With the rapid development of high-precision large-scale industry, especially in aerospace, shipbuilding, and machinery manufacturing, and the increasing size and complexity of products, the demand for precision position and dynamic attitude measurement of large-sized workpieces has also increased. In manufacturing, for example, large-size precision measurement technology has become the core detection means.
International aviation manufacturing enterprises, such as Boeing, Airbus, etc., have been on the manufacturing, assembly, and failure maintenance of the whole process of online automated measurement of technology certification, measurement also from the two-dimensional drawings to the development of three-dimensional CAD model. As a result, large-size precision measurement technology has emerged.
The technology is mainly based on high-precision and high-efficiency digital measurement systems, such as CMM, laser tracker, indoor GPS, etc., with different measurement principles and scopes for each system. The international application of this technology has enhanced measurement accuracy and efficiency, and further improved assembly accuracy and quality; while China’s is still in its infancy and under-researched.
Based on the analysis of different measurement systems, this paper focuses on key application technologies such as digital assembly, multi-system integration and data fusion, measurement-assisted assembly, model-based quality assurance, and, measurement uncertainty analysis, with a view to application for China.
Digital Measurement System
At present, for large-size precision measurement, advanced digital measurement systems mainly include CMM, articulated measuring arm, laser tracker, indoor GPS, LIDAR, digital photogrammetry, and so on. According to the contact measurement and non-contact measurement of the existing digital measurement system for classification.
(1) Coordinate Measuring Machine (CMM)
CMM is a high-precision contact measurement tool, usually used to measure complex surfaces and check the geometry of the workpiece. The principle is to form an orthogonal coordinate system through three mutually perpendicular guide rails, and the probe moves along the guide rails, thus realizing the three-dimensional coordinate measurement of the target point. CMMs are widely used for jig and fixture measurement, CNC machine tool inspection, and in-line measurement of flexible production lines.
The main advantages of this system are high accuracy and stability, which can provide very accurate measurement data and good adaptability to the measurement of large-size objects. Global CMM manufacturers, represented by Hexagon, dominate the market.
(2) Articulated measuring arms
ArtiAn articulatedsuring arm is a flexible measuring tool that calculates the coordinate value of the target point by the length and angle of the articulated arm. This measurement system can overcome some of the limitations of CMMs and is especially suitable for the measurement of narrow spaces or irregular objects. Compared with traditional CMMs, articulated measuring arms are smaller, lighter, and, have a wider measuring range.
Currently, the most common articulated measuring arm products on the market are 6-degree-of-freedom articulated arms with high mobility and portability.FARO, CimCore, and ROMER are currently the main manufacturers in this field.
(3) Laser Tracker
Through the laser interference ranging and mutually perpendicular high-precision encoder angular measurement, and with the ball prism and other cooperative targets for contact measurement, and then according to the principle of polar coordinate measurement to calculate the spatial three-dimensional coordinates of the target point, as shown in Fig.1 for the laser tracking instrument measurement principle diagram.
At present, the world’s laser tracker is mainly manufactured by three major manufacturers Leica, API, FAROand manufacturing, typical products are Leica AT901, FARO IO, API T3ATI Radian, etc., and China’s is currently blank.
Fig.1 Measurement principle diagram of laser tracker
(4) Indoor CPS
Establish a three-dimensional coordinate system, the use of the triangulation principle through infrared measurement to obtain the target point coordinates. Fig.2 shows that the transmitter transmits a shaped beam and the receiver receives the signal timing diagram.
The transmitter transmits three beam signals, respectively, two rotating fan beam signals around the transmitter and an LED pulse signal, through the receiver to receive the signal time parameter, you can solve the measured point of the horizontal azimuth and vertical azimuth.
The receiver receives two or more generator signals and uses the principle of triangulation to calculate the spatial three-dimensional coordinates of the target point. At present, the international manufacturer is Metric (now belongs to Nikon Metrology) company, China’s Tianjin University has also carried out the development of similar indoor GPS.
Fig.2 Measurement principle diagram of indoor GPS
(5) LIDAR
LIDAR is an advanced non-contact measurement tool, which accurately measures the three-dimensional coordinates of the target point through the frequency difference between the laser and fiber optic signals, combined with an angle encoder. With the functions of point measurement and profile scanning, LiDAR can quickly obtain a large amount of data, while having high accuracy and anti-interference ability. Its main application areas include automated production lines and non-contact measurement of large workpieces.
