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Car interior door panel computer-aided modeling

introduction

auto panel is a typical large sheet metal part with complex surface. The shape of this kind of parts is extremely complex, generally three-dimensional surface, in many cases complex free-form surface and sculpture surface. The inner door panel of the car is the most representative one of the automobile covering parts. It has the characteristics of multiple islands and pits, large blank size, mixed complex curved surfaces in many places, and various surface shapes. Therefore, the geometric relationship of its main parts can only be reflected by projection lines on the product drawing, and its shape characteristics can not be reflected generally. This makes the initial product model design difficult to be designed directly on CAD software, so the general method is to first manufacture the physical model (wood model, clay model, wax model, etc.), then measure the physical model, and then obtain the three-dimensional physical model through the processing of the physical model. In addition, there may already be a product, but there is no solid model of the product, so reverse engineering has become an effective method to solve this problem [1]. Taking the computer modeling of car interior door panel as an example, this paper discusses the application of reverse engineering in the computer modeling of complex panel materials, as well as the problems worthy of attention in computer-aided modeling

1 general steps of modeling

for complex sheets such as car interior door panels, the general steps of applying reverse engineering technology to establish CAD models are as follows:

(1) discrete digitization of initial shape, that is, the spatial position information of discrete points on the surface of the measured object is obtained by non-contact or contact scanning measuring machine. The non-contact measurement methods include grating method, holographic method, three-dimensional depth image measurement method, laser triangulation method, etc. these methods have their own application scope. The most commonly used method in contact measurement is the coordinate measuring instrument method (CMM). Using the coordinate measuring instrument method can achieve high measurement accuracy, but it also has some shortcomings, such as the probe is easy to be damaged, scratched parts, and seriously affected by the operation level and manual intervention. Because this method has been relatively mature in application, it is still widely used on the site, so enterprises have not reduced the price for sales and application due to high inventory [2]

(2) data processing after discrete digitization, including compensation of contact radius of contact CMM and conversion of scanning data into IGES, DXF or other data formats. If an effective non-contact measurement method is adopted, the compensation problem of the probe radius can be avoided. However, in parts with large size and particularly complex shape such as the inner door panel of a car, there are few practical and effective non-contact measurement methods, so the three-dimensional compensation of the contact radius of CMM is also a problem that can not be ignored. IGES (Initial Graphics Exchange Specification) defines a set of geometric and non geometric data formats and corresponding file structures commonly used in cad/cam. It is the most commonly used data exchange standard in cad/cam/cae systems at home and abroad

(3) transfer the compensated and converted data files into the modeling software to obtain the basic contour connected by the basic curves (straight lines and arcs). The commonly used commercial modeling software includes ug- Ⅱ, catia-cadam, Ⅰ -deas, pro/e, etc

(4) according to the modeling requirements, use the information provided by the product drawing and the contour line obtained in the modeling software to generate the three-dimensional surface of the model. The modeling process is shown in Figure 1 below

Figure 1 modeling process

2 key technical problems of modeling

in the process of establishing the CAD model of automobile panel, the following technical problems must be paid attention to. Whether these problems can be correctly handled is often the key to the accuracy of the final model

(1) the data measured by the contact CMM is actually the coordinate value of the probe center, so the compensation of the probe radius must be considered. Generally, the radius compensation method of the probe is two-dimensional. Because the surface of the covering part such as the inner door panel of the car is an irregular complex curved surface, and the contact point between the probe and the workpiece is not located in the measurement plane determined by the center of the probe, the two-dimensional compensation of the probe in the measurement plane is bound to have large errors, which cannot meet the conditions of high accuracy required by the car covering part, so the three-dimensional radius compensation method of the probe must be used. At present, there are many three-dimensional compensation methods for probe radius. A practical method is to regularize the measured data, and then take the double ordered nodes as the intersection points to form a network, which can reflect the characteristics of the measured surface. So as to simplify the measurement operation and eliminate the equipment for making this 2mm notch in the sampling points and positions in each section. What is needed is the impact specimen notch broaching machine! And it can make the three-dimensional compensation of probe radius suitable for a variety of different types of complex surfaces. According to the one-to-one correspondence between the probe center track surface and the points on the measured surface, the corresponding probe contact coordinates on the measured surface are found from the probe center coordinate points, so as to carry out three-dimensional compensation of the probe radius [3]

