A Linear Cross-Coupled Control System for High-Speed Machining

We present a linear cross-coupled controller to improve highspeed contouring accuracy independently of tracking accuracy in a biaxial machine tool feed drive servomechanism. Unlike conventional cross-coupled controllers, the cross-coupled controller presented here is a linear system, so it is very easy to perform the stability and steady-state error analysis, and to optimise the controller parameters. The proposed controller is evaluated experimentally on a CNC LOM machine and compared to an uncoupled controller and a conventional cross-coupled controller. Controller performance is evaluated for a circular contour at a feedrate of 30 m min _1 . The experimental results show that the proposed controller can greatly reduce the contour error at large feedrates. The linear cross-coupled controller is simple to implement and is practical.     

Disturbance observer and adaptive controller design for a linear-motor-driven table system

This paper presents a disturbance observer and adaptive controller design for a direct drive motion control system. An indirect adaptive controller is implemented to achieve desired tracking performance as well as deal with system parameters variation. To reduce tracking errors, a newly designed adaptive feed-forward controller is proposed based on an on-line estimated inverse model of the linear motor drive system. A digital disturbance observer is implemented to be included in the proposed feedback-feed-forward control structure to compensate for the undesired nonlinearity and external load disturbance of the direct drive system. Experimental results show that this control scheme can achieve superior contouring accuracy, disturbance rejection and robustness under the influence of friction and cogging force.     

System design of Maxwell force driving fast tool servos based on model analysis

A prototype of the Maxwell force driving fast tool servo (M-FTS) is designed and developed. The M-FTS is capable of accurately translating the cutting tool on a diamond turning machine. The FTS utilizes a flexure-based mechanism driven by Maxwell force to generate movements, a custom linear power amplifier to drive the armature, and a capacitance feedback system to measure accurate displacement. This paper describes the design of electromagnetic circuit, mechanical structures, and controller. The kinematic model of M-FTS is derived to instruct the designs; high-frequency electromagnetic finite element analysis is accomplished to assure the proper driving frequency ranges, and mechanical structure is analyzed to verify the proper displacement and stiffness of mechanical structure. Custom linear power amplifier with a bandwidth of 200 kHz is designed for the M-FTS. According to the experiment results, the frequency response of M-FTS system can be up to 100 kHz when it is open-loop driven. M-FTS has obtained a stroke of 35.5 μm with an open-loop driving. M-FTS can realize a stroke of 11.3 μm and frequency response of 3 kHz with the closed-loop control.     

基于轮廓最优预见的机床进给伺服控制研究

基于交叉耦合、预见控制设计出双轴进给伺服系统,并给出仿真评价。证明对普通机床引入交叉耦合、预见控制之后,系统在不改变单轴跟踪精度的情况下,提高了轮廓跟踪精度。     

Mechanical model and contouring analysis of high-speed ball-screw drive systems with compliance effect

The compliance effect of a high-speed ball-screw feed-drive system is modelled in this work. A mechanical model of a ball-screw feed drive system including the motor, ball-screw, coupling, supporting bearing, linear guide and machine structure was developed. It was found that at high acceleration, the mechanical compliance caused a significant contouring error. Smoothing the acceleration and deceleration control input command did improve the transient deviation in the contouring error. However, the steady state error caused by the elastic elongation in the mechanical elements was not improved. It was also shown that the inertia force of the slide and saddle comprised the dominant loading on a high speed machine tool drive mechanism. A topology structure optimisation method is proposed to reduce the moving weight. This method can help the designer efficiently reduce the moving weight of a machine tool axis in a systematic way. More than a 30% weight reduction was demonstrated in a spindle carrier structural design.     

Adaptive nonlinear contour coupling control for a machine tool system

The quality of products from a machine tool system is largely determined by the tolerances maintained, which is a function of how well the desired contour is tracked. To mitigate contour errors in a three-axis machine tool feed drive system, the control development in this paper is based on an error system that is transformed into tangential, normal, and binormal components to the desired contour (i.e., a cross-coupling controller (CCC)). Unlike previous CCCs, the controller developed in this paper does not assume exact knowledge of the inertia and friction matrices. Specifically, an adaptive estimate is developed to compensate for uncertain friction and inertial parameters. Lyapunov-based methods are used to craft the adaptive estimate and to prove global asymptotic contour tracking. Experimental results of the proposed controller on the x–y-axes of the high speed milling machine showed improvement of the contouring accuracy compared to proportional-derivative controller and a benchmark CCC.     

