Efficient inspection planning for coordinate measuring machines

The coordinate measuring machine (CMM) has been recognized as a powerful tool for dimensional and geometric tolerance inspection in the manufacturing industry. The power of the CMM depends heavily on an efficient inspection plan that measures a part in minimal time. This paper proposes CMM inspection planning that can minimize the number of part setups and probe orientations and the inspection feature sequence. In our planning, a greedy heuristic method is adopted to obtain the minimal number of part setups and probe changes. Meanwhile, a continuous Hopfield neural network is developed to solve the inspection feature-sequencing problem. The proposed method was successfully implemented and tested using a machine spindle cover part. The results show that the proposed method can achieve excellent performance compared to the other methods.     

基于多传感器集成的三坐标测量机自动检测规划系统研究

为了充分发挥多传感器集成的优势，提高检测自动化水平，提出了该环境下三坐标测量机自动检测规划系统的体系结构。构建了专家规则库来制定检测策略，选择检测设备。重点分析了接触式测量检测规划的流程，提供了零件定位、测点自动生成等模块的解决方案。讨论了光学自动检测规划的要点。通过中性文件的使用和映射，实现了CAD与三坐标测量系统的集成，从而为产品的自动检测和质量控制提供了保障。     

基于坐标测量机的自由曲面检测采样策略

为了实现自由曲面快速精确的检测和评定,研究了检测过程涉及的采样策略。提出采用随机Hamersley采样、简单随机采样和列系统采样3种方法设置自由曲面上的测量点。采用拟粒子群算法优化变换参数,实现被测曲面与设计曲面之间的定位,采用曲面细分法搜索设计模型上与被测点距离最近点以计算自由曲面轮廓度误差。为了对基于坐标测量机(CMM)在不同采样策略下的检测结果进行比较,采用非均匀有理B样条生成零件设计模型,选择相同的加工工艺在不同的加工中心加工多个零件得到粗糙度不同的自由曲面。最后,使用CMM在所设计的采样策略下实测零件,用提出的方法计算零件的轮廓度误差,并与CMM内置软件的计算结果进行比较。实验结果显示:选择随机Hammersley采样法和中等采样点数的采样策略检测效果最佳,不仅测量精度高,而且耗时少、成本低。应用提出的方法计算自由曲面轮廓度误差时,其精度比由CMM内置软件计算结果高10%~22%,表明提出的方法适用于自由曲面零件轮廓度误差的快速精确检测与评定。     

A novel artifact for testing large coordinate measuring machines

We present a high-accuracy artifact useful for the evaluation of large CMMs. This artifact can be physically probed by the CMM in contrast to conventional techniques that use such purely optical methods as laser interferometers. The system can be used over large distances; for example, over 4 meters, with an uncertainty of less than one part per million. The artifact is relatively inexpensive, robust for use in reasonable industrial environments, and significantly reduces testing time over traditional step gauge measurements.     

Calibration of coordinate measuring machines

For over three years the Physikalisch-Technische Bundesanstalt has been investigating the numerical error correction of cmms, and the relationships between problems in this area and in the calibration of these machines. It has been determined that calibration is possible, for pre-defined conditions, but that the calibration is applicable only to the particular measuring task and conditions. This paper describes the mathematical model used by PTB and gives results of tests on a cmm.     

A two-dimensional test body for calibration of coordinate measuring machines

A two-dimensional test body having cross-aligned hollow cones has been developed for automatic calibration of coordinate measuring machines (CMMs). Two hours are sufficient for the measurement of translatory and rectangularity errors of a CMM by means of the test body. In this paper the principles of measurement of such erros using the test body are described These principles can be utilized on the assumption that one knows the exact values of the test body, so the calibration procedures for the test body are also described.     

Development of a fast mechanical probe for coordinate measuring machines

To decrease inspection times of workpieces, not only do coordinate measuring machines (CMMs) need to operate faster, but also the mechanical probing process requires attention in this field. This paper shows that impact forces attributable to probing are much higher than generally accepted measurement forces, which can result in workpiece damage. Furthermore, it is shown that probe tip bouncing can slow down the probing process and requires mechanical damping to reduce it. Based upon these findings, a fast mechanical probe system has been developed, equipped with an optical measurement system able to measure six degrees of freedom at high speed. The probe system is suitable for high-speed, single-point measurements as well as scanning purposes. The measurement uncertainty of the prototype is approximately 1 μm for probing speeds up to 70 mm/s.     

