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Abstract: This paper mainly introduces the application process of reverse engineering technology in the design and development of modern body outer panels, and focuses on the research and discussion of key technologies such as the model segmentation of body outer panels, the creation of surface mathematical models, and the evaluation of surface quality

key words: reverse engineering evaluation of surface quality of body outer panel

1 overview of reverse engineering design of automobile body

reverse engineering is the process of obtaining the mathematical model of the product from the existing physical model or physical sample. At present, it is widely used in product development, mold manufacturing, product quality control and other fields, especially in the development and design of automobile body outer panels, reverse engineering technology has become an important technical means. Body reverse technology is one of the important methods and means of computer aided geometric design (CAGD). It mainly studies the geometric modeling problems in engineering. It is the computer representation, analysis and synthesis of various geometric shape information, and the mathematical description and control of the model through the computer. Its main mathematical theory is computational geometry, which is formed by function approximation theory, differential geometry, computational mathematics, computer graphics, etc. The reverse engineering application of body outer panel is the process of obtaining the surface mathematical model from the manual model. It is different from the general process of mathematical model of physical products. In the process of implementing reverse engineering in the product development stage of the new body outer panel, we should not only faithfully reproduce the design concept of the styling designer to the greatest extent, but also fully consider the requirements of structural design. In the process of surface modeling, we should also pay attention to eliminating the surface defects of the hand-made clay model and the discrete error of the measurement data. In a certain sense, The application of reverse engineering technology of body outer panel includes a certain positive design method

2 reverse engineering application process of body outer panel

reverse engineering design of body outer panel is the connection between modeling design and engineering CAD design, and is the key process in body conceptual design; In the design and implementation process of the outer panel of the whole car body, the collection of surface data and surface modeling account for a large proportion of time, and the surface quality is required to be high. In addition, taking the road of characteristic development, with the continuous change of the market, the requirements of time schedule are also gradually tense. After the reverse design work of some car types, we summarize the reverse workflow of the outer panel of the car body as follows

reverse engineering flow chart

3 key technologies of reverse engineering

there are three key technologies in the reverse engineering application of body outer panel

(1) quickly and accurately collect three-dimensional point cloud data from the 1:1 automobile body sludge model

(2) create mathematical models of surfaces quickly and with high quality

(3) correctly analyze and evaluate the mathematical model of the surface

3.1 collection of three-dimensional point cloud data

the three-dimensional point cloud data of the exterior panel of the car body is to discretize the physical surface of the clay model in the form of three-dimensional coordinates. The measured point cloud data is the basis of surface modeling and surface quality evaluation. At the same time, the process of physical data acquisition for the oil sludge model is a time-consuming key link, which is directly related to the speed and quality of surface modeling. How to finish data collection with high quality in a short time is the first key technology. At present, there are three measurement technologies widely used in China:

(1) contact coordinate measurement: the measurement method used in traditional contact coordinate measurement can generally achieve high accuracy, but due to the direct contact with the sludge model of the car body in the measurement process, the measurement efficiency is reduced first, and it is easy to scratch the sludge surface, causing bad points in the point cloud data. It is very suitable for measuring holes, curved surface feature lines, etc. in the measurement of body outer panels

(2) linear laser beam measurement: this measurement method is also known as phase measurement. The basic principle is to project periodic grating onto the measured surface, and calculate the required three-dimensional point cloud information through the modulation and demodulation of grating image. This measurement method has the advantages of large measurement range and fast speed, but it is only applicable to the measurement of flat surfaces with little fluctuation. It is mostly used for the measurement of large outer covering parts such as hood and door outer panel

(3) grating projection measurement (photographic measurement): grating projection is to project the grating onto the surface of the object to form an area to be measured. The surface data of the object is measured by the optical scanning system, and the three coordinate position of the point cloud data is obtained by the digital camera. This measurement method is non-contact measurement. There is no scratch on the surface of the measured object, and the complex surface can be measured in blocks. The measurement speed is fast, and the point cloud density is high

various measurement methods have their advantages and disadvantages. In reverse engineering, flexible selection can be made according to different products. In the data measurement of our company's body outer panel, the main body adopts the grating projection measurement method, and then uses the contact coordinate measurement to measure the surface feature line on the body, and finally obtains the complete point cloud data of the body outer panel. Figure 1 shows the point cloud data of the outer surface of the public car developed by our company

Figure 1 Songhua River Minyi car point cloud data

3.2 requirements for the surface of the body outer panel

in the design of the body outer panel, the most basic requirement for the surface is smoothness. The meaning of smoothness includes mathematical definition and subjectivity. Now let's talk about 1. What are the factors of the performance characteristics and quality of pressure testing machine? Aesthetic description. The mathematical condition of surface fairing is that there are no redundant convex and concave areas. Generally, the fairing of the section curve of any plane and surface is used as the criterion of surface fairing. The subjective aesthetic description of the body surface is mainly reflected in several local and overall visual effects:

1) the light reflection effect formed by a single surface itself

2) the light and shade changes formed at the splicing and turning points between surfaces. This description will play a decisive role in the final aesthetic review of the whole vehicle

3) highlight the boundary and transition surface of the surface with a highlight lamp to form a highlight and highlight outline. From the visual effect, it should be continuous and smooth, and local redundant convex concave, wrinkle or scattered light and shade contrast and other defects are not allowed

surface fairing mainly affects the following aspects:

1) modeling effect: under the irradiation of external light sources, the light reflection and shading effects should be better

2) stamping processability: the smooth surface has good material fluidity in the stamping process, especially for the outer body covering parts, wrinkles are absolutely not allowed

3) requirements of CAD, CAE and cam: if the surface is not smooth, it will have adverse effects when executing some functional modules. The simplest example is the offset module. In addition, if the surface is not smooth, the calculation time will be too long due to the dense lattice in the process of CAE dividing the finite element lattice; In some cases, it is even impossible to divide the lattice. The unsmooth surface also leads to poor processing technology of the mold, which makes it difficult to program NC machining, and may also cause knife pricking during machining

our company generally divides the surface quality into three levels: A, B and C. Class a surface is required to meet all the above fairing conditions. From the perspective of reverse engineering, A-level surface should be as close to point cloud data as possible on the basis of conforming to the design intent and continuous curvature. According to different software standards, the deviation between the boundary and internal surface and the measured point cloud data is not more than 0.5mm, and there is no change in geometric shape and difference in visual effect during lighting in the manufacturing process. Class B surface is required to meet the basic conditions of surface fairing, and the standard can be appropriately reduced in terms of curvature changes and local visual effects. Small deformation is allowed in the manufacturing process. However, it is mostly used for inner plate stamping parts of car bodies, such as wheel housings, floors, etc. Level C surface refers to other surfaces except level a and B

3.3 construction technology of mathematical model of curved surface

3.3.1 block of body panel

the surface of body panel is composed of several free-form surface patches. There are differences in the size and shape of these patches. How to divide the body panel surface into reasonable free-form surface blocks is the key preparation technology to construct the body panel surface mathematical model. Figure 2 describes the free-form surface segmentation of Songhua Jiangmin Yiche with specific examples:

Figure 2 Songhua Jiangmin Yiche free-form surface segmentation (the curve in the figure is a clear prism)

from the diagram, we can see that the main basis for dividing the surface patches of the body surface is to measure the curvature change law of the body characteristic prism and surface in the point cloud data. Among them, the characteristic edges of the body surface can be divided into clear edges and fuzzy edges. Clear curve is the intersection line of the main surfaces of the car body. For some shapes of modern car bodies, some clear edges on the car body can show the remarkable characteristics of the car body shape. It is mainly used to divide the functional areas of the car body surface to show the strength and dynamic visual effect of the shape. The fuzzy curve is the highlight contour formed by connecting the main surfaces with the transition surface. In the process of building the surface, the fuzzy edge line is an important basis for connecting the main surfaces. In addition, the following factors should also be considered when dividing the free-form surface of the car body:

1) modeling requirements

2) feasibility of body structure

3) the establishment of surface patches and the splicing method between surfaces

the division of body free-form surface directly affects the subsequent design work. In the whole process of division, stylists, measurement engineers and reverse engineering engineers need to fully discuss and study, and even repeat it several times. To a certain extent, some methods of concurrent engineering are adopted

3.3.2 surface establishment and fairing

in the process of establishing the surface model of the exterior panel of the car body, there are three common construction methods:

(1) lattice surface: also known as point to surface. That is, the surface patch is directly generated from the point cloud data. The software directly uses the crisscross point cloud lattice extracted from the point cloud data, and uses the method of u and V direction parameter lines to approximate the point cloud data to fit the surface. This method is mostly used in the construction of the surface model of windshield. Figure 3 shows an example of the construction of the front windshield of Minyi car in Songhua River

figure a windshield glass Point Cloud Figure B generate the final surface from the point cloud figure C

Figure 3 construction example of windshield glass in front of Minyi car in Songhua River

(2) section driven surface: it is the most widely used surface construction method. This method is the most widely used surface construction method, which is formed by controlling the cross-section shape at different positions of the generated surface and moving according to the determined ridge line after establishing the four boundaries of the generated surface. It is usually used to establish the main surface of body panels

(3) regular description surface: the surface generated by regularly changing a certain characteristic quantity of the surface, such as angle, radius, area, etc. according to the definition

in the practical engineering application of the mathematical model of the surface of the body outer panel, each method can be used separately, but in most cases, it is necessary to comprehensively apply different methods, repeatedly adjust the elements that generate the surface model, and finally get the ideal smooth surface

3.3.3 splicing and clipping of free-form surfaces

splicing of free-form surfaces

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