A three-dimensional triangular vision-based contouring error detection system and method for machine tools

Contouring error detection for machine tools can be used to effectively evaluate their dynamic performances. A triangular vision-based contouring error detection system and method is proposed in this paper, realizing the three-dimensional error measurement of an arbitrary trajectory in conditions of a high feed rate and wide motion range. First, a high-precision measurement fixture, which consists of high-precision circular coded markers and a highly uniform light source, is designed to accurately characterize the motion trajectory of a machine tool and realize the high-quality collection of an image sequence. Then, to improve the contouring error detection accuracy, a coded marker decoding and center location method for the automatic recognition and high-precision center positioning of the circular coded markers are applied. Using image preprocessing and matching, the markers’ three-dimensional coordinates in the camera coordinate system can be constructed. Moreover a data transformation method induced by the orthogonal motion of machine tools is proposed to obtain the three-dimensional trajectory in the machine tool coordinate frame and the contouring error can be calculated. Finally, a three-dimensional contouring error detection study of an equiangular spiral interpolation at different feed rates is performed in the laboratory. It is shown that the average contouring error for a feed rate of 1000 mm/min is about 3 µm, which verifies the vision measurement accuracy and feasibility.     

Effect of slice thickness on the profile accuracy of model maker rapid prototyping measured by a circular triangulation laser probe

The objective of this research is to investigate the effect of slice thickness on the profile accuracy of the model maker (MM) rapid prototyping (RP) system, layer by layer, through non-contact laser probe measurement. A circular triangulation laser probe, model OTM-3A20, made by Wolf & Beck Co., was mounted on a coordinate measuring machine (CMM), as the non-contact sensor. An adjustment device for the laser probe was designed to minimise the cosine error caused by assembly inaccuracy. The alignment test of the measuring laser beam was carried out using a calibrated specimen. The systematic accuracy of the circular triangulation laser probe with respect to the surface roughness and the surface slope of the RP workpiece was investigated using a HP5529A laser interferometer system. The maximum error of 21/2D RP part profile accuracy can be improved from 220 μm to 131 μm, and the average error can be improved from 78 μm to 46 μm as the slice thickness changed from 0.127 mm (0.005 in.) to 0.0127 μm (0.0005 in). However, the machining time increases by about seven fold based on the experimental results. An overall error of 197 μm as measured by the laser probe is attainable using the finest slice thickness 0.0127 mm (0.0005 in.) for the 3D profile accuracy. To verify the accuracy of non-contact laser probe measurement, the 3D profile of the RP part was also measured by a CNC CMM, with good consistency.     

基于PLC和直线电机的木材加工自动送料机

针对木材加工过程中工作强度大、工作风险高、加工精度低以及操作复杂等问题,设计研发了一台大推力直线感应电动机（LIM）,并采用可编程逻辑控制器（PLC）实现了整个木材加工过程中的复杂功能,最终提出并设计了一种木材加工自动送料机.该系统利用直线感应电机驱动工作台的运动实现了木材切割,采用伺服电机控制木材切割厚度方向的进给,并采用PLC分别控制直线电机和伺服电机实现了木材加工中的变速切割,同时通过PLC与人机交换界面通讯实现了系统的参数设置与显示等功能.实际运行结果表明,该系统不仅降低了劳动强度,减小了人工助力;另一方面,相比于传统的伺服电机加丝杆结构,该系统具有可靠性更高、寿命更长、使用更灵活等优点.     

Inverse determination of Johnson–Cook model constants of ultra-fine-grained titanium based on chip formation model and iterative gradient search

