Stylus tip envelop method: corrected measured point determination in high definition coordinate metrology

Scanning technology is now widely used in coordinate metrology and is increasingly used for the measurement of small features. The high density scans now possible results in a new context which can be called high definition coordinate metrology. The availability of such high point density allows the proposition of a new approach to the old problem of probe ball tip radius correction based on the successive positions of the probe tip envelop. The resulting outer boundary is made of a succession of arcs, each of which potentially defining a likely contact point with the measured surface. The method aims at selecting the most likely contact point for each successive arc by applying geometrical criteria and a fuzzy logic estimator. The method is particularly, but not exclusively, suitable for the metrology of features with small radii as well as metrological discontinuities. It does not require obtaining the normal vector traditionally used for probe tip correction. The algorithm performance was verified experimentally on a Mitutoyo Legex 910 CMM equipped with a MPP-300 scanning probe.     

NON-CONTACT MEASUREMENT OF SCULPTURED SURFACE OF ROTATION

A method for measuring the sculptured surface of rotation by using coordinate measuring machine (CMM) and rotary table is proposed. The measurement is realized during the continuous rotation of the workpiece mounted on the rotary table while the probe moves along the generatrix of the surface step by step. This method possesses lots of advantages such as simplicity of probe motion, high reliability and efficiency. Some key techniques including calibration of the effective radius of the probing system, determination of the position of axis of rotation, auto-centering of the workpiece, data processing algorithm, are discussed. Approaches for determining the coordinates on measured surface, establishing workpiece coordinate system and surface fitting are presented in detail. The method can be used with contact or non-contact probes. Some fragile ceramic and plaster parts are measured by using the system consisting of a CMM, rotary table, motorized head and non-contact laser triangulation probe. The measurin     

The selection of radius correction method in the case of coordinate measurements applicable for turbine blades

The objective of the research presented in the paper is the selection of suitable probe radius correction method in the case of coordinate measurements which can be applied for turbine blades. The investigations are based on theoretical analysis of geometric data and on further computer simulation of measurements and data processing. In the paper two methods for computing coordinates of corrected measured points are verified. Those so-called local methods of probe radius correction are based on Bézier curves. They are dedicated first of all to coordinate measurements of free-form surfaces which are characterized by big values of curvature, e.g. those surrounding the leading and trailing edges of a turbine blade. Numerical simulations are done for several models of the cross-sections of turbine blades with diversified magnitudes of radii of curvature. There are considered both manufacturing deviations and coordinate measurement uncertainty of each examined profile of a turbine blade. Numerical investigations based on the developed analytical models show the advantage of the algorithm which uses the second degree Bézier curves in probe radius correction. Moreover, in the paper there is explained the implementation of the developed algorithms for probe radius correction into the standard CMM software which amplifies the usability of the algorithms.     

双螺杆压缩机转子齿形三坐标测量的半径补偿

双螺杆压缩机转子齿形进行三坐标测量机测量时,需要对球形测头半径进行半径补偿.结合转子螺旋曲面的几何特点,对转子型线测量数据点进行半径补偿计算,得到三坐标测量机球形测头真实接触点,再将空间分布的真实接触点旋转到相同横截面上得到转子型线数据点.通过实例验证该方法的有效性,可以应用于双螺杆压缩机转子型线的检测及逆向工程等工作中.     

Measurement of form error of a probe tip ball for coordinate measuring machine (CMM) using a rotating reference sphere

This study presents a method of measurement of the form error of the tip ball in the tactile probing systems of a coordinate measuring machine (CMM) by using a rotating reference sphere. The measurement of the form error of the CMM probe tip was conducted without the use of additional external measuring instruments or sensors. The form errors of the probe tip ball and the reference sphere were separated from the probing coordinates of CMM by rotation of the reference sphere. The effectiveness of the proposed method was evaluated based on an uncertainty analysis. The uncertainty in measurement of diameter of the probe tip ball was estimated to be less than 0.5 μm.     

