Large-scale Dimensional Metrology Research Articles

Calibration of the Geometric Error Parameters of Laser Trackers in Site Environments

Laser trackers (LTs) are indispensable instruments in large-scale dimensional metrology. Nevertheless, geometric errors are prone to increase in site environments due to the occurrence of mechanical and optical misalignments, which deteriorates the measurement accuracy of the LT. The geometric error parameters are difficult to determine by traditional measuring or calibrating approaches. In this paper, a classification calibration method is proposed without any other etalons or instruments that accurately estimate the geometric error parameters in a site environment. First, to determine the environmental sensitivity error parameters, a refractive index correction model in a variable temperature environment is constructed, which performs well in reducing the interference of environmental temperature fluctuations. Subsequently, a spatial measurement network is established to determine two-face sensitivity error parameters using the difference between the frontsight and backsight measurements of the targets. Finally, the two-face non-sensitivity error parameters are solved by minimizing the residuals of the coordinate transformation from each LT station to the reference frame. Indeed, the effects of temperature fluctuation and geometrical misalignments on the measurement accuracy are deeply analyzed, which provide meaningful guidance for calibration and compensation of the geometric errors of the LT. The effectiveness of the proposed method is validated by a series of site experiments. The results show that the length error is reduced to 0.030mm on average, which is 44% lower than that of the network method.

The European GeoMetre project: developing enhanced large-scale dimensional metrology for geodesy

We provide a survey on the joint European research project “GeoMetre”, which explores novel technologies and their inclusion to existing surveying strategies to improve the traceability of geodetic reference frames to the SI definition of the metre. This work includes the development of novel distance meters with a range of up to 5 km, the realisation of optical multilateration systems for large structure monitoring at an operation distance of 50 m and beyond, and a novel strategy for GNSS-based distance determination. Different methods for refractivity compensation, based on classical sensors, on dispersion, on spectroscopic thermometry, and on the speed of sound to reduce the meteorological uncertainties in precise distance measurements, are developed further and characterised. These systems are validated at and applied to the novel European standard baseline EURO5000 at the Pieniny Kippen Belt, Poland, which was completely refurbished and intensely studied in this project. We use our novel instruments for a reduced uncertainty of the scale in the surveillance networks solutions for local tie measurements at space-geodetic co-location stations. We also investigate novel approaches like close-range photogrammetry to reference point determination of space-geodetic telescopes. Finally, we also investigate the inclusion of the local gravity field to consider the deviations of the vertical in the data analysis and to reduce the uncertainty of coordinate transformations in this complex problem.

Flexible field calibration of transmitter location and orientation in accurate large-scale positioning system

An accurate large-scale positioning system is a high-precision distributed large-scale dimensional metrology system that is widely used in smart manufacturing and assembly applications. The system comprises multiple transmitters with locations and orientations that are typically calibrated based on auxiliary measurement equipment. However, the calibration method is complicated and time consuming. We propose a rapid and flexible calibration method based on the highly precise three-dimensional coordinate control network established by a calibration bar with an accuracy of ± 0.005 mm, including both estimation and optimization algorithms. Exploratory experiments were performed to validate the feasibility of the proposed calibration method. The results demonstrate that the proposed calibration method enables the repeated measurement errors to be maintained at <0.05 mm in X and Y and <0.04 mm in Z. The standard deviation after calibration was 0.18 mm in comparison with the laser tracker.

Performance evaluation of laser trackers using the network method

Abstract Laser trackers (LTs) are widely used for large-scale dimensional metrology. Periodic LT performance evaluation is a key aspect in ensuring measurement reliability. The existing documentary standards for LT performance evaluation require scale bars and special artifacts that are often unavailable for many LT users. A network-based method is considered in this paper as a simple and effective approach to evaluate LT performance without the need for expensive artifacts or reference instruments. In this method, a set of fixed targets are measured from different locations of the LT. The reference values for the lengths between pairs of targets are determined by incorporating the geometrical error model of the LT (i.e., the LT data from each location are corrected for systematic geometric misalignments, quantities that are also obtained from the same data), thus the LT under test is itself used to calibrate the lengths in the network. Subsequently, these length values are used as references to evaluate the LT performance. We validate this method of establishing reference lengths by comparing the values for select lengths in the network against more traditional line-of-sight interferometry. We show that the uncertainties in the reference lengths obtained in this manner are sufficiently small in comparison to the maximum permissible error (MPE) specifications so that they can be used for performance evaluation. A case study using three LTs is presented in this paper. The results obtained from that case study is also verified by a simulation based on the ASME B89.4.19 standard, showing the feasibility of the proposed network-based LT performance evaluation method.

Dual-Comb Ranging

Absolute distance measurement is a fundamental technique in mobile and large-scale dimensional metrology. Dual-comb ranging is emerging as a powerful tool that exploits phase resolution and frequency accuracy for high-precision and fast-rate distance measurement. Using two coherent frequency combs, dual-comb ranging allows time and phase response to be measured rapidly. It breaks through the limitations related to the responsive bandwidth, ambiguity range, and dynamic measurement characteristics of conventional ranging tools. This review introduces dual-comb ranging and summarizes the key techniques for realizing this ranging tool. As optical frequency comb technology progresses, dual-comb ranging shows promise for various professional applications.

