Coordinate Measuring Machine Inspection Research Articles

A Voronoi diagram based framework for fast and accurate evaluation of 2D free-form profile errors

Free-form objects which have vast engineering applications are inspected using coordinate measuring machines (CMM) and non-contact scanners. The output from the devices, that is the measured point-set, must be registered with the reference profile and later inspected for the profile error. Registration is accomplished in two phases, that is coarse registration and fine registration. The most crucial step in the fine-registration stage is the profile error estimation at each point of the measured point-set. Generally, this is accomplished using the point-inversion method, which has relatively high time complexity. This work proposes a two-stage framework based on the Voronoi diagram concept for profile error estimation of 2D free-form profiles. A discretized profile is prepared for the measurement process in the first stage. The profile errors are estimated from the measured point-set using the discretized profile in the second stage. Two profile error estimation technique has been proposed based on Voronoi vertices and the Voronoi edges. The effective time-complexity of the proposed framework is O(lm log n). Implementations were executed on simulated NURBS profiles and practically measured CT-scanner data of two machined airfoils modelled as NURBS profiles. The validation results illustrate that the proposed framework’s accuracy is on par with that of the point-inversion method and is at least 56% faster than the latter; thus making it a fast and accurate alternative for CMM (operated in continuous scanning mode), CNC machine-based (in-situ) and vision-based 2D profile inspection.

Path Planning for 5-Axis CMM Inspection Considering Path Reuse

The 5-axis Coordinate Measuring Machine (CMM) is widely used for quality data collection of the machining parts, such as cylinder blocks and heads of the engines. High efficient inspection path planning for multiple feature groups from different stations is one of the key tasks for CMM application. In engineering practice, the inspection planning of diverse feature groups accounts for large labor cost and process development cycle. To improve the efficiency of path generation for the complex machining part, a five-axis CMM inspection path planning method considering path length, probe rotation and path reusability is proposed. Firstly, the measuring points (MPs) are classified based on feasible inspection direction cone and accessibility of the MPs to achieve the minimum times of probe rotation. Then, the rapidly exploring random trees with multi-root node (RRT-MRNC) algorithm is proposed to implement local path planning considering inspection path reuse. Furthermore, intra-group and inter-group path is generated simultaneously based on the proposed enhanced Genetic Algorithm (GA) algorithm. In order to evaluate the effectiveness of the proposed method, the cylinder block path planning case is used. Compared with the benchmark methods, the total planning time based on the proposed planning method for the dynamic tasks was reduced by 55.2% and 54.9% respectively.

Free-form surface inspection path planning using improved ant colony optimisation algorithm

To improve the inspection efficiency of free-form surfaces using coordinate measuring machines (CMMs), this paper proposes a free-form CMM inspection path optimization method based on an improved ant colony optimisation algorithm. The inspection path composition and its mathematical model are first analysed, and then the improved ant colony optimisation algorithm is applied to minimize the objective function. To address the problem that the ant colony optimisation algorithm has difficulty jumping out of local optimal solutions, this paper introduces a negative feedback mechanism in the pheromone update rule to improve the probability of the ant colony optimisation algorithm jumping out of the local optimal solution, so as to obtain the global optimal solution with a higher probability. Experiments showed that the inspection path length optimised by the improved ant colony optimisation algorithm was at least 5.2% and 8.6% shorter than those of the traditional ant colony optimisation and simulated annealing algorithms, respectively.

Towards the implementation of the Digital Twin in CMM inspection process: opportunities, challenges and proposals

The use of Digital Twin (DT) is adopted by manufacturers and have positive effects on the product manufacturing process. The aim of this paper is to define a Coordinate Measuring Machine (CMM) inspection DT model, based on inspection process digitalized functionalities, from one side, and Industry 4.0 opportunities (Digital thread, Big data, etc.), from the other side. A review about DT definition, is firstly presented. Secondly, we review related studies based on existing DT orientations and usages for CMM inspection. Thirdly, challenges related to the variation management are presented. Finally, a discussion about possible DT functionalities and opportunities is conducted then a CMM inspection DT model is presented.

Evaluation of lead time for contour and roundness measuring equipment

Measurement of roundness and contour is the essential process taking place in many manufacturing industries. Parts and components having round edges are typically produced in many industries for various applications like automobile shafts, gears, bearings and exhaust pipes. Incorporation of six sigma process can improve the quality of the measuring processes. Six Sigma methodology deals with using a strategy that attempts to reduce variation and to improve the quality through Six Sigma. This can be carried out by employing two different Six Sigma procedures. This article points out methods to reduce the lead time of Coordinate Measuring Machine inspection by introducing various measurement techniques and programming and design of holding fixtures. The methodology of execution of the measuring methods is initiated with the identification of problems, followed by analysis, identification, implementation and improvement. It is concluded that after the incorporation of the inspection lead time, there was considerable improvement in time spent for the entire measurement process.

