An Object Oriented Planner for Inspection of Prismatic Parts – OOPIPP

Coordinate measuring machines (CMMs) have been widely accepted and are increasingly used for carrying out inspections. With the increase in complex designs and tighter tolerances, the inspection process involving a CMM has become critical and there is a need to plan it effectively. In the present work, an object oriented planner for the inspection of prismatic parts (OOPIPP) has been developed. It incorporates interactive feature recognition and carries out all the essential steps of inspection planning, that is, selection of the most stable part orientation, arriving at number and distribution of inspection points, feature accessibility analysis, sequencing of probe orientations, removal of duplicate faces, and, finally, sequencing of faces. Various relationships between different entities of the system and distinct features of OOM have been used to develop the algorithms of different modules. Fuzzy logic for decision making has also been applied and a suitable method of combining fuzzy sets has been used for the selection of part orientation and for sequencing probe orientations.     

基于CMM测量路径优化算法的研究

随着计算机集成制造系统(CIMS)的深入发展,计算机辅助检测工艺规划(CAIP)已成为CIMS中集成质量系统(IQS)的关键环节。本文研究了基于坐标测量机(CMM)的测量路径优化算法,对坐标测量机的测量路径优化问题进行了合理的描述,分析了已有路径优化算法存在的不足,提出并实现了将遗传算法和禁忌搜索算法结合的策略用于测量路径优化的GATS算法,取得了良好的效果。     

A design advisor for the optimal inspection of circularity tolerance

In the wake of growing importance for quality and the need to reduce inspection costs simultaneously, the need for a scientific method of selecting an optimum inspection strategy for coordinate measuring machine (CMM) based inspection has become very important. The inspection error resulting from CMM inspection is greatly affected by the profile irregularities and the sampling strategy, which includes sample size, sampling methods, and algorithms used for form evaluation. This paper describes a system that can recommend an optimal inspection plan based on the needs of the user. A design of experiments (DOE) based approach is used to relate the inspection error with sampling strategies. Surface irregularities are included in the form of lobes formed on the profile. A new two-way model is proposed that works in both directions between the sampling strategy and the performance metrics. The results indicate that the number of lobes and the sampling method used have little impact on the inspection error, while the sample size and form evaluation algorithms have a significant influence. An inspection plan advisor is presented, which provides an inspection plan based on the estimated shape and acceptable measurement error.     

On-machine measurement using a noncontact sensor based on a CAD model

Once a machining process is finished, an inspection process is carried out to check whether the part is within dimensional tolerances. A coordinate measuring machine (CMM) is a general metrological device for assessment of dimensions on the shop floor. It cannot be ignored, however, that CMM measurements require significant resources in operating time and cost, which has led to many studies into on-machine measurement (OMM) systems. This study aims to develop an OMM system with a noncontacting laser displacement sensing apparatus and a computer-aided design (CAD) model for ease of operation, improved operating speed, and free form profiling. The system is composed of two software modules, one for sensor alignment with the machine tool and the other for measurement based on CAD/CAM (computer-aided machining). Consequently, the system was verified on the shop floor at a numerical control (NC) machining center.     

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.     

基于遗传算法的三坐标测量机测量路径规划方法

针对现有三坐标测量机检测路径规划方法的不足,提出和构建了零件检测特征群数学模型和求解流程,在此基础上,根据检测工作平面、检测测头及检测测针变动次数和检测路径构建优化目标函数,从宏观和微观两个层面分别应用矩阵交叉遗传算法和序列规划遗传算法进行零件检测特征路径的优化求解。最后以Hexagon公司检测零件为例,说明了零件初始检测信息的获取以及算法的优化求解过程。实践证明,该方法快速有效,且具有良好的工程应用价值。     

Fast genetic algorithm for roundness evaluation by the minimum zone tolerance (MZT) method

According to ISO 1101, “A geometrical tolerance applied to a feature defines the tolerance zone within which that feature shall be contained”.The main goal of the minimum zone tolerance (MZT) method is to achieve the best estimation of the roundness error, but it is computationally intensive. This paper describes the application of a genetic algorithm (GA) to minimize the computation time in the evaluation of CMM roundness errors of a large cloud of sampled points.Computational experiments have shown that by selecting the optimal GA parameters, namely a combination of the five genetic parameters related to population size, crossover, mutation, stop condition, and search space, the computation time can be reduced by up to one order of magnitude, allowing real-time operation.Optimization has been tested using seven CMM samples, obtained from different machining features. The performance of the optimized algorithm has been validated using four benchmark samples from the literature and with certified samples.     

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.     