Engineering survey planning for the alignment of a particle accelerator: part II. Design of a reference network and measurement strategy

The building blocks of particle accelerators are magnets responsible for keeping beams of charged particles at a desired trajectory. Magnets are commonly grouped in support structures named girders, which are mounted on vertical and horizontal stages. The performance of this type of machine is highly dependent on the relative alignment between its main components. The length of particle accelerators ranges from small machines to large-scale national or international facilities, with typical lengths of hundreds of meters to a few kilometers. This relatively large volume together with micrometric positioning tolerances make the alignment activity a classical large-scale dimensional metrology problem. The alignment concept relies on networks of fixed monuments installed on the building structure to which all accelerator components are referred. In this work, the Sirius accelerator is taken as a case study, and an alignment network is optimized via computational methods in terms of geometry, densification, and surveying procedure. Laser trackers are employed to guide the installation and measure the girders’ positions, using the optimized network as a reference and applying the metric developed in part I of this paper. Simulations demonstrate the feasibility of aligning the 220 girders of the Sirius synchrotron to better than 0.080 mm, at a coverage probability of 95%.

A novel method for improving the accuracy of coordinate transformation in multiple measurement systems

Large-scale dimensional metrology usually requires a combination of multiple measurement systems, such as laser tracking, total station, laser scanning, coordinate measuring arm and video photogrammetry, etc. Often, the results from different measurement systems must be combined to provide useful results. The coordinate transformation is used to unify coordinate frames in combination; however, coordinate transformation uncertainties directly affect the accuracy of the final measurement results. In this paper, a novel method is proposed for improving the accuracy of coordinate transformation, combining the advantages of the best-fit least-square and radial basis function (RBF) neural networks. First of all, the configuration of coordinate transformation is introduced and a transformation matrix containing seven variables is obtained. Second, the 3D uncertainty of the transformation model and the residual error variable vector are established based on the best-fit least-square. Finally, in order to optimize the uncertainty of the developed seven-variable transformation model, we used the RBF neural network to identify the uncertainty of the dynamic, and unstructured, owing to its great ability to approximate any nonlinear function to the designed accuracy. Intensive experimental studies were conducted to check the validity of the theoretical results. The results show that the mean error of coordinate transformation decreased from 0.078 mm to 0.054 mm after using this method in contrast with the GUM method.

Multivariate control charts for monitoring internal camera parameters in digital photogrammetry for LSDM (Large-Scale Dimensional Metrology) applications

Industrial non-contact dimensional measurements using photogrammetry rely critically upon stability in time of camera calibration. This is particularly relevant for multi-camera systems employed for continuous and/or long term monitoring of some dimensional process, e.g. dimensional checks of the same manufactured component as it comes off the production line. In most of these cases, camera calibration is updated regularly to ensure optimal accuracy. Specifically, the use of photogrammetric systems requires the knowledge of both internal and external camera parameters estimated by calibration. Constancy of both sets is required during use. Internal parameters, pertaining to camera-specific properties, require stability over the operational lifespan of the system, while external parameters, concerning location and orientation, may change between calibrations. A diagnostic method for internal parameters based on multivariate control charts is proposed. The purpose of this method is to provide a comprehensive stability control over all the performed calibrations, especially for those systems used for regular monitoring of production lines. By integrating chart building into calibration software, no additional steps are added to the operator's workload for the calibration process. A practical application of the described methodology is presented at the end of the paper.

Large-scale dimensional metrology (LSDM): from tapes and theodolites to multi-sensor systems

Last decades witness an increasing interest in Large-Scale/Large-Volume Dimensional Metrology (LSDM). Many fields of application, ranging from construction to shipbuilding and aerospace, have shown more and more accurate and versatile systems for geometric control and tolerancing. Especially in the last ten years, optical technology has registered a fundamental step forward both in terms of metrological performances, versatility and convenience of use. This is further demonstrated by the current large diffusion of laser tracker and photogrammetric systems. The growth is also complemented by the development of new standards, even though a comprehensive body specific for LSDM is still lacking. The twofold aim of the present survey is to present a bibliographical and bibliometric analysis of the field and to propose a scheme of classification of the main current approaches. As an output of this paper, a “LSDM Multi-perspective Model” is introduced, according to which dominant technologies and restrictions are analyzed with the aim of individuating current lacks and defining the roadmap for future development of new instruments and systems.

Cooperative diagnostics for distributed large-scale dimensional metrology systems based on triangulation

In the field of large-scale dimensional metrology, new distributed systems based on different technologies have blossomed over the last decade. They generally include (1) some targets to be localized and (2) a network of portable devices, distributed around the object to be measured, which is often bulky and difficult to handle. The objective of this article is to present some diagnostic tests for those distributed large-scale dimensional metrology systems that perform the target localization by triangulation. Three tests are presented: two global tests to detect the presence of potential anomalies in the system during measurements and one local test aimed at isolating any faulty network device(s). This kind of diagnostics is based on the cooperation of different network devices that merge their local observations, not only for target localization but also for detecting potential measurement anomalies. Tests can be implemented in real time, without interrupting or slowing down the measurement process. After a detailed description of the tests, some practical applications on Mobile Spatial coordinate Measuring System-II (MScMS-II) – a distributed large-scale dimensional metrology system based on infrared photogrammetric technology, recently developed at DIGEP-Politecnico di Torino – are presented.

