machine

Typically, the term “high speed drilling” is related to spindle capability of high cutting speeds. The suggested high speed drilling machine (HSDM) extends this term to include very fast and accurate point-to-point motions. The new HSDM is composed of a planar parallel mechanism with two linear motors as the inputs. The paper is focused on the kinematic and dynamic synthesis of this parallel kinematic machine (PKM). The kinematic synthesis introduces a new methodology of input motion planning for ideal drilling operation and accurate point-to-point positioning. The dynamic synthesis aims at reducing the input power of the PKM using a spring element.     

Thermal modelling of an HSC machining centre to predict thermal error of the feed system

Machine tools equipped with linear motors can achieve high feed speed as well as high accuracy. However, the direct feed drive system generates heat through power loss and friction. In combination with environmental influences such as machine shop climate, this can lead to a local deformation of the machine tool structure and induce a direct positioning error. This paper presents a thermal model, using the finite element method, to simulate the thermal behaviour of a high-speed cutting machining centre equipped with linear motors. This model considers the complex boundary conditions such as heat sources, contact and convective heat transfer. Transient changes in temperatures and deformations are allowed in the solution. The comparison of the experiments show that this model can predict the temperature distribution and positioning error under specified operating conditions very well.     

Thermal error analysis for a CNC lathe feed drive system

The development of high-speed feed drive systems has been a major issue in the machine tool industry for the past few decades. The resulting reduction in the time needed for tool changes and the rapid travel time can enhance productivity. However, a high-speed feed drive system naturally generates more heat and resultant thermal expansion, which adversely affects the accuracy of machined parts. This paper divides the feed drive system into two parts: the ball screw and the guide way. The thermal behavior model for each part is developed separately, in order to estimate the position errors of the feed drive system caused by thermal expansion. The modified lumped capacitance method (MLCM) and genius education algorithm (GEA) are used to analyse the linear positioning error of the ball screw. Thermal deformation of the guide way affects straightness and introduces angular errors, as well as affecting linear positioning. The finite element method is used to estimate the thermal behavior of the guide way. The effectiveness of the proposed models is verified through experiments using a laser interferometer.     

Feed rate optimization for 3-axis ball-end milling of sculptured surfaces

The aim of this research is to improve the productivity of CNC machine tools by optimizing feed rate. To optimize feed rate two programs were used: “ACIS” (with scheme language) and “Visual Basic”. The scheme program for modeling the work piece, tool, cutting edge, and calculating maximum cutting force and the Visual Basic program to control all the activities linked to the ACIS program for estimating optimized feed values. Laboratory tests were conducted to verify the results from the modeling, using an insert-type one-flute ball-end cutter on a CK45 carbon steel work piece. No coolant was used throughout the experimental works. Comparisons were made between the maximum cutting forces, in the “fix” feed rate tests. The results indicate significant increases in productivity, which can be achieved, by using the optimized feed rate method.     

直线电机在机床工业中的最新应用及技术分析

直线电机在机床进给驱动上的应用日益广泛,比起传统的进给驱动方式,它能提供非常高的速度、加速度和位置精度,大大提高了生产率和加工件表面质量。文章介绍了直线电机在机床上的最新应用情况,并就应用中的关键技术问题进行了分析。     

Theory and application of a combined feedback–feedforward control and disturbance observer in linear motor drive wire-EDM machines

This paper presents a combined two-degree-of-freedom controller and disturbance observer design for a direct drive motion control system actuated by permanent-magnet linear synchronous motors (PMLSM). A feedback controller based on pole-placement design method is proposed to achieve desired tracking performance as well as stabilize the closed-loop system. A newly designed feedforward controller is proposed to reduce tracking errors based on an inverse model of the direct drive system. A digital disturbance observer is implemented to be included in the proposed feedback–feedforward control structure to compensate for nonlinear friction, cogging effects, and external load disturbance. Furthermore, the proposed control scheme has been verified as being internally stable. Experimental results indicate that the proposed controller can achieve a high contouring accuracy of ±0.3 μm as well as provide disturbance rejection and robustness. The maximum contour error of circular trajectory was reduced from 8.5 to 3.2 μm in comparison with proportional-integral-derivative (PID) controller.     

冲压机床自动送料机构双电机同步控制技术研究

针对双电机同步驱动系统因电机特性、负载变化等因素引起的同步误差问题,对冲压机床自动送料机构双电机驱动系统进行研究.分析了电机的动态特性,找出影响同步控制的因素,为设计同步控制方案提供依据。提出一种基于单神经元PID控制策略,通过改变神经网络的权重系数对PID参数进行实时调节,以解决传统PID无法在线调整的问题。同时,在交叉耦合控制方法的基础上,以丝杠的进给速度、位置误差以及伺服电机的控制电流为控制指标进行仿真分析和实验。研究结果表明:与传统控制方法相比,所用方法能大幅提高两个电机的同步控制精度,有效减小丝杠之间的同步误差,抗干扰能力强,达到设计要求。     

Development of a micro-forming system for micro-punching process of micro-hole arrays in brass foil

Micro-forming technology poses much higher demands on positioning accuracy, velocity and mass production, and the common forming machines cannot satisfy these requirements using traditional drive methods. Micro-forming equipment using novel drive methods with high speed and high precision has become an important research field for industrial application. In the paper, a novel drive mechanism was designed with a symmetric distributed double linear motors and a micro-forming system was developed with micro-punching tools and automatic feeding apparatus for metal foil. The servo control system of this micro-forming system was designed using SIMOTION D445, and a parallel control model was adopted with a single motor module to solve the synchronous control problems of double linear motors. Micro-punching process of brass foil was studied with the micro-forming system, and micro-holes of 600 μm, 300 μm and 150 μm in diameter were manufactured with high dimensional accuracy. An array of 50 × 4 micro-holes, each 600 μm in diameter, was manufactured using an automatic feeding apparatus guided by a microscopic visualization system for assisted localization. The results indicate that the micro-forming system with high-accuracy and high-speed is suitable for the mass production of micro-scale parts.     