Automatic Sweep Scan Path Planning for Five-Axis Free-Form Surface Inspection Based on Hybrid Swept Area Potential Field

Sweep scan is an emerging five-axis inspection technology of retrieving geometric data from free-form surfaces. Compared with the traditional surface inspection by coordinate measuring machine (CMM) that works in a point-by-point manner, sweep scan is able to make the stylus tip continuously sweep at high speed on the surface to inspect, whilst acquiring the 3-D point data accurately and efficiently. However, at present, for an arbitrary free-form surface, it still heavily depends on humans to manually generate a suitable sweep scan path. This paper presents a practical sweep scan path planning method, which can automatically generate an efficient sweep scan path for an arbitrary free-form surface. The resultant sweep scan path is able to cater to the shape of the surface to inspect and the unique kinematic characteristics of the typical five-axis inspection machine, so as to achieve a tremendous increase in inspection efficiency. The experiments performed by us show that when compared with some existing automatic sweep scan path generation algorithms such as the simple zigzag method, the proposed method is able to reduce the total inspection time by as much as seven times. Note to Practitioners —This paper is motivated by the lack of effective automatic sweep scan path planning methods for arbitrary free-form surfaces. Although there is a rich body of five-axis CMM path planning methods for inspection of free-form surfaces, they are mainly designed for the traditional inspection mode, i.e., the point-by-point measurement. Compared with the traditional discrete way of CMM inspection, sweep scan has a quite different inspection mode, which tremendously boosts the inspection efficiency by maintaining the constant contact between the stylus and the surface to inspect. The traditional five-axis inspection path planning methods are no longer applicable to sweep scan. Up to date, only a very handful few of automatic sweep scan path planning methods are reported and they seem to be very difficult to implement and no optimality of inspection efficiency is discussed. We present a practical algorithm for automatic generation of sweep scan paths for inspection of an arbitrary free-form surface. The algorithm is novel in that, by considering both the surface shape and the unique kinematic characteristics of a typical five-axis inspection machine, it tries to globally minimize the total inspection time. In our experiments, the sweep scan paths, generated by the proposed method, are found to outperform that of some traditional simple sweep scan path generation methods such as the zigzag method by a large margin in terms of total inspection time (while the data acquisition accuracy is maintained at the same level). The method is particularly suitable for those industrial parts that are large but relatively smooth, such as windmill blades and automobile panels.

Automated Coordinate Measuring Machine Inspection Planning Knowledge Capture and Formalization

Capturing the strategy followed during a coordinate measuring machine (CMM) inspection planning session has been an extremely challenging issue due to the time-consuming nature of traditional methods, such as interviewing experts and technical documents data mining. This paper presents a methodology demonstrating how a motion capture-based system can facilitate direct and nonintrusive CMM operator logging for capturing planning strategies and representing in knowledge formats. With the use of recorded motion data, embedded knowledge and expertise can be captured automatically and formalized in various formats such as motion trajectory graphs, inspection plans, integrated definition (IDEF) model diagrams, and other representations. Additionally, a part program can be generated for driving a CMM to execute component measurement. The system's outputs can be used to help understand how a CMM inspection strategy is planned, as well as training aids for inexperienced operators and the rapid generation of part programs.

