Kinematic calibration of gantry hybrid machine tool based on estimation error and local measurement information

This paper presents the kinematic calibration of a four degrees-of-freedom (DOF) hybrid machine tool based on a novel planar 3-DOFs parallel mechanism and a long movement of the worktable. Closed-form solutions are developed for both the inverse and direct kinematics about the parallel mechanism. The error model is built and the mechanism accuracy is investigated. Two types of kinematic calibration method are proposed by a simple measurement device. The first type of calibration method is based on estimation error, and can easy improve the machine tool accuracy quickly by estimating the error trends. The second type of kinematic calibration method is based on local measurement information, which includes the position errors and does not include the pose errors of the machine tool. The calibration tests showed the effectiveness of the calibration methods, which can be useful for the similar types of parallel machine tool.     

A comprehensive method for calibration of volumetric positioning accuracy of CNC-machines

A comprehensive method for measuring the systematic errors of CNC-machine tools has been studied. The method used for measurement and calibration of machine tool errors should be general and efficient. The objectives of this study include:

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.     

七轴混联机床机构干涉校验算法及工作空间快速校验的研究

在加工具有复杂曲面的工件时,同六轴并联机床相比,七轴混联机床的加工能力有所提高,能够快速加工多个曲面,降低由于多次装夹造成的加工误差,缩短加工时间,有利于充分发挥并联机床的优势。为了避免加工过程中出现干涉碰撞问题,同时确保整个工件都在加工空间内,提出并详细讨论了七轴混联机床的机构干涉校验算法及工作空间快速校验算法。这些算法对混联机床的设计和控制系统的开发有着一定意义。     

Algorithm for detecting volumetric geometric accuracy of NC machine tool by laser tracker

Quick and accurate detecting the error of NC machine tool and performing the error compensation are important to improve the machining accuracy of NC machine tool.Currently,there are many methods for detecting the geometric accuracy of NC machine tool.However,these methods have deficiencies in detection efficiency and accuracy as well as in versatility.In the paper,a method with laser tracker based on the multi-station and time-sharing measurement principle is proposed,and this method can rapidly and accurately detect the geometric accuracy of NC machine tool.The machine tool is controlled to move in the preset path in a 3D space or 2D plane,and a laser tracker is used to measure the same motion trajectory of the machine tool successively at different base stations.The original algorithm for multi-station and time-sharing measurement is improved.The space coordinates of the measuring point obtained by the laser tracker are taken as parameter values,and the initial position of each base point can be determined.The redundant equation concerning the base point calibration can be established by the distance information of the laser tracker,and the position of each base point is further determined by solving the equation with least squares method,then the space coordinates of each measuring point can be calibrated.The singular matrix does not occur in calculation with the improved algorithm,which overcomes the limitations of the original algorithm,that the motion trajectory of machine tool is in a 3D space and there exits height difference between the base stations.Adopting the improved algorithm can expand the application of multi-station and time-sharing measurement,and can meet the quick and accurate detecting requirements for different types of NC machine tool.     

An artefact for traceable freeform measurements on coordinate measuring machines

The present paper describes an artefact based approach to obtain traceability of freeform measurements on coordinate measuring machines. First, the requirements for the traceability of freeform measurements and a strategy for the development of a feasible solution are presented. A new concept of artefact, called the “Modular Freeform Gauge” (MFG) has been developed. It is based on physical modeling of a given freeform surface by a combination of items with regular geometry, well calibrated on their dimensions and form. The relative position is accounted for during the procedure; this information is used to generate a “calibrated” CAD model as reference for freeform measurements. The architecture of the artefact, its collocation in the traceability chain, and the calibration procedure are described.Finally, a procedure for the uncertainty assessment of actual freeform measurements is presented. The work here described has been focused on implementation of the uncertainty assessment procedure for freeform measurements on turbine blades. A task-specific Modular Freeform Gauge was developed for this application.     