Dynamic calibration of the positioning accuracy of machine tools and coordinate measuring machines using a laser interferometer

This paper describes a method for evaluating the positioning accuracy of machine tools and coordinate measuring machines (CMM) under dynamic conditions. It is based on the Hewlett Packard 5519A laser interferometer which is capable of performing dynamic calibration. Such method uses the A-quad-B pulses from the machine encoder as the position trigger signals, thus enabling to make measurements “on-the-fly”. A software package has been developed so as to permit the data acquisition and presentation of the positional errors in accordance with ISO 230–2 standard. Thereby, the accuracy and repeatability of positioning of the machine axis on test are assessed considering the ISO parameters. This technique provides a more realistic and detailed picture of the errors of the machine. Such errors are measured in high resolution and dynamically. Application of this positional error calibrator on a moving bridge type CMM is undertaken. The results are presented and discussed.     

基于SERCOS-Ⅱ的机床数控系统实时通信与同步控制研究

现代数控机床和加工中心对实时接收数据和同步运动控制提出了更高的要求.论文对脉冲式数控系统进行简要述评的基础上,提出了基于SERCOSⅡ串行实时总线通信的机床数控系统架构:以运动控制器为主站,进给伺服驱动器为从站的主从式环形网络,并以通信周期为基准协调该多CPU结构.论文将提出的系统用于XKHL650型立式加工中心数字化控制,并提出了各进给轴伺服驱动电流环同步控制方法.该方法使各进给轴伺服驱动电流指令达到1μs的同步精度,从而保证了机床各进给轴动作的一致性.实验和加工运行表明基于SERCOSII总线通信的数控系统满足高实时性高精度的加工要求.     

Virtual prediction and constraint of contour errors induced by cutting force disturbances on multi-axis CNC machine tools

This paper presents a strategy to virtually predict and constrain the contouring errors contributed by cutting force disturbances on feed drives. The tracking errors on each feed drive are predicted as a linear function of tangential feed by evaluating the product of estimated power spectrum of cutting forces and disturbance frequency response function along the tool path in virtual CAM environment. The corresponding tool tip contouring and tool axis orientation errors are estimated and constrained by scaling the feed along the tool path. The algorithm is experimentally illustrated to improve the machining accuracy on a 5 Axis CNC machine tool.     

Manufacturing linear and circular contours using CNC EDM and frame type tools

In traditional electro discharge machining (EDM) the usual practice is to form a 3D female cavity in the work piece by making a corresponding male shaped tool and plunging this into the work piece. Recently, however, computer numerical control (CNC) EDM machines have become available which have the potential to generate a detailed shape without using complex electrodes. This paper describes a novel method of electro discharge machining which makes the generation of linear, circular and curved contours possible without any special electrodes. The experimental work employs CNC EDM, mild steel work pieces and inexpensive “frame type” copper tools. The results indicate, relative to the cavities produced, an improved effective material removal rate (MRR), a better surface finish and a lower powered machine. The paper also introduces the effect of EDM parameters on the process performance by modelling and analysing the experimental results using statistical techniques. Finally, a few applications are discussed.     

基于多线程技术的双直线电机同步驱动系统控制研究

基于Windows多线程技术,研究了由双直线电机驱动和固高运动控制卡组成的XY并联工作台系统中的同步控制问题,并进行了同步控制实验研究;利用MATLAB对采集的运动数据进行拟合分析.结果表明,在一定的精度要求范围内,采用多线程技术来实现双直线电机的同步驱动控制是合理可行的.     

预弯机位置速度双闭环同步控制

针对预弯机电液比例控制系统的高同步精度和速度控制要求,考虑到负载力对速度稳定和位置精度的影响,提出基于单轴带负载力补偿的位置速度双闭环控制的四轴同步控制策略。针对单轴同时控制位置和速度特点采用外环位置控制、内环速度控制的双闭环控制方法;对于四轴同步,采用两组主从同步、两组主轴之间均值同步的混合同步控制方式。仿真结果表明:该控制策略实现了较高的同步精度和速度误差,同步控制精度可达到0.24 mm,速度跟踪误差控制在2.2 mm/s以内,完全满足预弯机生产过程的控制要求。     

5轴卧式加工中心回转工作台设计与应用

大型5轴卧式加工中心机床回转工作台是机床在加工过程中驱动工件回转的部件,回转精度及可靠性直接影响机床的整体性能。针对5轴机床在联动切削过程中B轴移动的特点,选用大型可调间隙蜗轮蜗杆进行传动,传动机构配置有减速机构以增加驱动力矩,回转机构通过大型轴向径向组合轴承支撑,通过高精度光栅尺进行位置反馈,得到快速进给5 r/min、定位精度为3″、重复定位精度为1.5″的高精度回转工作台。     

龙门移动式数控机床的模糊交叉耦合补偿控制

以龙门移动式数控机床的双轴同步传动机构为背景,研究两套永磁直线同步电动机(PMLSM)伺服系统在具有一定动态指标和抗干扰性能前提下的同步控制问题。针对两套进给系统难以同步且易受干扰的特点,设计了一种基于模糊控制的交叉耦合补偿控制器。同时为了提高每套伺服系统的跟踪和抗扰能力,设计了最优预见前馈补偿。仿真结果表明该系统对不平衡负载具有良好的同步抗扰性能。