Curved Reflection Symmetric Axes on Free-Form Surfaces and Their Extraction

Feature detection on smooth free-form surfaces is much more difficult than that on shapes with sharp features. In this paper, we extract the curved reflection axes (CRAs) of an arbitrary free-form surface as features if they exist. The extraction result is robust to boundary noises and strongly sensitive to extrinsic properties of the surface such as projected normals and curvatures. Compared with the general reflection symmetry, curved reflection symmetry is defined to be a reflection symmetry along a smooth 3D embedded curve instead of a plane, where any point on the curve is a local reflection center for some surface points. The properties of the curved reflection symmetric axis are analyzed, and a novel computational model for detecting and extracting CRAs on free-form surfaces is presented. The experimental results are then compared with both the medial axis and the intrinsic symmetric axis, which are two popular feature representations of 3D shapes, and the advantages and uniqueness of the proposed method are convincingly demonstrated. An application of the proposed method in sweep scanning is also presented. Other applications of the proposed method include feature extraction, shape symmetrization, segmentation, and registration. Note to Practitioners —This paper was originally motivated by the stylus path planning problem in sweep scanning on curved surfaces (which is a new surface inspection technology), though it is also applicable to shape registration and matching. Traditional coordinate measuring machine inspection of curved surfaces depends heavily on manual intervention, resulting in a very slow and tedious process and the acquired data are prone to inconsistency due to the discontinuous nature of the operation. In contrast, sweep scanning adopts a configuration of a deflectable stylus mounted on a motorized articulating head with two rotary axes inside, which enables the stylus tip to follow a continuous path without having to leave the surface. The head itself is mounted on an xyz table. During a sweep scanning, the head slowly moves on the xyz table (which is heavy and has stringent dynamic constraints due to its large mass) along a nominal path, while the stylus (which is extremely light) quickly oscillates in a direction transverse to the path of the head. The proposed curved reflection axis offers to be a powerful algebraic model for generating a nominal head path for sweep scanning. The reported work is preliminary though, especially in terms of covering the entire surface and also considering the specific length of the stylus, which will be our future work.

Knowledge Capture in CMM Inspection Planning: Barriers and Challenges

Coordinate Measuring Machines (CMM) have been widely used as a means of evaluating product quality and controlling quality manufacturing processes. Many techniques have been developed to facilitate the generation of CMM measurement plans. However, there are major gaps in the understanding of planning such strategies. This significant lack of explicitly available knowledge on how experts prepare plans and carry out measurements slows down the planning process, leading to the repetitive reinvention of new plans while preventing the automation or even semi-automation of the process. The objectives of this paper are twofold: (i) to provide a review of the existing inspection planning systems and discuss the barriers and challenges, especially from the aspect of knowledge capture and formalization; and (ii) to propose and demonstrate a novel digital engineering mixed reality paradigm which has the potential to facilitate the rapid capture of implicit inspection knowledge and explicitly represent this in a formalized way. An outline and the results of the development of an early stage prototype - which will form the foundation of a more complex system to address the aforementioned technological challenges identified in the literature survey - will be given.

A frame for a computer aided inspection planning system

Coordinate Measuring Machine (CMM) inspection planning is an activity performed by well-trained operators, but different measurement techniques, using the same data analysis algorithms yield in different measurement results. This is a well-recognized source of uncertainty in coordinate measurement. A CMM, provided with an automatic inspection planning (CAIP) system, permits to implement more accurate and efficient operating procedures and to fit higher quality assurance standards and tighter production timings.In this paper we present a frame of a CAIP system, able to deal with almost all the decisional stages of CMM inspection. Moreover, original approaches have been developed and presented in inspection feature selection, part set-up, probe configuration, and path planning.

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).

Verification of Interim Check Method of CMM

In industry, the Coordinate Measuring Machine (CMM) is a major geometrical measurement method because of its precision and general versatility. The CMM is conventionally used in a thermostatic chamber, but its use in the production line environment is increasing. Many factors - dust, thermal fluctuation, and collision, to name a few - deteriorate measurement precision and make it necessary to inspect the CMM daily. However, the daily inspection method is not established. Study of CMM is mainly about calibration in controlled environment like a thermostatic chamber. It is unfeasible to calibrate the CMM everyday because of its cost. So some researchers have studied about daily inspection of CMM. This paper describes daily CMM inspection procedures and how to evaluate these uncertainty. This method treats axis scale errors and interaxis squareness errors from geometric errors of CMM. These factors influence the measurand. QuickCheck (Trapet Precision Engineering) is used as an artifact. In the linear model using QuickCheck, influences of alignment error is evaluated parameters.

Localization of parts with irregular shape for CMM inspection

The part coordinate setup is a necessary and important step in coordinate measuring machine (CMM) inspection. It usually counts on the precise machining of certain datum features for the determination of the part coordinate. In some manufacturing processes, such as die casting, stamping, pressing, injection, etc., precise machining of the datum features is difficult. Dimensional inspection of such parts, therefore, requires a reliable procedure for the determination of the part coordinate beforehand. The purpose of this study is to propose a new algorithm for the part coordinate setup of irregular shapes. The proposed algorithm is composed of a rough positioning and a fine positioning. The rough positioning is applied to bring the part coordinate into the neighborhood of the model coordinate. The fine positioning is then applied to assure that the part coordinate and the model coordinate match each other perfectly. A standard sphere is used for the verification of the proposed method. Several industrial examples are also presented to illustrate the feasibility of the proposed method.