Metric is a leader in LiDAR technology, especially in the aerospace field has a wide range of applications.
(6) Digital photogrammetry
Digital photogrammetry is a non-contact measurement technology that obtains the spatial 3D coordinates of a target by taking multiple digital pictures and using computer software for automatic processing (including automatic positioning of marking points, automatic matching, and stitching). This method is highly adaptable and capable of efficient measurement in complex environments.
However, digital photogrammetry also has some disadvantages, mainly reflected in the susceptibility to light source changes and external environmental interference, and when the number of targets is large, the complexity and time cost of data processing will also increase. Currently, GSI and AICON 3D are the main providers in this field.
Integration of Large-Size Precision Measurement and Assembly
With the continuous progress of manufacturing technology, the integration of large-size precision measurement and assembly processes becomes more and more important. Traditional measurement methods can no longer meet the high requirements of modern manufacturing for precision and efficiency. Therefore, the organic integration of measurement and assembly process has become the key to improving manufacturing quality and productivity.
The integration of large-size precision measurement and assembly not only requires accurate control of the measurement process but also requires the full integration of data, process, design, and manufacturing process, to realize the fully digitalized and automated high-efficiency assembly.
(1) Theoretical framework of measurement and assembly integration
Maropolous proposes a theoretical framework centered on the measurement process model to realize the integration of measurement and assembly. The framework emphasizes the data integration of assembly simulation, tolerance analysis, and measurement-assisted assembly to ensure the smooth connection of each link.
Specifically, the integration of measurement and assembly is divided into two main processes: the virtual design-assembly-verification process, and the physical design-assembly-measurement-automation process. This theoretical framework not only provides theoretical guidance for high-precision measurement but can also help manufacturers optimize the assembly process and improve overall efficiency.
As shown in Fig.3, the framework conceptually explores the properties of the measurement process model and clarifies the implementation steps of measurement and assembly integration, including the creation of the virtual design, the execution of the actual assembly process, and how to verify the accuracy of the assembly using precision measurement technology.
Fig.3 Integrated theoretical framework of measurement and assembly
(2) Assembly-driven method model based on key measurement characteristics
Based on the development of assembly and information technology, researchers have proposed a “measurement-driven assembly” method model based on key measurement characteristics. The model not only focuses on the application of measurement technologies and processes but also the integration of measurement concepts, data, and methods into product design, process design, and assembly processes.
Through this approach, manufacturers can use measurement as a driver of the assembly process, thereby effectively reducing errors and improving assembly accuracy.
As shown in Fig.4, the Measurement-Driven Assembly model demonstrates how data integration through key measurement characteristics ensures a high degree of synergy between the measurement and assembly processes. In this way, manufacturers can ensure that every part of a product is assembled to meet the design requirements, thereby reducing later adjustments and rework.
Fig.4Theoretical methodology model of measurement-assisted assembly based on key measurement characteristics
(3) Multi-measurement system synergy and data fusion technology
In the process of large-size precision measurement, a single measurement system is often unable to meet the high accuracy requirements. Therefore, the use of multi-measurement system synergy, combined with different types of measurement equipment, is an effective way to improve measurement accuracy and efficiency.
Through the fusion of heterogeneous measurement data from multiple sources, manufacturers can obtain more accurate measurement results and ensure that each step in the assembly process is fully verified.
At the heart of multi-measurement system integration lies data fusion technology. These measurement systems often come from different vendors and use different data acquisition standards and formats.
Therefore, how to realize the coordination between these systems and ensure the effective fusion of measurement data is one of the keys to the development of the technology.
As shown in Fig. 5, the model-based unified measurement dataset representation and interoperability technology provides a solid foundation for data fusion of multiple measurement systems.
First, the measurement process of a digital measurement system needs to be modeled, simulated and, layout optimized to ensure that individual measurement devices work together. Second, the fusion of measurement data must take into account the accuracy differences between different measurement devices, especially when the measurement environment is unstable, how to deal with uncertainty, bad points, and, invalid data is the key issue.
Fig.5 Basic framework for integrating the multiple measurement systems and data fusion
(4) Multi-source and heterogeneity of measurement data
When multi-measurement systems work together, multi-origin and heterogeneity of measurement data are two issues that need special attention.
Multi-source means that the coordinate data of the same target point may come from different measuring devices, while heterogeneity is manifested in the fact that the data provided by different measuring devices have different accuracies, especially in complex assembly environments, where the data may be interfered with by external factors.
