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Reverse engineering solid modeling technology based on STL model

Abstract: for the part model represented by STL data, based on the analysis of the geometric characteristics of the structural part model, it is proposed to continue to carry out in-depth international cooperation in the key fields of equipment, which is based on the separation of geometric elements and topology, Shandong Ruifu lithium, Huayou cobalt, jiangte Electromechanical, Yahua industry The STL model reverse engineering entity modeling technology based on the reconstruction of the relationship between Hunan Changchang lithium Co., Ltd. and other foreign mines. Through the combination of triangular patches, the separation of plane, cylinder, cone and other basic geometric voxels is realized, and the Parasolid system is used to complete the voxel reconstruction, and further extract the Boolean relationship between geometric voxels, so as to realize the product model reconstruction with topological relationship. Using this method, the rapid integration between re/rp system and general CAD system can be realized, and the smooth transfer of product data between different systems can be realized

Keywords: STL; Reverse engineering; Solid modeling; Model reconstruction

1 reverse engineering CAD technology and STL model

reverse engineering CAD technology generally take the output data of digital measurement equipment as the original information source [1]. Due to different measurement methods, digital measurement equipment can be divided into contact and non-contact. With the development of measurement technology, no matter what kind of measurement method, there are many measurement data, especially the non-contact laser measurement, which can produce hundreds of thousands or even millions of measurement points. We call this kind of data "point cloud" data. Generally speaking, digital measuring equipment is equipped with data processing software. The main function of this software is to preliminarily process the data output by the measuring equipment, such as removing obvious noise points, merging multiple pieces of data, data format conversion, etc. In addition to outputting data in user-defined format, general measuring equipment provides data output in IGES format. With the enhancement of software functions, many measuring devices can output triangle lattice data (that is, the data after triangulation) or STL format data at the same time of outputting measurement data. However, these STL format data are generally not tested (such as the sealing is not guaranteed, there may be cracks, etc.), and cannot be directly used for reverse engineering modeling or RP manufacturing. The STL data output by the measuring equipment must be repaired and corrected before it can be used for reverse engineering CAD modeling. Therefore, data preprocessing is an important part of reverse engineering

this paper focuses on geometric model reconstruction, so the data we use are effective data after preprocessing, rather than the original data from the measuring equipment

2 reverse engineering solid modeling based on STL model

stl model is a geometric model that represents the outer contour shape of an object with a set of triangles [2]. In practical applications, there are requirements for STL model data, especially in the RP field where STL model is widely used, STL model data need to be tested before use. This test mainly includes two aspects: the validity of STL model data and the closeness of STL model. Effectiveness inspection includes checking whether there are cracks, isolated edges and other geometric defects in the model; The closure inspection requires all STL triangles to form an inner and outer closed geometry [3]. The STL model in the STL model reconstruction technology discussed in this paper assumes that it has been tested for effectiveness and closure, and is a correct and effective STL model

since STL model only records the geometric position information of the object surface, without any topological information expressing the relationship between geometry, it is a very critical step to reconstruct the topological information based on the position information in the reconstruction of solid model. On the other hand, the vast majority of product parts (structural parts) in practical applications are obtained from regular geometric bodies (such as polyhedron, cylinder, transition arc) through topological operations, so the reconstruction of topological relations is particularly important for the reconstruction of structural part models. In fact, the commonly used B-rep model in cad/cam systems is expressed by Boolean operations based on this boundary representation of basic geometric primitives

therefore, the process of STL model reconstruction is as follows: first, rebuild the triangular topological relationship of STL model; Secondly, the basic geometric voxels are decomposed from the whole model; Reconstruction of regular geometric voxels; Then the topological relationship between these geometric voxels is established; Finally, rebuild the whole model

2.1 triangle topological relationship reconstruction

stl model only contains the geometric position information of products, and STL data contains a large number of duplicate data. Many advantages of intuitive and convenient triangle installation in STL are that the vertex of the shape is repeatedly recorded, and its repetition degree is n (n is the number of triangles with this point as the vertex). In order to improve the operation efficiency in the subsequent steps, it is necessary to first remove redundant data and improve the storage and operation efficiency [4]. At the same time, establish the triangle topological relationship of STL model, that is, establish the edge information structure of the triangle and record the left and right triangles of each edge. Considering the efficiency of sorting operation, we use a two-way linked list to record the left and right triangle information of each side

the establishment of triangular topological relations not only simplifies STL model data, but also is the basis for the separation of subsequent geometric voxels

2.2 STL model geometric voxel separation

because STL model is composed of triangle sets, it is impossible to separate different geometric voxels from each other directly using triangle data. If the triangles describing the plane part of the model are merged, and then the boundary of the plane is extracted, different geometric voxels can be distinguished by using the boundary

The plane is the simplest geometric element. According to the plane characteristics, we can get the plane boundary extraction criterion: if the normal vectors of two triangles with adjacent edges are parallel, the two triangles form a plane. With this extrapolation, all triangles with adjacent edges and normal vectors parallel to the plane belong to the plane. The process of plane recognition is a recursive cycle: let any triangle be a "seed" triangle, and its normal vector is N0. Ni is the normal vector of the triangle with adjacent sides to the "seed" triangle, and the included angle of N0 and Ni is calculated θ= Arccos (if θ
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