Improved machine tool linear axis calibration through continuous motion data capture

Machine tool calibration is becoming recognised as an important part of the manufacturing process. The current international standards for machine tool linear axes calibration support the use of quasi-static calibration techniques. These techniques can be time consuming but more importantly a compromise in quality due to the practical restriction on the spatial resolution of target positions on the axis under test. Continuous motion calibration techniques have the potential to dramatically increase calibration quality. Through taking several measurement values per second while the axis under test is in motion, it is possible to measure in far greater detail. Furthermore, since machine tools normally operate in dynamic mode, the calibration data can be more representative if it is captured while the machine is in motion. The drawback to measuring the axis while in motion is the potential increase in measurement uncertainty. In the following paper, different methods of continuous motion calibration are discussed. A time-based continuous motion solution is proposed as well as a novel optimisation and correlation algorithm to accurately fuse the data taken from quasi-static and continuous motion measurements. The measurement method allows for minimal quasi-static measurements to be taken while using a continuous motion measurement to enhance the calibration process with virtually no additional time constraints. The proposed method does not require any additional machine interfacing, making it a more readily accessible solution for widespread machine tool use than other techniques which require hardware links to the CNC. The result of which means a shorter calibration routine and enhanced results. The quasi-static and continuous motion measurements showed correlation to within 1 μm at the quasi-static measurement targets. An error of 13 μm was detailed on the continuous motion, but was missed using the standard test. On a larger, less accurate machine, the quasi-static and continuous motion measurements were on average within 3 μm of each other however, showed a standard deviation of 4 μm which is less than 1% of the overall error. Finally, a high frequency cyclic error was detected in the continuous motion measurement but was missed in the quasi-static measurement.     

Five axis machine tool volumetric error prediction through an indirect estimation of intra- and inter-axis error parameters by probing facets on a scale enriched uncalibrated indigenous artefact

The volumetric accuracy of five-axis machine tools is affected by intra-axis geometric errors (error motions) and inter-axis geometric errors (axes relative position and orientation errors). Self-probing of uncalibrated facets on the existing machine tool table is proposed to provide the necessary data for the self-calibration of the machine error parameters and of the artefact geometry using an indirect approach. A set of 86 non-confounded coefficients are selected from the ordinary cubic polynomials used to model both the intra- and inter-axis errors. A scale bar is added to provide the isotropic scale factor. The estimated model is then used to predict the actual tool to workpiece position. Experimental trials are conducted on a five-axis horizontal machining centre using its original unmodified machine table as an artefact. For validation purposes only, the estimated artefact geometry is compared to accurate coordinate measuring machine (CMM) measurements. A study of the volumetric error predictive capability of the model for selected subsets of estimated error coefficients is also conducted.     

一种新型钢卷尺零位标定方法（英文）

由于传统检定系统无法准确标定在零点位置具有可移动尺钩结构的部分钢卷尺的零位误差,提出了一种基于机器视觉的测量系统来完成钢卷尺零位误差的标定方法。结合钢卷尺钩量与顶量两种方式,设计了双基准面结构配合双基准线作为标准量,对比标准量与待测尺上尺头零位到500mm刻线之间示值距离的差值,实现了零位标定。线纹比对依托机器视觉实现,将图像处理过程结合双基准线计算了浮动像素当量值。实验结果表明,本方法的零位误差标定结果重复性可达0.006mm,满足国家标准钢卷尺检定规程要求。     

Influence of measurement noise and laser arrangement on measurement uncertainty of laser tracker multilateration in machine tool volumetric verification

This paper aims to present different techniques and factors that affect the measurement accuracy of a commercial laser tracker responsible for capturing checkpoints used in machine tool volumetric verification. This study was conducted to uncover various sources of error affecting the measurement uncertainty of the laser tracker, additional sources of error that further contributed to the uncertainty, and the factors influencing these techniques. We also define several noise reduction techniques for the measurements.The improvement in the accuracy of captured points focuses on a multilateration technique and its various resolution methods both analytically and geometrically. Similarly, we present trilateration and least squares techniques that can be used for laser tracker self-calibration, which is an essential parameter in multilateration.This paper presents the influence of the spatial distribution of laser trackers (LTs) in measurement noise reduction by multilateration, which produces an improvement in volumetric error machine tool reduction. A study of the spatial angle between LTs, the distance and the visibility of the point to be measured are presented using a synthetic test. All of these factors limit the scope of multilateration. Similarly, a comparison of self-calibration techniques using the least squares and trilateration methods with which to determine the relative position of the laser tracker employees is presented. We also present the influence of the relationship between the radial and angular measurement noise self-calibration processes as it relates to the volumetric error reduction achieved by the machine tool with multilateration. All studies were performed using synthetic tests generated using a synthetic data parametric generator.     