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.

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.

Application of fuzzy logic in the selection of part orientation and probe orientation sequencing for prismatic parts

With an increase in complex designs and tighter tolerances, the Coordinate Measuring Machine inspection process has become increasingly more advanced. By inspection planning, design data can be transferred to an inspection system and an entire inspection operation can be carried out with a minimum of time and with reduced uncertainty. The current need is to automate this process completely so that the inspection plan can be generated directly from the design information. Two modules of inspection planning, i.e. selection of part orientation and probe orientation sequencing, have not been dealt with properly. Also, some important factors for the selection of part orientation have been neglected and proper weights have not been given to the probe-orientation sequencing criteria. An attempt was made to overcome these limitations. Both problems have been approached as the ranking of a number of alternatives based on multiple criteria, where each criterion has unequal importance. To get the optimum probe-orientation sequence and stable part orientation, fuzzy logic was applied. Fuzzy sets were obtained and combined using a suitable methodology. To explain and validate the proposed methodology, an example part was taken. As a practical case, an engine block was considered and the results presented.

Comprehensive design of experiments-based framework for optimal CMM inspection and uncertainty analysis of form tolerances

In the coordinate measuring machine (CMM) measurement environment, uncertainty can arise from various sources. Previous literature has focused on how to measure uncertainty in a CMM system using segregated approaches. We developed a framework to verify bias and variance tendencies deriving from form errors. Two types of form tolerances, namely flatness and circularity, are addressed here. By studying the nature of the CMM measurement uncertainty impact, this paper targets three objectives: (1) developing a design of experiments (DOE)-based measurement performance approach to integrate critical measurement strategy factors; (2) optimizing identified controllable measurement factors; and (3) synthesizing the impact of CMM uncertainty issues. Consequently, the paper fully explores the influence and interplay of these contributing factors, resulting in robust and optimal guidelines for the CMM flatness and circularity measurement strategy. Extensions of the framework for other form tolerances are discussed.

Automatic sampling for CMM inspection planning of free-form surfaces

Coordinate measuring machines (CMMs) are used to examine the conformity of the produced parts with the designer's intent. The inspection of free-form surfaces is a difficult process due to their complexity and irregularity. Many tasks are performed to ensure a reliable and efficient inspection using CMMs. Sampling is an essential and vital step in inspection planning. Efficient and reliable approaches to determine the locations of the points to be sampled from free-form surfaces using the CMM were developed. Four heuristic algorithms for sampling based on the NURBS features of free-form surfaces are presented. The sampling criteria are equiparametric, surface patch size and the surface patch mean curvature. An algorithm for automatic selection of sampling algorithms performs complexity checks on NURBS surfaces, including the surface curvature changes and surface patch size changes, and selects the suitable sampling algorithm. Extensive simulations were performed using the developed methodologies to evaluate their performance using free-form surfaces with different degrees of complexity and compared with the uniform sampling pattern. The CMM measurement errors and manufacturing form errors have been simulated in these studies. The developed algorithms provide a useful tool in selecting the effective sampling plans for the tactile CMM inspection planning of free-form surfaces.

On the selection of flatness measurement points in coordinate measuring machine inspection

Inspection of form tolerances using the coordinate measuring machine (CMM) presents two distinct problems: data collection and data fitting. The former problem deals with the selection of the sample size and the sample point location while the latter involves the determination of the tolerance zone enveloping these points. Four types of strategies and five different sample sizes were studied in this work to address the former problem. The accuracy of flatness measurement was investigated using realtime experiments with respect to the above two factors and their respective levels. In addition, the length of the probe path was studied with respect to the two factors using a simulation study. A priority coefficient was developed to combine the influence of accuracy and path while selecting sampling strategies and sample size. Preliminary observations made suggest that any one sampling method may not be the best solution in all cases, while considering the accuracy of flatness and the shortest CMM probe path.

Inspection point placement and path planning algorithms for automatic CMM inspection

This paper discusses development of automatic inspection point placement and path planning methods, for use in the integration of computer-aided design with coordinate measuring machine (CMM) inspection. The proposed system demonstrates that optimal collision-free inspection paths can be efficiently generated for geometrically complex parts consisting of multiple intersecting features. This is achieved by using iterative subdivision of surfaces for point placement coupled with, efficient 3D collision avoidance and path planning. This planning module is an integral part of an overall inspection planner to fully automate rapid and reliable inspection using computer-controlled CMMs. The paper discusses the structures and algorithms used, and presents experimental results.