IN-PROCESS AND POST-PROCESS QUANTIFICATION OF MACHINING ACCURACY IN CIRCULAR CNC MILLING

ABSTRACT

In-process part inspection using a spindle touch probe has gained a significant importance, mainly because parts can remain on the machine without disrupting the machine setup while inspection is being conducted. This practice leads to a shorter inspection time, improved part accuracies, and reduction of scraps. Recently, intense domestic and international competition has put more importance on part quality in terms of producing parts right the first time and maintaining the consistent quality standards. A literature review revealed that a comparative analysis between in-process gauging using a touch probe and post-process inspection using a coordinate measuring machine (CMM) to ascertain part quality has not been adequately studied. Therefore, there is a need for a study to measure the characteristics of the two inspection techniques. To address the problem, cutting experiments were conducted and measurement data were analyzed using a state-of-the-art CNC machine, a CMM, a touch probe, and a high-precision ballbar system. The experimental data show that machined features and touch probe measurements are affected by the inherent shortcomings in machine tool structure, suggesting a machine tool capability analysis be undertaken in tune with the required tolerance specifications prior to machining operations, rather than solely relying on the touch probe inspection for part quality assessment.     

Measuring external profiles of porous objects using CMM

Coordinate measuring machine (CMM) has been extensively used in surface measurement and inspection. CMM produces more accurate and reliable results compared with non-contact measurement devices, since CMM measures target surfaces in a tactile way and is not affected by surface reflection quality. However, when there is porosity on the outside surfaces, CMM measurement will have errors because part of the probing stylus will come into the porosity spaces. This paper presents a method of using CMM to measure external profiles of objects with external porosity spaces. The center of a CMM probing stylus will be compensated to avoid porosity spaces and located above external surface areas along surface normal directions. Such a probing strategy can be implemented in both computer-aided design (CAD)-guided mode and non-CAD-guided mode. When the CAD model is available, the probing styli are guided to approach the surface along its normal directions successively to identify and avoid porosity spaces. When the CAD model is unavailable, surface normal directions will be estimated first and then adjusted. The presented method is able to avoid porosity spaces in CMM measurement regardless of the availability of CAD models.     

A New Algorithm for CAD-Directed CMM Dimensional Inspection

The recent developments in computer integrated manufacturing (CIM) systems have made the traditional dimensional inspections bottlenecks in the production line. To overcome these bottlenecks, computer integrated dimensional inspection was proposed with the coordinate measuring machine (CMM) being the key device. In this investigation, a framework for integrating manufacturing (CAD/CAM) environment is developed to automate the process of design, manufacturing and inspection. An algorithm to generate an optimum collision-free CMM probe path is proposed. This algorithm uses the ray tracing technique to locate the collision of the possible paths with the workpiece to be inspected, between the initial probe point and the target point. If there is a collision, the algorithm walks through the topological structure of the part and selects the midpoint of the edge shared by the face with which the path collides and the adjacent face nearest to the target point, as the next probe point. This procedure is followed until the target point is reached. The first half of the proposed algorithm is implemented using Mechanical Desktop as the CAD system and AutoCAD Runtime Extension (ARX) as the application programming interface, running on a Windows NT 4.0 platform. The effectiveness of the proposed algorithm is verified by the results of the implementation which demonstrate optimum collision-free dimensional inspection path generation for three prismatic parts.     

基于检测状态模型的飞机结构件在线检测路径规划

针对飞机结构件在线检测路径规划中零件工艺模型伴随加工过程动态变化、检测工艺复杂的难点,提出了基于检测状态模型的飞机结构件在线检测路径规划方法。检测状态模型以特征为载体,以干涉特征和虚路径表达干涉规避、路径优化等知识和经验,并随着加工过程动态扩充。基于检测状态模型,按照检测操作级、检测特征级、特征自身级三级实现了在线检测路径规划。根据以上研究,基于CATIA V5开发的在线检测数据自动生成系统已在某大型航空企业中得到成功应用。     

Wear Measurement and Assessment of Explanted Cross-Linked PE Acetabular Cups Using a CMM

Wear has been considered the main limiting factor in the longevity of hip replacements. Wear analysis is thus essential for determining wear-related failure mechanisms and prediction of wear, which will consequently enable biomedical engineers to improve the design, material, and service life of the bearing components. This article presents wear measurement and assessment of the explanted conventional cross-linked polyethylene (XPE) and second-generation highly cross-linked polyethylene cups (X3) using a coordinate measuring machine (CMM). An expanded uncertainty analysis was performed to assess the performance of wear measurement. Wear measurement using the CMM method was validated with the gravimetric technique. The normalized error between volumetric wear measurement of the CMM method and that of the gravimetric technique was estimated to be always less than 1, suggesting that the CMM method applied to explanted hip wear measurements under the specific conditions was accurate and reliable. The approach to CMM measurement with uncertainty analysis was shown not only to locate 3D wear scar and wear direction but also to accurately quantify linear and volumetric wear with a maximum volumetric uncertainty of ±3.15 mm³ (95% confidence level). It is shown that identifying the key uncertainty components involved in the measurement process including validation, which contributes to an overall expanded uncertainty budget, is crucial to improve the confidence and the reliability of hip wear measurement results using a CMM.     