In this paper, a systematic approach on how to predict kinematic errors based on tolerance of machine tools’ guideways is introduced. Firstly, the truncated Fourier series function is applied to fit curve of guideways surface. Since geometric profile errors are regarded as a bridge between tolerance and kinematic errors of machine tools’ guideways, the mapping relationship between tolerance and geometric profile errors of machine tools’ guideways is formulated, and the mapping relationship between geometric profile errors and kinematic errors of guideways is established. Then, kinematic errors prediction model based on tolerance of guideways is subsequently proposed. Finally, simulation verification is conducted with this method. Simulation results show the range of the predicted kinematic errors (positioning error, y direction and z direction straightness error, roll error, pitch error, and yaw error) is 17.12 μm, 56.57 μm, 70.71 μm, 28.28 μrad, 141.42 μrad, and 113.14 μrad, respectively. In order to verify the feasibility and effectiveness of the presented method, a measuring experiment is carried out on guideways of a gantry-type five-axis milling machine tools by using a dual-frequency laser interferometer. The measured and identified discrete data can be fitted precisely by Fourier curve fitting method. The fitting results show the range of the measured kinematic errors is 16.96 μm, 59.43 μm, 68.63 μm, 28.65 μrad, 135.40 μrad, and 111.58 μrad, respectively. The maximum residual errors between the predicted and measured values of kinematic errors are 1.67 μm, 5.19 μm, 5.50 μm, 1.87 μrad, 9.81 μrad, and 7.07μrad, respectively. Comparing with the measured results of kinematic errors, residual errors are considerably small and can be neglected. Therefore, there is no doubt that this method is effective enough for predicting kinematic errors and can be used to replace the measurement of kinematic errors. In the design stage of machine tools, this approach is convenient for engineers to derive the distribution of kinematic errors. And its basic idea can be applied to other type of machine tools’ guideways.     

RESPONSE SURFACE ANALYSIS OF SLICING OF SILICON INGOTS WITH FOCUS ON PHOTOVOLTAIC APPLICATION

Polycrystalline silicon wafers are widely used in Photovoltaic (PV) industry as a base material for the solar cells. The existing silicon ingot slicing methods typically provide minimum wafer thickness of 300–350 μm and a surface finish of 3–5 μm R_a while incurring considerable kerf loss of 35–40%. Consequently, efficient dicing methods need to be developed, and in the quest for developing new processes for silicon ingot slicing, the wire-EDM (electric discharge machining) is emerging as a potential process. Slicing of a 3′′ square silicon ingot into wafers of 500 μm in thickness has been performed to study the process capability. This article analyzes the effect of processing parameters on the cutting process. The objective of the experimental study is improvement in slicing speed, minimization of kerf loss and surface roughness. A central composite design-based response surface methodology (RSM) has been used to study the slicing of polycrystalline silicon ingot via wire-EDM. A zinc-coated brass wire, 100 μm in diameter, has been used as an electrode in the slicing experiments. It has been observed that the optimal selection of the process parameters results in an increase of 40–50% in the slicing rate along with a 20% reduction in the kerf loss as compared to the conventional methods. The machined surfaces on the sliced wafer were free of micro-cracks and wire material contamination, thereby making it useful for electronic applications.     

基于直线电机的高速滚珠丝杠副寿命试验机设计

针对高速滚珠丝杠副性能指标的测量需求,设计了一套基于直线电机的高速滚珠丝杠副寿命测试试验系统,系统以直线电机为加载机构、旋转伺服电机为丝杠驱动机构、多轴运动控制卡为运动控制器。据此系统设计的滚珠丝杠副高速加载跑合寿命试验机在连续稳定变负载的情况下,可以对滚珠丝杠在高速、高加速度、大负荷等条件下进行长时间运行测试,满足寿命试验机对滚珠丝杠副寿命试验和相关参数的测试要求。试验结果证明了系统的可行性。     

基因测序仪运动平台的高精度定位控制

为了实现对基因测序仪运动平台的高精度定位控制,建立了基因测序仪运动平台控制系统。对该系统所采用的数学建模、模型辨识、控制器设计、输入整形等方法进行研究。根据运动平台动力学方程和永磁同步直线电机电压-推力关系构建了运动平台数学模型,利用频域扫描法在实物实验的基础上辨识出运动平台的模型参数。最后,基于运动平台模型设计了双闭环控制器和前馈控制器组成的复合控制器来保证运动平台的稳定性和高精度,同时根据整个系统的主导极点设计了输入整形器以抑制运动平台的残余振荡。实验结果表明:加入了输入整形的复合控制器将运动平台的稳态重复定位精度从±1.47μm提高到±0.354μm。较传统复合控制器,本文提出的方法能使基因测序仪运动平台更快进入可用重复定位精度范围,并基本满足基因测序仪采集图像时所需的稳定性强、精度高等要求。     

Research on the control systems of a precision linear motor drive device

The model of a precision linear motor drive device (PLMDD) and its control requirements are analyzed. In order to enhance the tracking and anti-disturbance performance of the system, its third-order model is established, and disturbance-observer based input revising feedforward error compensation robust control algorithm, combined with integral-separated proportional integral derivative (PID) control algorithm is proposed. This includes feedback control algorithm and feedforward control algorithm. The feedback controller improves system tracking performance and suppresses load and mechanical disturbance while the feedforward controller compensates phase hysteresis introduced by feedback control. Theoretical analyses, simulations and experiments show that this control method increases the tracking precision of PLMDD from ±5 μm to ±2 μm and dramatically improves its anti-disturbance ability.     