An accuracy design approach for a multi-axis NC machine tool based on reliability theory

Accuracy design constitutes an important role in machine tool designing. It is used to determine the permissible level of each error parameter of a machine tool, so that any criterion can be optimized. Geometric, thermal-induced, and cutting force-induced errors are responsible for a large number of comprehensive errors of a machine tool. These errors not only influence the machining accuracy but are also of great importance for accuracy design to be performed. The aim of this paper is the proposal of a general approach that simultaneously considered geometric, thermal-induced, and cutting force-induced errors, in order for machine tool errors to be allocated. By homogeneous transformation matrix (HTM) application, a comprehensive error model was developed for the machining accuracy of a machine tool to be acquired. In addition, a generalized radial basis function (RBF) neural network modeling method was used in order for a thermal and cutting force-induced error model to be established. Based on the comprehensive error model, the importance sampling method was applied for the reliability and sensitivity analysis of the machine tool to be conducted, and two mathematical models were presented. The first model predicted the reliability of the machine tool, whereas the second was used to identify and optimize the error parameters with larger effect on the reliability. The permissible level of each geometric error parameter can therefore be determined, whereas the reliability met the design requirement and the cost of this machining was optimized. An experiment was conducted on a five-axis machine tool, and the results confirmed the proposed approach being able to display the accuracy design of the machine tool.     

Error analysis and auto-calibration for a Cartesian-guided tripod machine tool

This paper focuses on the error analysis and calibration methodologies for a parallel kinematic machine (PKM) called a Cartesian-guided tripod (CGT). The CGT volumetric error due to the geometric error, kinematic parameter error and nonlinear machine stiffness were studied. It is well known that the PKM nonlinear machine stiffness can produce significant volumetric errors from several tens to several hundreds of micrometres depending on the averaged value and deviation range for the machine stiffness. For most PKMs, joint level sensors are used to estimate the virtual Cartesian movements of the cutting tool. The nonlinear stiffness effect is not detected by this indirect metrology method and must be compensated for by a calibration methodology. A solution for the nonlinear stiffness effect implemented on the CGT involves using a passive Cartesian guiding/metrology leg that is independent of the driving legs to directly measure the Cartesian movement of the motion platform. Because the metrology loop of the Cartesian guiding/metrology leg is separated from the kinematic loops of the driving legs, the volumetric accuracy of the CGT is immunised against thermal errors and load deformations on the drive mechanisms. The passive Cartesian guiding/metrology leg is also used for the auto-calibration of the CGT kinematic parameters. The auto-calibration methodology and simulation results were studied and reported.     

圆度最小二乘评定结果的不确定度估计

目前,利用最小二乘原理进行圆度误差评定时,一般只是给出圆度误差的评定结果,并没有给出评定结果的不确定度。本文在对圆度最小二乘评定模型线性化处理后,根据不确定度传递公式,推导出了不确定度估计公式,不仅保证了圆度评定结果的完整性,而且符合新一代产品几何规范(GPS)标准的要求,提高了工件检验的准确性。     

基于灰关联的加工中心可用性需求重要度研究*

为快速、准确地响应加工中心用户的可用性需求,需要准确评估加工中心各组件对用户可用性需求的满足程度,从而有针对性地构建可用性保障机制。针对加工中心产品的用户可用性需求的特点,以及加工中心各组件间自相关关系,引入质量功能展开理论(Quality function deployment,QFD),建立可用性功能展开屋模型(Dynamic availability of the house,DHOA)。由于用户需求与加工中心各组件间的相关系数具有不确定性、呈区间数值的特点,采用区间灰关联分析与可用性功能展开融合方法,求解可用性功能展开模型的输出项,确定各组件对可用性需求重要度。以国产某型号加工中心为实例,验证该模型的合理性与有效性。     

测量不确定度的验证及在材料试验机不确定度评定中的应用

本文介绍了测量不确定度的几种验证方法,并且给出了在材料试验机校准结果和金属材料拉伸试验检测结果测量不确定度评定验证的应用实例。     

Computational model for the control, performance evaluation, and calibration of a parallel mechanism

This paper presents a computational model that defines a methodology for the control, performance evaluation, and calibration of a parallel mechanism by means of the kinematic model, the kinematic parameter identification, and the control of the system actuators and sensors. The developed software has special relevance to the calibration of parallel mechanisms, allowing us to perform the system control, performance evaluation, and mechanism calibration in a single application. Parallel computing techniques are used to overcome the high computational cost involved with solving complex nonlinear kinematic models and parameter identification, obtaining a significant computational cost saving. Finally, the developed procedure is validated, obtaining an important improvement in the accuracy of the mechanism. This methodology can be used in mechanisms for metrology applications and machine tool processes.     