In the process of data fusion, the accuracy weights of different measurement systems must be reasonably assigned. This is not only a fusion of the data itself, but also of the data accuracy. With this approach, manufacturers can ensure high accuracy in the final measurement results and can better support the precise execution of measurement-assisted assembly.
As shown in Fig.6, in an aircraft segment docking application, the indoor GPS serves as a feedback signal to guide the robot in adjusting its position, while the LiDAR is responsible for performing measurement compensation. This multi-measurement system working in tandem not only realizes high-precision target positioning but also adjusts the assembly process in real time through data fusion technology.
Fig.6 Multiple measurement systems applied in the alignment of aircraft component
(5) Uncertainty analysis and measurement-assisted assembly
In the process of large-size precision measurement and assembly, uncertainty analysis is one of the core technologies to ensure the efficient operation of the measurement system. Different measurement equipment and different measurement environments may lead to errors in measurement results, so how to analyze and handle these errors is an important topic in measurement and assembly integration.
Through uncertainty analysis, manufacturers can identify factors that may affect measurement accuracy and take appropriate technical measures to compensate. At the same time, building a knowledge base of the measurement system’s process capabilities can support subsequent measurement system coordination and data fusion to ensure efficient execution of the measurement process.
Ultimately, the attitude control technology of measurement-assisted assembly also plays an important role in this process. By accurately controlling the attitude and position of the measurement equipment, the manufacturer can ensure that each component is assembled at a precise location, thereby improving overall assembly accuracy and reducing errors in production. As shown in Fig.7.
Fig.7 Cooperation measurement of laser tracker and digital photogrammetry
Measurement-assisted assembly
Measurement-assisted assembly is an important application of large-size precision measurement technology in the digital assembly of products, which integrates the application of product digital definition, digital simulation, automatic tracking measurement, automatic control and mechanical positioning, and other technologies, the use of digital measurement systems on the assembly components position measurement feature point tracking measurement, and guide the components to adjust the posture to complete the assembly.
The measurement-assisted assembly process, assembly relationship data m, model and data processing, and analysis algorithms are always running through the product digital assembly of various stages, to achieve the transfer of data in the assembly process. The data, algorithm,thms, and operation processes involved in the assembly are integrated and supported by each other, as shown in Fig.8.
Fig.8 Relationship among the core algorithm of measurement-assisted assembly, data, and process
The raw data of the digital assembly process are the coordinate measurement values and theoretical values of the measurement target points in each coordinate system, and all the coordinate data constitute a raw data pool.
First of all, based on the corresponding data processing algorithms for pre-processing and integration, to get the key measurement characteristics of the data set, and then get the assembly relationship data model; and then, through the analysis and solution of the assembly relationship data model, to provide driving data or decision-making guidance for the digital assembly process.
According to the type and role of the data it handles, the core algorithms are divided into digital measurement field construction algorithms, measurement data preprocessing algorithms, assembly relationship analysis algorithms, and, assembly execution and control algorithms.
Fig.9 shows the Airbus A380 fuselage segment docking using LiDAR MV260 to accurately measure the connection position on the docking segment, using Brunson’s lifting bracket to adjust the fuselage segment position, and the actual position of the spacer and the deviation are shown in the SA software after data processing.
Fig.9 Laser radar applied in the alignment of fuselage
Model-based inspection planning and quality assurance
To improve and ensure product quality, the implementation of quality inspection of product parts is an essential part. Before the implementation of parts inspection, it is particularly important to develop a reasonable inspection plan.
Inspection planning is to ensure that the parts testing process, testing steps, testing methods, testing tools, and other important means of consistency and effectiveness, through the development of a reasonable sequence of testing processes, clear testing objects, and testing requirements, for parts on-site inspection to provide guidance and basis.
Based on the three-dimensional model of the inspection planning information directly from the design model or process model, but also to eliminate the previous inspection process duality, inspection planning, and design changes are not synchronized and other issues. Drive the digital measurement system to perform inspection tasks, realize the effective connection between digital inspection and design and manufacturing, and improve and ensure product quality.
To ensure that the digital inspection technology can be fully utilized, the authors propose a basic architecture for model-based inspection planning and quality assurance, as shown in Fig.10. The architecture is divided into computer-aided three-dimensional inspection planning and inspection data and business management through the construction of a unified model of three-dimensional inspection to achieve effective integration between the two parts.