(2) the standard used for data exchange is also a noteworthy issue. Before the scanning data measured on the CMM is transferred into the special modeling software, it must be converted into a standard data format file. After the final modeling work is completed, the data exchange is also involved before the model is transferred into other cam or CAE systems. Considering the convenience of data conversion between systems, general standard data format should be adopted as far as possible. It is recommended to use the most commonly used IGES format. General cad/cam/cae systems have special IGES interfaces

(3) in the modeling work on the modeling software, the model to be established must be carefully pondered for a long time, and the design drawing of the product and the original model (or product) as well as the contour line obtained from the initial scanning must be used as much as possible to obtain an overall understanding and grasp of the maintenance and use structure of its general instruments and meters. In addition, we should fully understand and make use of various functions of modeling software

(4) finally, the quality of the model depends not only on the fitting degree of the model to the shape of the real object, but also on whether the surface is smooth and whether the connection of the surface is smooth. Since the shape value points are directly taken from the contour line points obtained by scanning the surface of the real object, as long as the selection of the shape value points can ensure accurate position and sufficient quantity, the resulting curves and surfaces can meet the requirements of the fitting accuracy of the real object. In addition, the fairing of curves and surfaces is a problem that should be paid special attention to. Generally speaking, the conditions to meet the fairing of curves are: ① second-order smoothness, that is, continuous curvature; ② There is no redundant inflection point in the curve; ③ The curvature changes evenly. The conditions for surface fairing are: ① fairing the key curves that make up the surface; ② The grid of the surface has no redundant inflection points; ③ The Gaussian curvature of the surface changes uniformly [4, 5]. Generally, the smoothness of the surface is tested by rendering the surface. Through perspective rendering, specular reflection, transparency, reflection, multiple light sources and other processing means, realistic and visually good images can be generated. Observing the smoothness of the generated surface from all angles can comprehensively judge the smoothness of the entire surface

3 establishment of CAD model of car inner door panel

3.1 discrete digitization of door inner panel shape

measurement is carried out by contact coordinate measuring machine. Due to the influence of some factors such as probe radius and measured spacing, the measured data must be three-dimensional compensated by probe radius error, and then converted to IGES format data file

3.2 the modeling system accounts for 68% of the global output

the system hardware adopts SGI indigo2 workstation (memory 128M, hard disk capacity 2GB). The operating system is irix6.2. The modeling software is EDS' general cad/cam software Unigraphics (UGII) (version v13.0), which has strong modeling functions and rich modification functions, and can easily generate various complex and accurate 3D surface models

3.3 modeling steps

the scanned data is transferred to UG Ⅱ software after three-dimensional compensation of probe radius. The generated contour line is shown in Figure 2, and then the modeling work is carried out according to the modeling flow chart 1 described above. Finally, the three-dimensional CAD model of car interior door panel is shown in Figure 3

Figure 2. The scanned data is transferred to the outline of UG Figure 3. The three-dimensional CAD model of the inner door panel of the car

4 conclusion

(1) the computer-aided modeling of automotive panels is a difficult work. In order to complete this work well, we must have sufficient understanding and grasp of the established model in space on the basis of mastering the theory of computer graphics and skilled use of modeling software, The process of modeling is actually a process of reproducing the model in the mind of the modeling staff on the computer

(2) the ultimate goal of reverse engineering is to establish a system that can completely reproduce the physical geometric model. However, the current technology is still a long way from the realization of this step, and a lot of work still needs to be completed by manpower. Therefore, to a certain extent, work experience is still a decisive factor

(3) the CAD model of car interior door panel established by computer-aided modeling has laid the foundation for subsequent cam processing and CAE simulation analysis, so it is of great significance for the design and manufacture of auto panel dies. (end)

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