Kinematic and geometric error compensation of a coordinate measuring machine

In this paper, kinematic modelling of a Coordinate Measuring Machine (CMM) is carried out and the methodology followed in modelling is explained in detail. The model is simplified by certain assumptions which may result in over-simplification of the model. Consequently, the model is investigated and enhanced by adding the relevant and suitable geometric error terms. Different approaches are employed to evaluate the model coefficients. In the first approach, a commercial ring gauge is measured in a structured lattice in the work volume of the CMM. Resulting errors in these measurements are used in conjunction with some statistical methods to arrive at sets of model coefficients values. The second approach is based on measurement of the individual 21 error terms in the CMM by means of laser interferometry. These measurements are used to evaluate another set of model coefficients. A compensation strategy is proposed and tested using the model and the sets of coefficients obtained. Volumetric Performance of the CMM is evaluated according to ASME standards, before and after compensation. Improvement in the CMM volumetric performance is demonstrated and compared.     

Geometric and quasi-static thermal error compensation for a laser digitizer equipped coordinate measuring machine

An emerging trend in dimensional inspection of manufactured parts is the use of coordinate measuring machines (CMMs) equipped with non-contact laser digitizers instead of traditional contact probes. Error mapping is generally proprietary to the CMM controller, and the accuracy of such a combined system is limited by the lack of integrated CMM/laser digitizer error compensation. This paper reports progress towards achieving such a system.Geometric error compensation measurements are made on a test machine at 20, 25 and 30 °C. Performance of the system was tested by measuring a special optically coated sphere ballbar artifact. The scan coordinates, the CMM axis scale positions and temperature were recorded simultaneously in real time. The CMM error compensation data was then used to post process the laser digitizer coordinates to obtain improved global part coordinates.Inclusion of the error compensation terms is shown to reduce the dispersion of results by 65 to 80%.     

An efficient error compensation system for CNC multi-axis machines

To enhance a machine’s ability for a precision machining task, an efficient compensation scheme that takes all error sources into account is needed. In this study, an effective static/quasi-static error compensation system composed of an interpolation algorithm based on shape functions for error prediction, and a recursive software compensation procedure, was developed. The new error prediction scheme is efficient enough to be implemented on-line, taking into consideration the fact that the machine structure is non-rigid. In addition, an automatic NC code converting software was also developed so that the developed system could be applied to practical machining for CNC multi-axis machines. To verify the algorithm and system, cutting experiments were conducted, and the results have shown the success of the proposed error compensation system.     

Accuracy enhancement of five-axis CNC machines through real-time error compensation

Although error modeling and compensation have given significant results for three-axis CNC machine tools, a few barriers have prevented this promising technique from being applied in five-axis CNC machine tools. One crucial barrier is the difficulty of measuring or identifying link errors in the rotary block of five-axis CNC machine tools. The error model is thus not fully known. To overcome this, the 3D probe-ball and spherical test method are successfully developed to measure and estimate these unknown link errors. Based on the identified error model, real-time error compensation methods for the five-axis CNC machine tool are investigated. The proposed model-based error compensation method is simple enough to implement in real time. Problems associated with the error compensation in singular position of the five-axis machine tool are also discussed. Experimental results show that the overall position accuracy of the five-axis CNC machine tool can be improved dramatically.     

基于CORBA多关节测量机接口技术研究

网络制造要求测量系统和设备具有高度柔性 ,可以实现与系统内外进行数据通讯与信息集成。传统笛卡尔坐标机不可移动的安装使其在网络制造中的使用受到限制。多关节测量机具有体积小、可便携的特点 ,成为网络制造中潜在的主要测量设备。本文提出可便携基于CORBA多关节测量机的新概念 ,通过研究基于CORBA(CommonObjectRequestBrokerArchitecture)的DMIS (DimensionalMeasure mentInterfaceStandard)接口 ,并把DMIS接口与传统的多关节测量机系统相结合 ,形成面向网络制造的多关节测量机系统 ,为在网络制造模式下坐标机的集成和通讯问题进行了初步研究和探索     

动平衡机系统误差分析及标定方法研究

在动平衡机刚性转子动力学建模的基础上,以统一的矩阵形式给出目前动平衡机存在的四种标定方法-永久标定法、影响系数法、复影响系数法以及多元线性回归法的理论公式,分析了它们对平衡机机械支承系统和测量系统的要求以及对系统误差的补偿能力,指出动平衡机机械支承系统和测量系统的频响特性对动平衡机的标定有较大影响,并对其优缺点进行了对比分析,指出它们的使用前提、应用场合以及目前动平衡机系统误差研究的不足.     