Laser scan planning based on visibility analysis and space partitioning techniques

This paper presents a methodology to develop an automatic process planning system applied for scanning parts with free-form surfaces by using a laser stripe system mounted on a coordinate measuring machine (CMM). The part has been modelled using a STL format that permits the automatic recognition of any part surface. The valid orientations of the scanning device are obtained in order to guarantee the visibility of the zone to be scanned and also to be compatible with the constraints imposed by the process. With the aim to speed up the calculation of valid orientations, we apply different methods like space partitioning techniques base on kd-tree as well as back-face culling algorithms. Once the space occupied by the part is partitioned in regions, recursive ray traversal algorithms are used in order to exclusively check for intersection the part triangles of the STL model that can potentially be traversed by each laser beam direction. In order to reduce the scanning time related to laser orientation changes, part triangles must be classified into a set of clusters based on their common visibility orientations. Finally, the scanning paths for each cluster are generated as well as the joining paths between them by taking into consideration depth of field and laser beam width.     

Genetic Algorithm Modelling and Solution of Inspection Path Planning on a Coordinate Measuring Machine (CMM)

A multiple component inspection path planning problem (MCIPP) can be formulated as an optimisation problem, referred to as a non-deterministic polynomial complete problem (NP). An MCIPP consists of testing points, which will be visited by a CMM probe only once, and dummy points which are set to avoid collision and may be visited by a CMM probe more than once. This paper considers the application of genetic algorithms (GAs) acting as optimisers for optimal inspection path planning systems. The paper explores the techniques used in the GA optimal inspection path planning system. The paper also discusses the comparison of integer programming models and genetic models.     

An Automated Inspection System

An automated inspection system for manufactured parts is proposed using a cloud of 3D measured points of a part provided by a range sensor, and its CAD model. In spite of the high precision attained by coordinate measuring machines (CMM), range sensors offer significant advantages for dimensional inspection: a high speed of digitisation and the capacity to take 3D measurements on the whole surfaces of a part without physical contact. The system first registers a cloud of 3D points with a CAD model of the part, then segments the 3D points in different surfaces by using the CAD model, and finally measures the control of the specified tolerances on the part. Results of the inspection are displayed in two ways: visually, using a colour map to display the level of discrepancy between the measured points and the CAD model, and a hardcopy report of the evaluation results of the tolerance specifications. Two range sensor technologies have been tested and inspection results are compared with the results obtained with a CMM.     

Automated dimensional inspection planning using the combination of laser scanner and tactile probe

Combining multiple sensors on CMMs (Coordinate Measuring Machines) is useful to fulfil the increasing requirements on both complexity and accuracy in dimensional metrology. Yet, the methodology to plan measurement strategies for systems combining different types of sensors is still a major challenge. Such planning is commonly done in an interactive way. This paper presents a methodology which can create inspection plans automatically for CMM inspection combining a touch trigger probe and a laser scanner. The inspection features are specified based on the extracted geometry features and the associated PMI (Product and Manufacturing Information) items from a CAD model. A knowledge based sensor selection method is applied to choose the suited sensor for each inspection feature. For touch trigger measurements, the sampling strategy considers the measurement uncertainty calculated by simulation. A geometry-guide method is developed for collision-free probing path generation. For laser scan measurements, the required view angles and positions of the laser scanner are determined iteratively, based on which the scan path is generated automatically. The proposed methodology is tested for several cases and validated by measurement experiments. The methodology provides suited planning results and can be used for automated dimensional inspection, i.e. Computer Aided Quality Control (CAQC).     

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.     

复杂曲面测量数据最佳匹配问题研究

针对复杂曲面类零件加工余量分析过程中的测量数据匹配问题,提出通过初始匹配和精确匹配来实现曲面测量数据的最佳匹配。交互式的初始匹配过程决定后续算法的变量范围,精确匹配确定测量数据与曲面的最佳匹配姿态。精确匹配采用最小二乘法构造评估函数,应用边界约束BFGS方法对问题涉及的曲面匹配变换矩阵的6个未知量进行优化求解。对通过三坐标测量机获取的数据,提出了一种测头半径补偿方案。实验结果表明,该方法与遗传算法相比具有运算速度快和精度高等特点,能较好地解决复杂曲面类零件测量数据的匹配问题。     

Automated inspection of PCB components using a genetic algorithm template-matching approach

Automated inspection of surface mount PCB boards is a requirement to assure quality and to reduce manufacturing scrap costs and rework. This paper investigates methodologies for locating and identifying multiple objects in images used for surface mount device inspection. One of the main challenges for surface mount device inspection is component placement inspection. Component placement errors such as missing, misaligned or incorrectly rotated components are a major cause of defects and need to be detected before and after the solder reflow process. This paper focuses on automated object-recognition techniques for locating multiple objects using grey-model fitting for producing a generalised template for a set of components. The work uses the normalised cross correlation (NCC) template-matching approach and examines a method for constraining the search space to reduce computational calculations. The search for template positions has been performed exhaustively and by using a genetic algorithm. Experimental results using a typical PCB image are reported.