Measuring large 3D structures using four portable tracking laser interferometers

A new concept that allows measuring 1D–3D objects in the range of several centimeters to 5 m × 5 m × 5 m is presented. In general terms the concept can be seen as a task specific correction of geometrical errors of coordinate measuring machines (CMMs). The developed system comprises a commercial CMM, its measurement and its evaluation software and a set of at least four high accurate tracking laser interferometers. The CMM is simply used as a mover which allows to capture points on the surface of a measuring object. In parallel the tracking laser interferometers follow a retro-reflector located close to the stylus tip of the tactile probe of the CMM. Based on a multi-lateration algorithm 3D-positions are calculated from the measured interferometric distances. Finally, two sets of coordinates emerged, namely, one by the CMM and the second from the metrological frame of the tracking laser interferometers. The interferometrically measured positions are usually more precise than the positions measured by the CMM. This is due to the high accuracy of the interferometric system and also due to the fact that the measurement positions are taken in a manner which almost avoids Abbe errors. Because of that, the measurement positions of the CMM are substituted with the more accurate measurement points calculated from distance measurements of the tracking interferometers. The position coordinates thus obtained are used for the further computerized evaluations, which yield the geometric parameters of the object measured. First measurements under laboratory condition show very promising results. It has been demonstrated that the concept is suitable for the high precision calibration of large workpieces with small tolerances, for instance, for the calibration of large gears for the windmill industry.     

Aspects of tactile probing on the micro scale

This paper discusses the aspects that influence the interaction between a probe tip and a work piece during tactile probing in a coordinate measuring machine (CMM). Measurement instruments are sensitive to more than one physical quantity. When measuring the topography of a work piece, the measurement result will therefore always be influenced by the environment and (local) variations in the work piece itself. A mechanical probe will respond to both topography and changes in the mechanical properties of the surface, e.g. the Young's modulus and hardness. An optical probe is influenced by the reflectivity and optical constants of the work piece, a scanning tunneling microscope (STM) responds to the electrical properties of the work piece and so on (Franks, 1991 [1]).The trend of component miniaturization results in a need for 3-dimensional characterization of micrometer sized features to nanometer accuracy. As the scale of the measurement decreases, the problems associated with the surfaceprobe interactions become increasingly apparent (Leach et al., 2001 [2]). The aspects of the interaction that are discussed include the deformation of probe tip and work piece during contact, surface forces during single point probing and scanning, dynamic excitation of the probe, synchronization errors, microfriction, tip rotations, finite stiffness effects, mechanical filtering, anisotropic stiffness, thermal effects and probe repeatability.These aspects are investigated using the Gannen XP 3D tactile probing system developed by Xpress Precision Engineering using modeling and experimental verification of the effects. The Gannen XP suspension consists of three slender rods with integrated piezo resistive strain gauges. The deformation of the slender rods is measured using the strain gauges and is a measure for the deflection of the probe tip. It is shown that the standard deviation in repeatability is 2 nm in any direction and over the whole measurement range of the probe. Finally, this probe has an isotropic stiffness of 480 N/m and a moving mass below 25 mg.     

光学非球面三坐标测量中的像散补偿

利用三坐标测量仪在光学非球面镜研磨与粗抛阶段进行面形检测时,测量结果常由于补偿程序不完善而出现像散误差。本文分析了非球面三坐标测量得到的数据,指出测量结果中出现像散误差是测头半径补偿不准确所致。然后,提出了一种离线数据处理方法对测量数据进行补偿来消除像散误差。该方法通过计算网格排列的测头中心点行和列方向的切向量得出曲面上每个点的法向矢量;根据测头半径计算出测头球心到接触点的偏移量,从而实现三坐标测量仪的三维测头半径补偿。球面样板实验显示这种方法可以将该样板测量中的像散峰谷值(PV)由4.921 9μm减小到0.065 2μm,基本消除了测量结果中的像散误差,提高了三坐标测量结果的准确度。实验结果验证了提出的三维测头半径补偿程序的有效性。     

Influence of measured objects parameters on CMM touch trigger probe accuracy of probing

Influence of the measured objects parameters on coordinate measuring machine (CMM) touch trigger probe inaccuracy of probing is investigated. Factors such as material stiffness of measured workpiece, its surface shape and roughness are taken into consideration. The measuring method out off CMM, based on a rotary table and a reference axis is applied for tests. The concept is based on measurements of the distance between reference and triggering points in various directions. The reference points are established by the rotation of a precise rotary table. The statistical analysis of variance and regression is applied for data analysis.     