Application of homogenous transformation matrix to measurement of cam profiles on coordinate measuring machines

A kinematic model is presented to aid the cam profile design task and to determine the NC data required to move an inspection probe to the necessary locations to inspect the surface profile of a spatial cam on a coordinate measuring machine (CMM). The cam profile is designed in accordance with the principles of conjugate surface theory. Homogenous transformation matrices are then employed to derive the CMM ability function matrix and the measuring probe location matrix. The required NC data are obtained by equating the elements in the ability function matrix with the corresponding elements in the probe location matrix in order to solve the CMM link variables. The validity of the proposed approach is verified by carrying out the automatic inspection of a machined cam on a CMM using the NC data computed by the developed algorithm. The methodology presented in this study combines the cam design, machining and inspection activities, and makes possible a flexible and automatic spatial cam design and manufacturing process.     

Selection of sampling points for accurate evaluation of flatness error using coordinate measuring machine

Dimensional inspection, in integrated manufacturing environments, requires accurate inspection while minimizing the cost and time of inspection. The selection of sampling plan—sample size and sample point locations, the method of evaluating the form error and the nature of the manufactured surfaces will play an important role in deciding the best inspection strategy to be adopted. This paper deals with the strategy for evaluation of flatness error which is one of the most commonly used form tolerances for control of manufactured surfaces. Investigations have been carried out to ascertain the influence of surface quality (surface roughness) in determining the sampling strategy for accurate determination of flatness error while inspecting on a coordinate measuring machine (CMM). The sampling plan utilizes the Hammersley sequence for point location and the flatness error is evaluated using the minimum zone method (MZM) based on computational geometry techniques. Results indicate that the surface roughness influences the accuracy of inspection and can be used as a parameter for determining an initial sample size for the determination of flatness error.     

Selection of an optimum sample size for flatness error estimation while using coordinate measuring machine

In the present day manufacturing environments it is becoming increasingly important to be able to deliver quality products at the right time to the market at competitive costs. The quality, cost and time to market depend not only on the design and manufacturing but also on the inspection process adopted. Design specifications rely on extensive usage of form tolerances to ensure that the functionality of surfaces and features of the product are maximized. The use of the coordinate measuring machines (CMM) has greatly improved the efficacy of form tolerance measurement and is also used as the key device in this work. The focus of this work is to deal with the method and strategies for measurement of flatness error so as to be able to predict the flatness error accurately at reduced sample sizes in batch and mass production setups. Accurate evaluation of flatness will require large sample sizes which increase the cost and time of inspection and hence a need to reduce the sample sizes without compromising on the accuracy. In the absence of robust models that can predict the errors due to manufacturing processes, an alternative technique has been devised to arrive at a reduced sample size. The procedure involves using large sample data inspected on the first component as the basis for evolving smaller sample sizes for subsequent components.Experimental verification of the developed algorithm shows that flatness error can be predicted with sufficient accuracy at small sample sizes.     

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.     

Active compensation for portal machines

The cutting performance of a machine tool is mainly determined by its static and dynamic behavior. During a cutting process high static and dynamic forces may occur, which result in instabilities of the cutting process. Especially, long protruding structure components, such as z-slider of a portal machine, represent a distinct weak spot concerning their structural behavior. The structure integrated compensation modules represent a new approach to compensate static and dynamic deformations of slider structures and help to achieve a considerable improvement of their compliancy behavior.     