Fig.10 Inspection planning and quality assurance based on the model
Computer-aided 3D inspection planning is built based on 3D modeling software, which reads the product model obtained from the 3D design system or 3D process system for inspection planning.
According to the study the inspection characteristics can be obtained from the product model inspection needs, including inspection objects, tolerance values, and other inspection information. For different inspection needs, select the appropriate inspection method and inspection tools. According to the sequence of the inspection process for the inspection sequence planning.
Reasonable layout of measurement points on the geometric surface of the inspection object, followed by measurement path planning, and finally inspection protocols and measurement programs. The unified mathematical model of 3D inspection integrates the inspection model, lightweight model, inspection protocols, measurement procedures, inspection reports, etc. in a unified information framework to achieve uniform and standardized management.
The purpose of inspection data and business management is to realize the unified, standard, size,d and traceable management of digital inspection business and inspection information. Measurement data to analyze, assess the processinerroror and generate different forms of inspection reports.
As shown in Fig. 11, a Leitz CMM equipped with a precision rotary table is used to inspect the leaf disk after inspection planning. As shown in Fig. 12, the literature introduces an error model based on precision inspection planning for repeated machining of free-form surfaces, where the maximum error position is determined by measuring fewer points to realize efficient measurement.
Fig.11 Blisk inspected by coordinate measuring machine Leitz equipped with a precise turntable.
Fig.12 Maximum error measurement for freeform surface based on CMM
Task-oriented measurement uncertainty analysis
In conventional measurement methods, the evaluation of measurement results is intuitive and reliable. However, for coordinate measurement, the evaluation of measurement results is more abstract and less convincing.
Measurement uncertainty theory is a more scientific evaluation of the quality of measurement results developed on the traditional error theory. It is a covariate associated with the measurement results, characterizing the degree of dispersion of the reasonably assigned value of the measurement is a statistical concept, and is also an inherent property of digital measurement systems.
In the task-oriented measurement uncertainty analysis, not only the simple addition of each uncertainty factor but bu also the ccodegree of impact on the task and the correlation between the factors have a certain transmission law.
Yang Jingzhao of the National University of Defense Technology described the task-oriented measurement uncertainty analysis program based on the existing uncertainty assessment methods, proposed a measurement uncertainty Monte Carlo assessment method based on the simulation of the measurement process as well as the uncertainty assessment method based on the gray theory and neural network, as shown in Fig.13.
Fig.13 Uncertainty analysis of the task-oriented measurement
In the pre-processing stage of measurement data, the influence of deterministic systematic errors or regular systematic errors can be compensated or reduced according to the corresponding methods. For the uncertainty of random errors, a suitable method is chosen to evaluate the measurement uncertainty.
For example, the Guidelines for the Evaluation of Uncertainty in Measurement (GUM) include the Class A and Class B evaluation methods, the experimental method proposed in IS011530, and the model-based GUM uncertainty framework method and Montthe e Carlo method.
The literature uses the Monte Carlo method to give the corresponding measurement uncertainty to the laser tracker’s measurement results in multi-station time-sharing measurement. The measurement uncertainty of CMM is evaluated using simulation and experimental methods.
The mathematical definition, analytical algorithm, and the analysis of uncertainty sources of position measurement uncertainty in the assembly process of large parts of aircraft are introduced in the literature. The API laser tracker T3 tracks the motion of the spindle of the measuring machine, collects the three-dimensional coordinates of the spatial points, and then completes the tuning of large 5-axis and 6-axis machine tools through spatial error checking and compensation technology.
Conclusion
Large-size precision measurement technology has become a research hotspot in the manufacturing field, although its core technology is still accumulating and verifying. The digital measurement system is developing in the direction of portable, networked, and, efficient precision, and the e-size precision measurement has transformed from single technology to multi-sensor fusion.
This paper analyzes the application of large-size measurement technology and digital assembly integration, proposes a multi-measurement system integration and data fusion architecture, introduces the core algorithms of measurement-assisted assembly of data and process, proposes a model-based inspection planning and quality assurance framework, and summarizes the task-oriented measurement uncertainty analysis method, which provides technical support for engineering applications.
In the future, the collaborative work of multiple digital measurement systems will promote the change of high-precision large-scale industrial production mode and enhance the international competitiveness of the manufacturing industry.