In order to adapt to the needs of the market, more is to adapt to the development of advanced manufacturing technology, the need to use real CAD data, in order to take advantage of rapid prototyping system (RF), computer-aided manufacturing (CAM) systems, product data management (PDM) and other advanced The technology processes and manages it and further modifies and redesigns it. Because of this demand, a solution is needed: sample-data-product. Reverse engineering was born, and reverse engineering is also called Reverse Engineering (RE). Reverse engineering provides a new and efficient means of reconstruction for the manufacturing industry, enabling direct conversion from actual objects to geometric models. Next page
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The requirements of physical reverse engineering mainly have two aspects: on the one hand, as the research object, the product is the most extensive and most popular design result for the consumer market. It is also the most easily available research object; on the other hand, in product development and manufacturing. In the process, although computer geometry modeling technology has been widely used, there are still many products. For various reasons, they are not originally described by computer-aided design models, and the designers and manufacturers are faced with physical samples.
In reverse engineering, the reconstruction of a 3D CAD model refers to the use of scattered point data on the surface of the product to construct an approximate model to approximate the prototype by interpolation or fitting. Reconstruction of 3D CAD model is the most critical and complex part of the whole reverse engineering, and is the basis for subsequent product manufacturing, rapid prototyping, engineering analysis and product redesign. With the deep research of reverse engineering and its theoretical research on CAD reconstruction, the commercial application of its results has been paid more and more attention, and a large number of commercial reverse engineering CAD modeling software systems have emerged.
1 CAD modeling technology and software system
The 3D CAD modeling of the product refers to the process of generating the corresponding CAD model from an existing physical model or physical part, including the meshing of the discrete points of the object, feature extraction, surface segmentation and surface generation, etc. The most critical and complex part of the RE process also provides mathematical model support for subsequent engineering analysis, innovative design and manufacturing. Its contents include computer, image processing, graphics, computational geometry, measurement and numerical control processing, and many other interdisciplinary and engineering fields. It is a hot and difficult issue that is widely concerned in the academic circles at home and abroad, especially in the CAD/CAM field.
In the actual product, there are not many cases where only one curved surface is formed, and the product shape is often composed of a plurality of curved surfaces. Because of the different types of surface types, the general steps of CAD model reconstruction are: firstly segment the point cloud data according to the geometric features, then fit each surface patch separately, then through the transition, intersect, crop, and round the surface. By means of other means, the multiple surfaces are "stitched" into a whole, that is, the reconstructed CAD model.
According to different surface topological forms, free-form surface modeling methods fall into two categories: the first is a surface construction method based on triangular Bezier surfaces; the second is NURBS (non-uniform rational B-spline) curves and surfaces. A rectangular surface parameter surface fitting method based on rectangles. These two surface reconstruction schemes have their own advantages and disadvantages.
The triangular domain Bezier surface fitting has the advantages of flexible structure and good boundary adaptability. It has great flexibility in fitting complex shape surfaces and is widely used in surface fitting of scattered data points. This method is best suited for objects with irregular, complex profiles, especially objects such as faces. However, the surface model constructed by this method does not conform to the product description standard, and data exchange with the general CAD/CAM system is difficult. Therefore, the surface generated by this method must be converted into a NURBS surface. However, in the conversion process, some information will inevitably be lost. Certain errors will occur. At the same time, this method has shortcomings such as insufficient modification ability to the surface and poor controllability, which limits its application in practice.