建立坐标测量机计量标准方案的探讨

依据计量标准考核的要求,通过阐述计量标准的工作原理,不确定度的评定并进行重复性、稳定性指标和不确定度的验证考核,建立坐标测量机量值传递的标准器组。     

Towards a haptic virtual coordinate measuring machine

In this paper, a novel coordinate measuring machine (CMM) inspection path planning environment, named haptic virtual coordinate measuring machine (HVCMM), is proposed, which makes use of haptic modeling technique for CMM off-line programming. HVCMM is an accurate model of real CMM, simulating a CMM’s operation and its measurement process in a virtual environment with haptic perception. It enables CMM off-line programming to take place exactly as if an operator were in front of a real CMM and moving a real CMM probe. To generate the inspection path of a part, teach pendant programming is made by pointing a probe at the 3D computer-aided design (CAD) model of a part using a haptic device. With the HVCMM system, it is much easier to generate collision-free probe path than using other off-line inspection planning methods. Furthermore, when a contact occurs between the CMM and the object, a force is generated based on a proposed mechanics model besides visually showing the contact in the HVCMM environment. Surface voxel is used for quick collision detection. The HVCMM not only facilitates the inspection path planning, but also speeds it up. The proposed HVCMM can also be used for the training of CMM operation.     

Direct volumetric error evaluation for multi-axis machines

A generalized methodology, referred to as direct error analysis, for the evaluation of the position and orientation errors in the workspace of a multi-axis machine of arbitrary configuration was formulated based on two important new concepts. First, a modified coordinate frame assignment rule was defined. This rule offers the ability to assign the origins of the joint frames at the physical joint so that the corresponding homogeneous transformation matrices can overcome the problem with the conventional Denavit-Hartenberg rule with respect to small variations in link geometry. Second, the concepts of fixed and moving error reference frames for kinematic error motions were introduced. Depending on the joint configuration these frames facilitate the representation of the true kinematic error motions at both ends of the physical link.     

基于PC-DMIS的三坐标测量机测量距离评价的探讨

介绍了PC-DMIS软件中常用的三种距离评价方式以及实际测量中的选用原则,结合实例,简述了三坐标测量距离的不确定度评定。     

Position error compensation of robotic contour end-milling

Three-dimensional cutting force dynamics of end-milling is developed, and position error sources of robotic machining is studied. A strategy to compensate cutting force in force/motion control of robotic machining is established in order to achieve better positioning accuracy. Position error is analysed in the cutting process through reference points by using force sensor. The tip position error estimated with the aid of wrist-force sensor is compensated in feedforward control form. The quantitative results are obtained from the digital computer simulations of the dynamic model of the studied system.     

凸轮轴磨削的误差补偿新研究

介绍了凸轮轴磨削的数学模型，提出了当量磨削厚度误差补偿技术，对运动模型进行修正，以保证磨削过程中任意时刻的当量磨削厚度相等。首先由误差补偿模型得出叠加于砂轮架上的位移修正量，再将这一修正量通过砂轮架与头架的联动关系叠加到头架的角速度修正量，从而实现了恒金属去除率磨削，提高了凸轮轮廓的加工精度。     

用坐标变换计算“一面两孔”的定位误差

用坐变换的方法对工件一面两孔定位时的定位误差进行了合理地分析计算，给出了定位误差对工序尺寸的函数关系式。     

Small-wavelength form error compensation during hydrodynamic polishing

This paper proposes a process control strategy for removing small-wavelength form error during hydrodynamic polishing, that is, by planning a tool dwelling time appropriate to accurately remove the form error. Volume removal analysis suggests that removal of an arbitrary error profile requires the tool to dwell at a given position for a period that is a linear function of the error profile. The dwelling-time distribution of the tool is solved by the non-negative least squares method. The residual error between the actual and desired profile is induced by this strategy. Residual error occurs mainly at the peaks and valleys of the profile, in addition to the boundaries of the machining area. Results indicate that the dominant factors in deciding residual error are the size of the machining zone, tool step, and wavelength and amplitude of the error profile. It is shown that larger residual error occurs in bands with wavelengths smaller than the machining zone, and vice versa. If the wavelength is sufficiently large, a small tool step effectively reduces the residual error. Furthermore, large variations in the amplitude of the error profile can be effectively reduced when the wavelength is large. Experimental results confirm that the proposed polishing strategy can remove an arbitrary profile and automatically reduce the small-wavelength errors. However, it is not effective when the wavelength of the error profile is near the size of the machining zone.     

Applications of 3-D solid modelling to coordinate measuring inspection

This paper describes a set of algorithms for the automation of component inspection using coordinate measuring machines. Solid modelling techniques are used to describe the component, and this model is then used to generate a set of inspection points and a collision-free probe path. The model is also used in the analysis of the resulting measurements. The algorithms have been implemented in a system that will handle the range of form, location and attitude tolerances, as defined by BS 308, for planar, cylindrical, conical and spherical features.