Summary:Using existing hardware and software facilities, such as three-dimensional measuring instrument ATOS measurement system, pointware processing software Imageware, three-dimensional modeling software UG, etc., to reverse the design process of some product lines as the main line, combined with reverse engineering and industrial design Characteristics, discussion and research on the precision control of reverse engineering and rapid prototyping in industrial design, proposed a new error evaluation method, analyzed the causes of various errors and gave a way to improve the accuracy.
1 Accuracy detection in reverse engineering
With the continuous development of computer software and hardware, CAD/CAM technology has provided more and more support for the design and manufacture of freeform products. However, some free-form products tend to exist initially as hand-made models, ie, no CAD model actually exists. In order to use advanced numerical control processing technology and rapid prototyping manufacturing technology, it is necessary to inversely construct a CAD model from existing physical objects. This is the concept of reverse engineering. Under the environmental conditions in this paper, after comparison and analysis, a feasible system solution can be used as shown in Figure 1. First, the surface of the prototype was measured using an ATOS optical scanner and other devices to obtain a 3D point cloud of a complex-shaped free-form surface product. Then, the CAD model is constructed by surface fitting software Imageware software and three-dimensional modeling software UG, etc., thereby driving the manufacture of rapid prototyping machines.
The research of rapid reverse engineering is mostly focused on point cloud data processing and surface fitting of point clouds, but less research is done on the accuracy evaluation and error analysis of each link. In fact, it is often necessary to detect the precision of manufacturing parts based on a CAD model. It is also very important to evaluate the precision of the reconstructed model at the same time. This is the process of detecting the reconstructed model based on the prototype. This article analyzes each aspect that affects the accuracy of the selected program for these characteristics.
2 Structural light measuring instrument error analysis
GOM recently introduced a digital camera-based optical 3D measurement system. It includes a measuring head, a tripod, a controller and a computer. The middle of the measuring head is an ordinary light source with CCD cameras at both ends (as shown in Figure 2). The data acquisition principle is the projection grating method (as shown in Figure 3).
The meter is free to move around the object under test, enabling fluid operation without any platform support. Each scan photograph can obtain three-dimensional coordinates of 400 000 points on the surface of the workpiece, with a scan accuracy of 0.03 mm and an overall scan accuracy of 0.1 mm/m.
The error produced by ATOS measurement is divided into the error of the machine itself, the influence error of the external factors before shooting, the error when the instrument is calibrated, and the error when shooting.
2. The error of the machine itself
The intensity and illumination uniformity of the projection light source affect the quality of the image stripe. If the illumination of the light source is not uniform, the brightness of the captured stripe image is not uniform, so that the edge of the detected stripe is likely to be broken, which causes troubles for the post-processing. A lamp with good uniformity of illumination and high brightness can be used as a light source to reduce the influence of intensity and illumination uniformity of the projection light source.
2. 2 Influence of external factors
2. 2. 1 Influence of ambient light.
Since the ATOS measurement is performed by projecting light and acquiring light reflected from the surface of the solid body, the ambient light that varies greatly in the measurement will affect the measurement light, thereby affecting the accuracy of the data we measure. In order to prevent the influence of ambient light changes, the meter's own software can pre-set the specified change value. When the specified change value is exceeded, the prompt dialog box will appear.
2.2.2 The effect of a tiny movement of a lens or entity.
This effect is mainly caused by the vibration of the surrounding environment caused by the movement of the lens or the object to be measured at the time of shooting. In order to reduce this error, the software of the measuring instrument's white belt can also be set in advance.
2. 2. 3 Imaging agent effects.
When measuring a poorly reflective body, the developer is sprayed for measurement. When measuring, the surface of the solid should be sprayed as evenly as possible, and as thin as possible; otherwise, the error of the physical size and the measurement size will be increased. In addition, since the particles of different types of imaging agents are different, they also have different effects on the measurement. When some reflective entities are measured, the imaging agent can be used to reduce the measurement error.
2. 3 error in instrument calibration
After the system is installed, a calibration is required, including hardware calibration and software calibration. The hardware calibration mainly calibrates the projected image, adjusts the focus of the intermediate projection lens, and adjusts the focal length and aperture of the left and right lenses. The software calibration can be divided into standard calibration, rapid calibration and extended calibration. The standard calibration is generally performed. When the error of the reference point occurs more than 0.1 times in the scanning process, rapid calibration can be used. The extended calibration is used when scanning very small objects (less than 50 mm x 40 mm). This kind of correction is artificial, because the visual error of the operator makes the measurement system inevitably produce the error.
