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Research and application of 3D digital dimension detection in reverse engineering

Abstract: the emergence of a large number of class a surfaces and the popularization and use of CIMS put forward higher requirements for traditional detection methods. This paper comprehensively expounds the method of size detection, and puts forward a new concept of data mining in reverse engineering. Based on Geomagic qualify, it enumerates examples of the application of 3D digital size detection. With the help of computer 3D full size detection, it is bound to set off a new upsurge of research in reverse engineering

Keywords: data mining; Reverse engineering; With the development of science and technology and the improvement of manufacturing level, a large number of class a surfaces have appeared in home appliances, aircraft, automobiles and other parts. Users have higher and higher requirements for the accuracy of surface parts, while the shape of surface parts is complex, it is difficult to test with traditional inspection tools, and the detection accuracy is not high. How to effectively improve the detection accuracy of surface parts is an urgent problem to be solved in scientific research and actual production. Moreover, with the popularization and application of CIMS, many enterprises put forward the demand for computer-aided quality system. Computer aided inspection planning (CAIP) system has become an important part of CAQ system. Its core is to solve the problem of how to inspect parts. Therefore, 3D digital detection technology is a high-tech with broad application prospects, which is of great significance for the flexibility and automation of detection means and the application of product design [1]

reverse engineering is the process of transforming product prototypes into digital models. It refers to the development of similar or more advanced products based on the theories and methods of system engineering, product design methodology and computer-aided technology with the product prototype, physical objects, software (drawings, procedures, etc.) or images as the research object [2]. The basis of its existence is that there must be point data that accurately reflect the product characteristics. That is, first measure the real object or model with certain measurement means, and then reconstruct the digital model of the real object with the measurement data through the three-dimensional geometric modeling method. The three-dimensional digital dimension detection arises at the historic moment

2 classification of detection methods

the development of detection technology has experienced from human detection, manual detection, electrical detection to computer-aided detection. For a long time, due to the limitations of manufacturing level and underdeveloped process, general measuring tools and special inspection tools have been widely used in various industries as the main inspection means. However, there are many disadvantages, which can not accurately inspect some key parts of the tested parts

2. 1 Classification of measurement methods

2 1.1 contact measurement

the contact mechanical probe measurement method is a contact measurement method developed earlier and fully studied. The mechanical probe is used to contact the surface of the object to be measured. When the object surface moves, the probe moves along the surface of the object to be measured. Its movement is measured by the displacement sensor combined with the probe. After proper processing of the measured data, the movement of the surface to be measured can be obtained. Coordinate measuring machine (CMM) and articulated CMM are representatives of widely used contact measuring equipment. In reverse engineering, CMM is the main means of data acquisition. It has the advantages of high measurement accuracy and strong adaptability, but the measurement efficiency is low. It is suitable for objects with certain hardness

2. 1.2 non contact measurement [4]

according to different measurement principles, non-contact measurement includes optical measurement, ultrasonic measurement, electromagnetic wave measurement, infrared measurement, photogrammetry, etc

(1) optical measurement methods can be divided into laser triangulation method, machine vision method, moire profile method, phase measurement profile method, scanning tunneling microscope (STM) method, optical fiber method, laser interference method, laser scanning method, etc. [5]

① optical triangulation method: it is divided into direct optical triangulation method and oblique optical triangulation method. According to the measurement method, it can be divided into point light source incidence, line light source incidence and grid light source incidence [6]; ② Moire profile method: it is a full field measurement method that detects the shadow grid or image grid of the profile surface through a reference grating, and calculates the shape of the profile surface according to the distribution law of moire pattern. The light field distribution function of the deformed grating projected on the three-dimensional contour plane can be expressed as a spatial phase modulation signal. When the object to be measured deviates from the reference plane, the spatial phase change caused by the change of the shadow grating fringe position, and the height information of the object to be measured is included in this phase modulation term [7]; ③ Phase measurement profile method: Satoru Toyooka and tuuji iwaasa proposed the spatial phase detection method for three-dimensional profile surface in 1986. The TV camera is used to collect the gray information of the deformed raster image on the object, and then the computer demodulates the phase and calculates the height according to different FTP rules, so as to obtain the coordinates of the required points; ④ Scanning tunneling microscope method: using the tunneling effect in quantum theory, the function of the scanning motion system is to generate the three-dimensional scanning motion of the probe relative to the sample. The linearity, orthogonality, repeatability and stability of the moving parts have reached unprecedented high requirements; ⑤ Optical fiber method: optical fiber sensor can realize the combination of "transmission" and "sensing"; Small size, light weight, strong anti electromagnetic ability, high sensitivity, easy to bend, and can be observed and telemetered from a long distance; ⑥ Laser interferometry: interferometry has always been an effective means of measuring precision linear displacement and angular displacement. The application of laser light source completely overcomes the limitation of monochromatic light source on interferometer. The rapid development and application of photoelectric converter and electronic technology have changed the traditional interference fringe processing method; ⑦ Laser scanning method: the laser scanning method is mainly used to measure the geometric dimensions of the workpiece, such as length, thickness, outer diameter, etc. With He Ne laser or semiconductor laser as the light source, the laser beam is scanned to the measured object from top to bottom through a rotating polyhedron, and the measured results are obtained through the conversion of photoelectric receiving devices and data processing. The theodolite measurement system, total station coordinate measurement system, laser tracking measurement system, indoor GPS measurement system and laser tracking measurement system are all based on this principle [8]