Calculation of the virtual pitch thread diameter using the cloud of points from CMM

This paper presents a new calculation procedure of the virtual pitch thread diameter using the cloud of points from coordinate measuring machines (CMM). The procedure involves scanning of thread profiles using a CMM, calculation of points sets belonging to the left and right thread flanks, next determination of the thread profile which belongs to the adjoining helical surface. Its parameters guarantee the required diameter calculation. Estimation of this diameter comparing it with the standard one measured by CMM “Lapik KIM-1000” and the theoretical maximum pitch thread diameter shows that this approach gives a reliable result.     

Metrological feasibilities of CMM touch trigger probes. Part I: 3D theoretical model of probe pretravel

New 3D model of the inaccuracy of a touch trigger probe used to collect coordinates of a measured object by coordinate measuring machine (CMM) is proposed. The analysis is performed both for one and two stage types of probes. The influence of a stylus length and diameter, a spring force and direction of a contact point approach are taken into consideration. The effects of frictional interaction between the stylus ball and a measured surface are also taken into consideration. The theoretical analysis of a tough trigger probe hysteresis is presented. Finally a computer simulation of mathematical modeling in the XYZ space is shown.     

The hybrid contact-optical coordinate measuring system

This paper presents a hybrid contact-optical coordinate measuring system which was designed to measure and analyze the geometry of objects. It combines the accuracy of contact measurement regarding the identification of measurement points and the speed of full-field structured light optical scanning methods. The measurement process starts from fast optical acquisition by structured light system(s), positioned around the working space, then numerical analysis is performed to calculate a set of surface points that should be finally re-measured by the CMM. Both set-ups work in a common coordinate system created according to existing metrological strategies. Final metrological analysis is performed using classic, certified metrological software. In this paper we also describe the maximum permissible error of length measurement of the hybrid system.     

Evaluation of a multi-sensor horizontal dual arm Coordinate Measuring Machine for automotive dimensional inspection

Multi-sensor coordinate measuring machines (CMM) have a potential performance advantage over existing CMM systems by offering the accuracy of a touch trigger probe with the speed of a laser scanner. Before these systems can be used, it is important that both random and systematic errors are evaluated within the context of its intended application. At present, the performance of a multi-sensor CMM, particularly of the laser scanner, has not been evaluated within an automotive environment. This study used a full-scale CNC machined physical representation of a sheet metal vehicle body to evaluate the measurement agreement and repeatability of critical surface points using a multi-sensor horizontal dual arm CMM. It was found that there were errors between CMM arms and with regard to part coordinate frame construction when using the different probing systems. However, the most significant effect upon measurement error was the spatial location of the surface feature. Therefore, for each feature on an automotive assembly, measurement agreement and repeatability has to be individually determined to access its acceptability for measurement with a laser scanner to improve CMM utilisation, or whether the accuracy of a touch trigger probe is required.     

Mapping and correction of the CMM workspace error with the use of an electronic gyroscope and neural networks—practical application

The article presents the application of neural networks in determining and correction of the deformation of a coordinate measuring machine (CMM) workspace. The information about the CMM errors is acquired using an ADXRS401 electronic gyroscope. A test device (PS‐20 module) was built and integrated with a commercial measurement system based on the SP25M passive scanning probe and with a PH10M module (Renishaw). The proposed solution was tested on a Kemco 600 CMM and on a DEA Global Clima CMM. In the former case, correction of the CMM errors was performed using the source code of WinIOS software owned by The Institute of Advanced Manufacturing Technology, Cracow, Poland and in the latter on an external PC. Optimum parameters of full and simplified mapping of a given layer of the CMM workspace were determined for practical applications. The proposed method can be employed for the interim check (ISO 10360–2 procedure) or to detect local CMM deformations, occurring when the CMM works at high scanning speeds (>20 mm/s). SCANNING 35: 222‐231, 2013. © 2012 Wiley Periodicals, Inc.     