2. 4 errors when shooting
The multi-view flattening error is mainly caused by the error of the common reference point. ATOS should not be pasted in a straight line when pasting reference points. At least three points (geometric knowledge, not a point on a straight line determine a plane). And as far as possible not to put the reference point in the isolated position or curvature of the larger changes in the position, but to stick in the area of ​​small curvature changes. Avoid elongated triangles, and try to make the most basic three reference points close to the positive triangle.
When attaching a reference point during the actual measurement process, the following points should be noted:
1) AB, C can not be collinear three points, should avoid â–³ ABC becomes a narrow triangle â–³ ABC area is also large enough.
2) The marker point can be either manually set or intrinsic to the measurement object, but it must be a point that can be accurately identified, matched and positioned in the measurement data point set.
3) In practical engineering applications, a circular sticker with a certain area is attached to the surface of the measured object. Since the slanting circle is transformed into an ellipse, and the system has a certain degree of ellipticity, the three small round faces are on a practical plane as much as possible. When they are unsatisfiable, they must take into account the effects of different ellipticity as much as possible. .
How to improve the measurement accuracy is a combination of theory and practice. In addition to the theoretical analysis, it is more important to be cautious and cautious in the actual measurement, and constantly explore the experience in order to reduce various errors as much as possible and improve the measurement. Accuracy lays a good foundation for subsequent processing.
3 Accuracy of Surface Reconstruction Molding
In the reverse engineering, the real surface is unknown, and the CAD model reconstructed from the measurement point set only approximates the real surface. Model reconstruction involves multiple links such as data processing, data partitioning, multi-view flattening, surface fitting and stitching, and solid model building. The error of any one link will affect the accuracy of the final CAD model.
The ATOS non-contact optical scanner can obtain high-density measurements. The point cloud contains a large amount of data and fully expresses the characteristics of the measured surface. Therefore, it can be used as a true surface. Based on this understanding, the comparison between prototype and manufacturing parts must also consider the influence of model reconstruction accuracy. According to this, a two-step comparison strategy of “prototype part point set-refactoring model-rapid prototyping point set†is proposed. The "prototype component point set-refactoring model" is used to evaluate the reconstruction accuracy of the model, and the results can be fed back to the model reconstruction process; "reconstruction model-a rapid prototyping point set" is used to evaluate the manufacturing precision of the manufactured parts. The results can be used to guide improvement of the process. Through this two-step comparison strategy, the shape error between the prototype and the manufactured part can be more accurately analyzed, and the source of the error can be clearly indicated, which is conducive to adopting targeted improvement measures.
3.1 Comparison of Prototype Point Sets and Reconstruction Models to Evaluate the Accuracy of Reconstructed Models
For propeller surfaces, the comparison between the physical model and the mathematical model is generally represented by the distance between the corresponding points of different models. Here, we use the line and plane coincidence detection method to introduce the reconstructed 3D model into Imageware software to analyze the corresponding points of the reconstruction model and the prototype part set. As shown in Figures 4, 5 and 6.
In the case of reverse propellers, the error range of reverse calculation is t0.6mm. Within the allowable error range, the accuracy evaluation of prototype set and reconstruction model has achieved satisfactory results.
3.2 Comparison of Reconstruction Model and Rapid Prototyping Point Set to Evaluate Machining Accuracy
The propeller is a typical thin-walled complex curved part. For thin-walled and complex surface parts, how to choose the right molding process is crucial to ensure its molding accuracy and surface quality. This article uses the SPS600' speed-deformation molding machine developed by Xi'an Jiaotong University, and its maximum molding size is 600 mm x 600. Mm x 500 mm, layer thickness is 0.1-0.2mm, forming accuracy is 0.lmm.
For thin-walled parts, the part placement and support, laser power, preheat temperature and forming temperature, and scanning speed have an effect on the machining accuracy and surface quality of the part.
The placement of parts has a great influence on the forming quality of the parts. For each part, there are numerous ways to place it. The basic principle of part placement is to shorten the length of the scan path in one direction as much as possible and to make the scan paths in both directions have similar lengths. Shown in Figure 7 is the propeller placement and support map.
The three-dimensional scanning of the rapid prototyping parts was carried out. The reconstructed three-dimensional model was introduced into Imageware software using line and plane coincidence detection methods to analyze the corresponding points of the reconstructed model and rapid prototyping parts set. As shown in Figures 8, 9, and 10. Has achieved satisfactory results.
4 Conclusion
In this paper, based on a systematic study of the complex surface measurement technology in reverse engineering and quality control in reverse engineering and rapid prototyping, the product is explored through the reverse design of a typical thin-walled complex curved surface product. Accuracy in the process.
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