In order to meet the needs of the market, more is to adapt to the continuous development of advanced manufacturing technology, the need for solid CAD data, in order to take advantage of advanced forming systems (RF), computer-aided manufacturing (CAM) systems, product data management (PDM) and other advanced Technology handles and manages it, and further revise and redesign optimization. Because of this demand, a solution is needed: sample-data-product. Reverse engineering was born, and reverse engineering is also known as reverse engineering (RE). Reverse engineering provides a brand-new and highly efficient reconstruction method for the manufacturing industry, realizing the direct conversion from actual objects to geometric models.
There are two main requirements for the reverse engineering of entities: On the one hand, as the object of research, the physical products are the most extensive and largest type of design results for the consumer market, and are the most accessible research objects; on the other hand, they are developed and manufactured. In the process, although the computer geometry modeling technology has been widely used, there are still many products. Due to various reasons, the computer-aided design model is not initially described. The designers and manufacturers face physical samples.
In reverse engineering, the reconstruction of 3D CAD models refers to the use of scattered point data on the surface of the product to construct an approximate model to approximate the product prototype by interpolation or fitting. The reconstruction of 3D CAD models is the most critical and complex part of the entire reverse engineering, and is the basis for subsequent product manufacturing, rapid prototyping, engineering analysis, and product redesign. With the deep research of the relative technical theory of reverse engineering and CAD reconstruction, the commercial application of the results has been increasingly emphasized. A large number of commercial reverse engineering CAD modeling software systems have emerged.
1 CAD Modeling Technology and Software System
Product's 3D CAD modeling refers to the process of generating a corresponding CAD model from an existing physical model or physical part. It includes gridding, feature extraction, surface slicing, and surface generation of discrete measurement points of an object. The most critical and complex part of the RE process is to provide mathematical model support for subsequent engineering analysis, innovative design and manufacturing applications. Its contents include computer, image processing, graphics, computational geometry, measurement and numerical control processing and many other cross-disciplinary and engineering fields. It is a hotspot and difficult issue in the academic community at home and abroad, especially CAD/CAM.
In an actual product, there are not many situations where only one surface is formed. The product surface is often formed by mixing multiple surfaces. Due to the different types of the formed surfaces, the general steps of the CAD model reconstruction are: firstly segment the point cloud data according to the geometric features, then fit each surface patch separately, and then through the transition, intersection, clipping and rounding of the surface. By means of other means, "sew" multiple surfaces into one whole, namely a reconstructed CAD model.
According to the different topological forms of the surface, free-form surface modeling methods are divided into two categories: the first one is based on a triangular Bezier surface construction method; the second is NURBS (non-uniform rational B-spline) curves and surfaces Based on rectangular domain parameter surface fitting method. The two surface reconstruction schemes have their own advantages and disadvantages.
Triangular Bezier surface fitting has the advantages of flexible structure and good boundary adaptability. It has great flexibility in fitting complex shapes and surfaces, so it is widely used in surface fitting of scattered data points. This method is best suited for objects with irregular, complex shapes, especially objects such as faces. However, the curved surface model constructed by this method does not conform to the product description standard and is difficult to exchange data with the general CAD/CAM system. Therefore, the surface generated by this method must be converted to a NURBS surface. However, in the process of conversion, some information will inevitably be lost. There will be some errors. At the same time, this method has the disadvantages of insufficient ability to modify the surface and poor controllability, which limits its application in practice.
The NURBS method is based on the B-spline method, which achieves the unification of analytical geometry and free-form curves and surfaces. It has become the mainstream of surface mathematical expression in CAD/CAM software. This type of surface can be applied to four-parameter surface patch interpolation, stretching, rotation, lofting or skinning, sweeping, blending, and four-boundary constructions. A complete set of surface delays (adaptation, reduction, transformation, splicing, and smoothing algorithms). However, this method can only obtain light if the measurement point data satisfies the requirements of a tensor product and the data does not change drastically. The smooth surface is not suitable for surface reconstruction of large-scale scattered data.
With the deep research of reverse engineering and its CAD reconstruction theory, the commercial application of its achievements has also been increasingly valued, and a large number of commercial reverse engineering CAD modeling systems have emerged. At present, there are dozens of softwares that provide reverse modeling function on the market, but they can be roughly divided into two categories: First, dedicated reverse software such as Imageware, Geomagic, Polyworks, CopyCAD, ICEMSurf, and RE-Soft; The second is to provide forward CAD/CAM software for reverse processing modules, such as PTC's Pro/Scan-tools module, UG's Point cloudy function, and so on.
The reverse engineering software adopts NURBS or triangular domain surface modeling. It has strong point cloud processing function, but it is difficult to recognize features and depends on the operator's experience. At the same time, the function provided by the surface fitting model is provided. Better than advanced forward CAD/CAM software. The forward CAD/CAM software mostly adopts B-spline or NURBS method for surface modeling, and has a strong surface modeling function. Although there are special modules for the processing of point cloud data, compared with the reverse software, the processing function is not yet rich and powerful.
At present, the application of CAD reconstruction technology in reverse engineering is still a very professional job. How to choose the reverse construction that suits the needs of the enterprise according to the actual situation (such as the level of the model reconstruction staff, the characteristics of the enterprise product, etc.) The modular software system still has more practical significance.The literature compares the commercial reverse software on the market, but focuses on the basic operation of the software, the literature from the data point processing, curve and surface fitting, editing and quality The evaluation, processing and engineering analysis, etc., compared and evaluated the reverse engineering function of commercial CAD/CAM software, and the literature was based on the surface modeling technology in reverse engineering, and analyzed the commercial reverse software (module). Regardless of whether it is dedicated reverse software or forward CAD/CAM software that provides reverse modules, in the entire reverse engineering CAD model construction, it still does not have excellent and ideal universal processing capabilities, in the current reverse CAD modeling. In combination with the reverse characteristics of different requirements in practice, the use of special reverse modeling software based on forward CAD software is Processing method popular.
2. Application examples
The process of constructing a surface model of a car surface in Imageware is: firstly, using I}nagew's point cloud data processing function to construct a feature line, and segmenting the point cloud data. Reuse image. The facet function builds the final surface model based on the feature line and the segmented point cloud.
3. Conclusion
CAD modeling in reverse engineering involves many fields of research. Its theory and method are of great significance both in deepening the theoretical understanding of three-dimensional object recognition and in engineering practice. Because of this, it has received extensive attention and investment from domestic and foreign researchers. However, the current work is still generally limited to the study of special objects, special measurement data, and special modeling methods. There is a lack of communication in different fields. There are still difficulties in the study of objects and data in complex topologies, and there are no ideals. The universal method.
Reverse engineering technology, especially the reconstruction of three-dimensional models, is still a highly professional work. In addition to understanding product features, manufacturing methods, and skilled use of CAD software and reverse modeling software, it should also be familiar with upstream measurement equipment and The measurement process to understand the data structure, as well as the requirements of the reconstruction model for subsequent processing. Therefore, the quality of model reconstruction is still directly affected by the operator's experience and level. How to increase the degree of intelligence in software and reduce the reliance on designers is the direction of future development.
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