Reverse engineering and rapid prototyping
abstract with the rapid development of RP industry, reverse engineering plays an increasingly important role in the field of rapid prototyping and manufacturing. As a new product design idea and method, it is more and more widely used. Therefore, we should try our best to avoid the above phenomena in the field of rapid prototyping and manufacturing, and have achieved a lot of results. This paper comprehensively discusses the emergence, development, current research and application of reverse engineering and its important role in the field of rapid prototyping manufacturing, and briefly introduces the measurement principle of its equipment
key words reverse engineering rapid prototyping manufacturing three-dimensional measurement
in the modern information society, the competition in the manufacturing industry is becoming increasingly fierce. The speed of product updating is accelerating, and the production mode is becoming small batch and multi variety. Under this background, reverse engineering technology came into being and developed rapidly. In China, represented by Huazhong University of science and technology, Tianjin University, Xi'an Jiaotong University and Tsinghua University, the reverse engineering equipment developed has reached the product level and occupied a considerable market share in China
II. Basic concept of reverse engineering
for an existing sample, use a 3D digital measuring instrument to accurately and quickly measure its contour coordinates, and reconstruct the 3D CAD surface. After modification, it is transmitted to the general cad/cam system, and then the NC machining path of the tool generated by cam is sent to the mold required by the CNC machining equipment, or sent to the rapid prototyping machine (RP) to make the sample. This process is called reverseengineering
the traditional design process is to carry out conceptual design first according to the function and purpose of the product, then output the design drawings of the product through CAD, compile the NC code and input it into the CNC processing equipment for product processing or make samples through the rapid prototyping machine after review. Reverse engineering starts with an existing part or product model. First, it carries out digital measurement to obtain its contour coordinate value, and then obtains its 3D CAD model through 3D CAD surface reconstruction and outputs drawings. After passing the review, it is processed by CNC processing equipment or rapid prototyping machine. As shown in Figure 1
III. The significance of reverse engineering in rapid prototyping manufacturing
to survive and develop in the modern manufacturing industry full of fierce competition, the speed of product development and the flexibility of manufacturing technology play a decisive role. The emergence of RPM (Rapid Prototyping & Manufacturing) technology has greatly accelerated the speed of product development and significantly improved the flexibility of product design. The processing process of rapid prototyping is essentially a layer stacking process in the z-axis direction. The rapid prototyping process starts with the three-dimensional solid model, after appropriate processing (triangulation, slicing, smoothing, lattice division, etc.), and finally obtains the processing data of each layer. Therefore, it is one of the key problems in RPM technology to obtain the 3D solid model of the product. For most products, its 3D model can be designed with general CAD software (such as UG, pro/e, I-DEAS, etc.). But drawing a complex part model with CAD software is a very time-consuming and laborious work. And sometimes what the engineering community provides is often a physical object, which needs to be used to manufacture molds or make design improvements based on it. Reverse engineering technology provides a way to obtain 3D CAD models directly from real objects. Using reverse engineering technology is much faster than using CAD software to draw. Generally, complex small and medium-sized parts can be completed in a few hours or even tens of minutes, while drawing with CAD software often takes several days to complete. At the same time, it also greatly reduces the requirements for the technical level of staff
IV. research and application of reverse engineering technology
in reverse engineering, the traditional measurement method is contact measurement. Although this method has high measurement accuracy, it also has some inherent defects. The measurement speed is slow, it is easy to scratch the surface of the tested parts, and it is impossible to measure some surfaces that cannot be touched by the probe. With the development and maturity of photoelectric technology and computer technology, non-contact laser measurement method is usually used now. The laser measurement method avoids the inconvenience of probe radius compensation in contact measurement, and can realize high-speed three-dimensional measurement of various surfaces. There are many ways of laser measurement, including point scanning, line scanning and grating scanning. In rapid prototyping manufacturing, laser line scanning method based on optical triangulation principle is the most widely used measurement method (see the fifth part of this paper for the principle)
at present, many foreign reverse products have entered the Chinese market. For example, the replica laser rapid scanning measuring instrument of the UK 3D scanner company, the American "laserdesign" surveyor1200 stereo laser scanning system, and the ATOS measuring instrument of the German GOM company. These measuring devices have perfect functions, but they are quite expensive. In recent years, many universities in China, such as Huazhong University of science and technology, Tianjin University, Xi'an Jiaotong University and Tsinghua University, have also strengthened the research and development of reverse engineering measurement equipment, which has been commercialized and widely used in machinery, automobile, household appliances, medical treatment, light industry and other industries. As the price of domestic equipment is far lower than that of foreign equipment, and its functions are continuously enhanced, it has great development potential and competitiveness
v. laser line scanning process and its triangulation principle
the basic process of laser line scanning method is as follows: the line laser generator sends out line laser to scan the scanned entity, the CCD camera collects the diffuse reflection image of the surface of the measured entity and sends it to the computer. Instead, the equipment image processing software performs binarization, thinning and other processing on the input image, and then uses the triangulation principle to calculate the Z coordinate value and combines x The Y coordinate value (x and Y coordinates are provided by the motion system) generates a dense data cloud stored in a certain data format. The 3D CAD reconstruction software uses this data cloud to reconstruct the 3D solid graphics. The final generated graphics are stored in STL format, so that the rapid prototyping machine can process the tested entity
the hardware of triangulation method consists of line laser generator, CCD camera, image acquisition card, corresponding connecting line and power supply, and microcomputer. If two CCD cameras with identical parameters are placed symmetrically, the measurement blind area can be reduced and the measurement accuracy can be improved. The triangulation method calculates the Z coordinate value of the object by using the relationship between the datum plane, image point, object distance, image distance, etc. (as shown in Figure 2)
in the figure, i-incident light, l-lens, n-imaging screen, u-object distance of lens L, v-image distance of lens L, intersection of o-l optical axis and incident light I, A-light point on the object plane, a ', O' are image points of a and O respectively, h-height of light point on the object plane relative to the reference plane, a-angle between incident light and optical axis, M '- target plane, m-reference plane
according to the lens imaging principle, taking the plane m where the intersection of the incident light and the lens optical axis is located as the reference plane, the calculation formula of the height h of the light point a relative to the reference plane m is:
where: u, V and a are system parameters and are fixed values, so the value of H can be calculated from H '
the author believes that reverse engineering has fast measurement speed and high accuracy (± 0.005~ ± 0.01mm), which can meet the requirements of rapid prototyping manufacturing. However, from the current development level of reverse engineering, there are still many problems. For example, the accuracy of some algorithms in each link of reverse engineering is not high, and the measurement of three-dimensional entities with high accuracy requirements can not meet the requirements; The amount of data obtained from 3D solid measurement is very large, and it takes a long time for data processing and 3D reconstruction. Therefore, it is necessary to continue to strengthen the improvement of reverse engineering so that it can be continuously improved and developed
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