Double Position Servo Synchronous Drive System Based on Cross-Coupling Integrated Feedforward Control for Broacher

Synchronization errors directly deteriorate the machining accuracy of metal parts and the existed method cannot keep high synchronization precision because of external disturbances. A new double position servo synchronous driving scheme based on semi-closed-loop crosscoupling integrated feedforward control is proposed. The scheme comprises a position error cross-coupling feedforward control and a load torque identification with feedforward control. A digital integrated simulation system for the dual servo synchronous drive system is established.Using a 20 t servo broacher, performance analysis of the scheme is conducted based on this simulation system and the simulation results show that systems with traditional parallel or single control have problems when the worktable works with an unbalanced load. However, the system with proposed scheme shows good synchronous performance and positional accuracy. Broaching tests are performed and the experimental results show that the maximum dual axis synchronization error of the system is only 8 μm during acceleration and deceleration processes and the error between the actual running position and the given position is almost zero. A double position servo synchronous driving scheme is presented based on crosscoupled integrated feedforward compensation control,which can improve the synchronization precision.     

数控机床用直线电动机及其控制技术

直线电动机直接驱动系统具有结构简单、动态响应快、速度和加速度大等一系列优点,是各类数控机床进给运动的理想传动方式。介绍了直线电动机直接驱动技术原理、特点及其在数控机床中的应用,阐述了直线电动机直接驱动系统控制技术的研究现状及发展趋势。     

Robust iterative learning contouring controller with disturbance observer for machine tool feed drives

In feed drive systems, particularly machine tools, a contour error is more significant than the individual axial tracking errors from the view point of enhancing precision in manufacturing and production systems. The contour error must be within the permissible tolerance of given products. In machining complex or sharp-corner products, large contour errors occur mainly owing to discontinuous trajectories and the existence of nonlinear uncertainties. Therefore, it is indispensable to design robust controllers that can enhance the tracking ability of feed drive systems. In this study, an iterative learning contouring controller consisting of a classical Proportional-Derivative (PD) controller and disturbance observer is proposed. The proposed controller was evaluated experimentally by using a typical sharp-corner trajectory, and its performance was compared with that of conventional controllers. The results revealed that the maximum contour error can be reduced by about 37% on average.     

Research on accuracy analysis and performance verification test of micro-precise five-axis machine tool

In order to meet the increasing requirements of manufacturing complex micro-structures, a configuration method of small five-axis machine tool is proposed. The preliminary design of the machine tool includes the total layout, the counterweight part, tool align and monitor device, and so on. By analysis of finite element method, the paper focuses on the influence of position changing of counterweight force on the static displacement of the tool nose. The affect of X-axis unit load in the limit position on the deformation of X-axis unit is also studied. The result provides reasonable parameters for the device optimization. In order to meet requirement of the device’s total error, geometric accuracy of the gantry and position accuracy of the axis are analyzed theoretically, and targets of machining accuracy of the gantry and assembly accuracy of the axis are given. According to the theoretical analysis, a micro-precise five-axis machine tool is developed. Spherical surface and complex micro-structure optical compound eye is manufactured by the device. After processing, the deviation of morphology of the spherical surface, with width of 400 μm and depth of 20 μm, is about 0.133 μm, which is measured by surface profiler. Diameter of each ommatidium in the compound eye is 1.31 mm with the surface roughness R _a less than 0.12 μm. Results show that the performance of the device is good and the theoretical analysis is correct.     

Control of linear motor machine tool feed drives for end milling: Robust MIMO approach

Linear motors become promising for use as high speed, high accuracy machine tool feed drives. The cutting force in the machining process are directly reflected to the linear motor due to no gearing mechanism. To achieve highly accurate machining, the controller for the linear motor system should be designed to compensate for the cutting force.     

快走丝电火花线切割机床动刚度研究

国产快走丝电火花线切割机床加工精度一般均不高.通过对机床结构的分析和激振试验,找出影响该类机床加工精度的主要原因--结构不合理、动刚度差,并提出改进办法.