Problems with software for running coordinate measuring machines: The use of virtual volumetric standards

The validation of software for coordinate measuring machines is increasinly being seen to be as important as the assessment of the machine construction and its coordinate system. Virtual volumetric standards are the coordinates of points on the surface of an imaginary workpiece which can be used to evaluate cmm software. This paper gives some examples of virtual volumetric standards and their use.     

Ball tip–stylus tilt correction for a stylus profilometer

Ball tip and stylus tilt are two error sources found in coordinate measurement machines (CMMs). The magnitude of these errors is dependent on the ball tip radius and the degree of misalignment between the stylus motion axis and lateral referencing plane determined by the x and y axes of the CMM used. It is shown that these two errors are actually coupled and that their correction can be accomplished in one process. The overall benefit and versatility of this correction routine on a more general basis are given.     

Usability assessment of selected methods of optimization for some measurement task in coordinate measurement technique

This paper presents the comparison of selected methods of optimization on the example of some measurement task in coordinate measurement technique. The task consists in the computation of coordinates of the center and radius of a circle which approximates some set of measured points using least squares. The application of the least squares method follows an approach used in conventional analysis of an associated element in coordinate measurement technique. The coordinates of points are measured on a CNC machine tool equipped with a measuring probe.     

<Emphasis Type="Bold">Determination of the Optimal Parameters for Freeform Surface Measurement and Data Processing in Reverse Engineering</Emphasis>

One objective of this work is to determine the optimal combination of the probe diameter and grid distance for freeform surface measurement, and another is to determine the optimal parameters for the local Shepard interpolation. The optimal combination of the probe diameter and grid distance for freeform surface measurement was determined through a Taguchi matrix experiment. The smaller the probe diameter and grid distance, the better the accuracy of the surface normal based on the configured matrix experimental result. The optimal parameters, namely the exponent and the radius R, for the local Shepard interpolation were determined by using the minimisation method of the root-mean-square normalised error (RMSNE) between the measured data points and the theoretical data points on a standard steel ball surface. The optimal parameters determined were actually applied to the measurement of a freeform surface (mouse surface) on a coordinate measuring machine (CMM). The local Shepard interpolation method was used to interpolate 16 control points from 1054 measured data points. Bi-cubic Bezier- and B-spline surface CAD models were constructed through these interpolated control points.     

三坐标测量机驱动的摄像机标定技术

为了降低摄像机的标定费用和提高标定精度,提出了一种基于三坐标测量机平动的摄像机标定技术,对该技术的原理、数学模型、标定步骤和标定精度进行了研究。根据摄像机标定的基本原理,利用三坐标测量机沿X、Y、Z轴移动和正交精度都很高的特点,以三坐标测量机带动待标定摄像机产生和白色陶瓷标准球球心(标定特征点)在X、Y、Z轴方向上的相对平动,得到标定特征点在测头坐标系中不同位置的坐标;在每一个确定位置,摄像机拍摄标定特征点的像,经图像处理后,计算出像点在计算机帧存坐标系中的坐标。引入测头坐标系,建立了该项摄像机标定技术的数学模型,给出了标定步骤,组建了标定系统。比对标定实验结果表明,基于三坐标测量机平动的摄像机标定系统与专用标定系统的标定精度相当,数据相差在±1μm以内,满足工程实际精度要求,成本低、标定效率高。     

三坐标测量机在高速测量过程中的动态误差研究

本文研究了三坐标测量机在高速测量过程中的动态误差。以一台移动桥式三坐标测量机为例,分析了其动态误差的产生原因,建立了其动态误差的数学模型,并对动态误差进行了全面的测量和补偿。实验证明,三坐标测量机的动态误差具有一定的重复性,可用软件补偿,从而提高三坐标测量机快速测量的精度。