Flatness error evaluation and verification based on new generation geometrical product specification (GPS)

New generation geometrical product specification (GPS) links the whole course of a geometrical product from the research, development, design, manufacturing and verification to its release, utilization, and maintenance. Measurement process is one of the most important part of verification/inspection in the new generation GPS. With the knowledge-intensive and globalization trend of the economy, unifying the evaluation and verification of form errors will play a vital role in international trade and technical communication. Considering the plane feature is one of the most basic geometric primitives which contribute significantly to fundamental mechanical products such as guide way of machine tool to achieve intended functionalities, the mathematical model of flatness error minimum zone solution is formulated and an improved genetic algorithm (IGA) is proposed to implement flatness error minimum zone evaluation. Then, two evaluation methods of flatness error uncertainty are proposed, which are based on the Guide to the Expression of Uncertainty in Measurement (GUM) and a Monte Carlo Method (MCM). The calculating formula and the propagation coefficients of each element and correlation coefficients based on GUM and the procedures based on MCM are developed. Finally, two examples are listed to prove the effectiveness of the proposed method. An investigation into the source and effects of different uncertainty contributors for practical measurement on CMM is carried out and the uncertainty contributors significant are analyzed for flatness error verification. Compared with conventional methods, the proposed method not only has the advantages of simple algorithm, good flexibility, more efficiency and accuracy, but also guarantees the minimum zone solution specified in the ISO/1101 standard. Furthermore, it accords with the requirement of the new generation GPS standard which the measurement uncertainty characterizing the reliability of the results is given together. And it is also extended to other form errors evaluation and verification.     

A kinematic model for machine tool accuracy characterisation

A method for characterising machine tool accuracy is presented in this paper, using a compact quasi-static error model of a three-axis horizontal machining centre. Rigid-body kinematics and homogeneous coordinate transformation were used in this model. A metrology pallet was designed so that coefficents in the error model can be updated periodically. Ten measurement points were allocated on two perpendicular planes of the metrology pallet. One of the measurement planes was on the work table and the other was perpendicular to the table. Two different length touch trigger probes were required for error measurements. Establishing or updating the error model can be carried out in-process without disturbing the workpiece setup. The proposed compact error model is an improvement over past work for a production environment because of its robustness and convenient calibration procedure. The error model was tested in two dimensions and a good error prediction capability was observed in warm machine states.     

基于灰自助最大熵法的机床加工误差的调整

基于灰自助最大熵法,进行了机械制造工艺中输出的误差分布及机床加工误差调整研究。在分析误差分布时,首先运用自助法对当前少的信息量进行自助再抽样得到大量样本数据,然后利用灰色系统理论建立误差的灰自助动态预报模型GBM(1,1),最后用最大熵法获得了输出误差的概率分布。根据输出误差的概率分布,对机床的加工误差进行调整,使加工的产品误差满足要求。计算机仿真和实际案例的研究表明,灰自助最大熵法能够对机床进行准确调整,且预报准确率高。     

点光源测头光束方向的标定

指出在采用点光源测头测量物面测点坐标时 ,因光束方向相对于机床坐标系的安装偏角而引起的测量误差问题。提出了一种标定光束方向的方法。设计 V形块做标定的靶标平面 ,通过在 V形块随各坐标轴做等距移动时采样 ,实现了点光源测头光束方向的标定。给出了标定光束方向后的测点坐标计算方法。并设计了仿真程序验证了该方法的可行性。仿真结果表明 ,当测头精度为 0 .0 1m m,光栅尺对机床位移的检测精度为 0 .0 0 2 mm时 ,光束方向的标定误差可以控制在 0 .1mrad的水平