Calibration-based thermal error model for articulated arm coordinate measuring machines

The influence of thermal changes in the accuracy of articulated arm coordinate measuring machines (AACMMs) is one of their main error sources. Usually, the effects of this problem are minimized by using low thermal-expansion materials in the arm design or by implementing an empirical error correction model based on the outputs of several temperature sensors placed inside the arm. These models, inherited from temperature correction models for robot arms, are based on the empirical arm positioning error characterization of several temperatures inside its measuring range and by posterior corrections implemented using an average error approximation. Considering this approximation as a correction valid for all the temperature range violates the AACMM calibration conditions, which are defined by a kinematic parameter identification procedure at 20 °C. This implies that, by applying these models, the AACMM will work outside of calibration conditions and will not meet the nominal accuracy values obtained from the identification procedure. In this work a new empirical correction model for thermal errors is presented. This model keeps the calibration conditions established in the parameters identification procedure unaltered. This fact makes it possible to apply other correction models for geometric and non-geometric errors obtained at the calibration temperature after having applied the temperature correction. The algorithm and objective functions used, which are based on a new approach including terms regarding measurement accuracy and repeatability and using the measurements of a ball-beam gauge artifact are shown. The empirical correction model and the arm accuracy and repeatability results before and after correction are also explained, showing an important accuracy improvement in the arm performance for typical arm operation at temperatures different from 20 °C.     

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.     

Factors affecting component measurement on coordinate measuring machines

It can often be difficult to find a complete relationship between the specified performance of a coordinate measuring machine (cmm) (see Fig 1) and actual results obtained, because of unexplained differences between them. This report explain one method of identifying these differences and discusses some sources of error which may give rise to them.     

Modeling of the residual kinematic errors of coordinate measuring machines using LaserTracer system

It is complex and divergent for the conventional motion controllers to process various G codes using different interpolation algorithms. This impairs programming efficiency and robustness of the controller. In this paper, we propose the universal non-uniform rational B-splines (NURBS)-based interpolator which can simplify the architecture of interpolation in spite of interpreting different kinds of inputs. Direct conversion of long G01 and G02/G03 numerical control (NC) segments to NURBS segment is first implemented. The fitting of multiple short segments into a continuous and smooth NURBS segment is then carried out. More importantly, the universal NURBS-based interpolator utilizes the Cox–de Boor algorithm which is highly efficient and can take advantage of the parallel computing scheme to accelerate the processing speed. Furthermore, due to the construction of real-time environment, the proposed algorithm enables interpretation, look-ahead functions, and motion control to work simultaneously. A 2D NC program possessing hundreds of short G01 segments and long segments (i.e., G02, G03, and long G01) is tested on an in-house developed XY-table with a PC-based motion controller. The results have shown the effectiveness and feasibility of the proposed real-time NURBS-based interpolator.     

Designing small samples for form error estimation with coordinate measuring machines

We propose a method for designing inspection plans of small size for use in form error evaluation of parts from batch or mass production using coordinate measuring machines. The method exploits a priori information coming from large sample inspection of one or a few parts of the lot. It is based on a regression model fitted to the large sample. The model is used twice. First, it provides the dominant pattern of the surface. Then, the deviations from the model that are relevant to form error are captured by weighting the points of the convex-hull of the large sample with the regression residuals. Based on two case studies, we show that the method provides a good accuracy in the estimation of straightness and flatness with very few measurement points. Comparative results also indicate that the method outperforms both the typical sampling schemes used in industry (random sampling, latin hypercube sampling), which do not exploit a priori information, and a recently proposed method using the same kind of a priori information.     

Measuring probe alignment errors on cylindrical coordinate measuring machines

This note describes a method for measuring probe alignment errors on precision cylindrical coordinate measuring machines. Specifically, this method is used to determine the minimum distance between the line of travel of the center of a spherical probe tip and the axis of rotation of a rotary axis. Within the Timken facility, we refer to this error as intersection error [Bryan JB. Private communication; 7 June 1992]. In addition to intersection error, this method determines the position along the probe line of travel at which the center of the probe tip passes nearest to the axis of rotation. This position is commonly referred to as probe offset error. It is used to adjust the probe location such that its radial position is zero where it intersects (or nearly intersects) the axis of rotation. In this way, the probe tip location is datumed to the axis of rotation.     

关节类坐标测量机关键技术及进展

关节类坐标测量机具有便携性好、质量轻、测量范围大等优点,广泛应用于汽车工业、航空航天、模具制造、大型装备装配等领域的质检工作。关节类坐标测量机由6或者7个旋转关节串联而成,靠近基座的关节一般装有力平衡机构,提高了关节类坐标测量机的易操作性;其测角系统由安装在各旋转关节上的高精度角度编码器实现;光学扫描测头应用于关节类坐标测量机上实现了复杂曲面的非接触快速测量;为得到精确的测量模型,研究了多种建模和参数辨识方法并由不同国际组织提出了关节类坐标测量机的性能评价标准。最后介绍了关节类坐标测量机的关键技术和未来发展趋势。     

Research on optimal measurement area of flexible coordinate measuring machines

When we measure a workpiece in the optimal measurement area of the flexible coordinate measuring machines (FCMMs), the measuring precision can be improved. The flexible CMM’s measuring space is given. Then its spatial error distribution model is build by using support vector machine theory. Finally, the two solutions of the optimal measurement area are discussed and solved through experiments. The high precision measurement results can be realized by using a low-precision measuring machine without any increase in hardware manufacturing cost. This is of great theoretical value and practical significance for the flexible CMM’s further development.