The evolution of large-scale dimensional metrology from the perspective of scientific articles and patents

In the last two decades, there has been a great development of the measuring systems in the field of large-scale dimensional metrology (LSDM), as also proved by the significant growth of the scientific literature. The aim of this paper is to analyze the LSDM scientific literature, from the dual perspective of scientific articles and patents, and estimate their impact in terms of amount of documents and relevant citations. In detail, this study investigates similarities and differences between articles and patents, as regards their dominant technologies and temporal distribution, and identifies (1) the major scientific journals and conference proceedings containing LSDM articles and (2) the major assignees of LSDM patents. Two main results emerge from the analysis: (a) important inventions concerning new LSDM systems were deposited before the end of twentieth century, while scientific articles have “bloomed” only in the last decade, and (b) the vast majority of patents concern inventions related to the laser-interferometry technology, while articles are divided more evenly among available technologies, with an important role played by the less accurate but more affordable ones, such as photogrammetry or structured-light scanning.

Corrective algorithms for measurement improvement in MScMS-II (mobile spatial coordinate measurement system)

Abstract This paper presents a set of algorithms for the correction of measurement errors of a prototype system designed for large scale dimensional metrology (LSDM) applications. The system, developed in the Quality and Industrial Metrology Laboratory of Politecnico di Torino, is based on the principles of photogrammetry and consists of a set of cameras wirelessly connected to a central unit able to track the position of a portable contact probe. Due to its architecture the system is affected by several systematic error sources. This paper addresses some of them: the distortion of the lenses, the dimension of the probe tip and the kinematic of the probe. By means of the implementation of appropriate mathematical correction models, the overall system performance is significantly improved as shown by the conducted tests.

A wireless sensor network-based approach to large-scale dimensional metrology

In many branches of industry, dimensional measurements have become an important part of the production cycle, in order to check product compliance with specifications. This task is not trivial especially when dealing with large-scale dimensional measurements: the bigger the measurement dimensions are, the harder is to achieve high accuracies. Nowadays, the problem can be handled using many metrological systems, based on different technologies (e.g. optical, mechanical, electromagnetic). Each of these systems is more or less adequate, depending upon measuring conditions, user's experience and skill, or other factors such as time, cost, accuracy and portability. This article focuses on a new possible approach to large-scale dimensional metrology based on wireless sensor networks. Advantages and drawbacks of such approach are analysed and deeply discussed. Then, the article briefly presents a recent prototype system – the Mobile Spatial Coordinate-Measuring System (MScMS-II) – which has been developed at the Industrial Metrology and Quality Laboratory of DISPEA – Politecnico di Torino. The system seems to be suitable for performing dimensional measurements of large-size objects (sizes on the order of several meters). Owing to its distributed nature, the system – based on a wireless network of optical devices – is portable, fully scalable with respect to dimensions and shapes and easily adaptable to different working environments. Preliminary results of experimental tests, aimed at evaluating system performance as well as research perspectives for further improvements, are discussed.

Recent developments in large-scale dimensional metrology

With ever-more demanding requirements for the accurate manufacture of large components, dimensional measuring techniques are becoming progressively more sophisticated. This review describes some of the more recently developed techniques and the state-of-the-art in the more well-known large-scale dimensional metrology methods. In some cases, the techniques are described in detail, or, where relevant specialist review papers exist, these are cited as further reading. The traceability of the measurement data collected is discussed with reference to new international standards that are emerging. In some cases, hybrid measurement techniques are finding specialized applications and these are referred to where appropriate.

Optical measurement techniques for mobile and large-scale dimensional metrology

Dimensional inspection tasks are often carried out on conventional coordinate measuring machines (CMMs). These CMMs can differ in layout and size depending on the application area. They can have measurement volumes up to 100 m 3. However, when measuring large objects it is not always possible to bring a large object to these conventional CMMs. That is why for these applications mobile measuring systems are an ideal solution. These systems often measure the dimensions through optical techniques, like interferometry and optical triangulation. After a short survey of common optical measurement techniques for mobile and large-scale measurements, this paper focuses on two industrial cases where different techniques were used to solve a measurement problem. The first case covers the measurement of a large iron casting with an optical LED-based triangulation system. It was possible to predict problems that would occur with the machining of the part, like unfinished surfaces after milling due to material shortage, and adapt the alignment of the part to prevent these problems. The second case covers the measurement of a double-decker train by means of photogrammetry, as an alternative for the currently used total station. Despite some specific drawbacks of photogrammetry systems it was possible to obtain the same accuracy and to reduce the overall inspection time significantly in comparison with the current situation.