(2) during ultrasonic measurement, the probe emits a wave of a certain rate, records the time of the reflected wave of the measured object to calculate the coordinates of the measured point, but it has the disadvantage of poor continuous acquisition force, which belongs to non-destructive testing, and is mostly used for crack detection of composite materials and ceramic materials [9]

(3) electromagnetic wave measurement use CT scanning equipment to obtain a series of cross-sectional image slices and data after CT tomography of parts. These slices and data provide complete information about the part profile and its internal structure, and can be used to analyze the shape, structure and function of the workpiece

(4) infrared detection the development of infrared detectors is the most eye-catching aspect of the development of infrared technology. The multi array detection technology of imaging detection makes the infrared detection technology continuously innovated. It is widely used in military guidance and detection

2 under the condition of non overloading 2 chromatography

this method needs to fix the parts on the workbench of the machine tool (milling machine or grinder), wrap and fill them with wrapping materials, and then mill them layer by layer (z direction) with a milling cutter according to a certain size. After milling off each layer, a two-dimensional measurement is carried out on the newly obtained cross section to obtain the two-dimensional coordinate data (x, y) of the cross section contour. In this way, after milling layer by layer, the complete three-dimensional data (x, y, z) of the product can be obtained

2. 3 digital photogrammetry

a photogrammetric computer system based on digital images is composed of standard computer hardware, image input/output equipment and special photogrammetric software. The digital image stored in the computer is obtained by processing the captured photos with image scanning technology. Digital orthophoto, digital elevation model (DTM) and digital map are the results processed by the workstation in the digital photogrammetry system according to the photogrammetry principle (using a certain number of photo control points and necessary connection points for relative orientation of image pairs, absolute orientation of models, stereo measurement of models, etc.) [10]. In reverse engineering, its advantage is to import global landmarks, which is especially suitable for objects with large shapes that need to be assembled after multiple shots

3 definition of data mining in reverse engineering. The author thinks that there is something wrong with data collection in reverse engineering. In reverse engineering, it tends to discretize the complete area spread over the object surface, extract and digitize the volume points, surface points, line points and single points representing the model features with the help of computers and measuring equipment, so as to achieve the purpose of global discretization, find, judge and extract the used data points, and finally play a necessary role in 3D surface reconstruction. This is defined as data mining. Because of the large amount of data acquisition required by data acquisition (especially non-contact measurement), "carpet" acquisition leads to the excavation of a large number of noise points, flying points and abnormal points. This kind of undifferentiated overall "absorption", or active data acquisition, is to avoid the loss of feature points, but it causes unnecessary trouble in the later point cloud processing, some of which are even "fatal" defects, The reconstructed digital model is deformed and deviates from the original shape of the object

therefore, it is the top priority in reverse engineering to correctly mine feature point data, that is, to extract potential and valuable knowledge from the feature surface of an object, and to dig out necessary, correct and representative feature point data, which is also one of the hot spots of scholars at home and abroad

4 the application of 3D digital dimension detection in reverse engineering the development of mechanical engineering detection technology is closely related to the development of production and science and technology. The development of production needs to constantly put forward new tasks and new topics of detection technology, which is the driving force for the development of detection technology. For example, the mold manufacturing of aircraft, automobile and other parts in China mostly depends on imports because of the complex shape, high precision requirements and advanced manufacturing technology of the parts. This kind of mold needs a lot of financial, human and material resources. How to pass the acceptance of moulds has always been a headache for these companies [12], because it is also the inevitable future of the transformation of Jixi, a traditional coal city, with the help of traditional methods, which are difficult to solve, and there is no good judgment standard. This problem can be solved by applying the data mining technology of reverse engineering to accurately obtain the three-dimensional full-scale value of the object, and using the computer to construct the corresponding digital model. In China, Xi'an Jiaotong University, Zhejiang University, Tsinghua University, Huazhong University of science and technology, etc. have made active research in this area, and developed application equipment and corresponding software. In post-processing, Geomagic qualify, imagewave, PolyWorks, CopyCAD and other software are mostly used to compare and analyze measurement models and digital models, and provide detection reports to help engineers and inspectors judge

this paper takes Geomagic qualify as an example. It is a reverse engineering post-processing software developed by American raindrop Geomagic software company, which can realize the rapid inspection of products. Automatic 3D comparison and error analysis can be carried out between product part models generated by computer aided design (CAD) system and existing actual parts. Geomagic qualify

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