Hybrid modeling technology of traditional design i

2022-08-08
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Hybrid modeling technology to promote traditional design

in the past decade, digital shape sampling and processing (DSSP) has evolved into complex computer-aided design and product development, helping thousands of customers convert 3D scanning data into digital models, so that they can be used for design, analysis, manufacturing and visualization

dssp software automatically generates NURBS surfaces from point cloud data, so that users can accurately capture and reconstruct shapes. It is this function that makes it have the following uses: (1) capture physical design and prototype; (2) Recycling profit and loss parts, molds and tools; (3) Copy complex and organic shapes; (4) Prepare for modeling and CAE application as construction model; (5) Enable unique parts to be customized on a large scale; (6) Preserve historical relics

in recent years, the complementary relationship between existing CAD technology and digital shape acquisition and processing has been introduced into hybrid modeling. The so-called hybrid modeling is the combination of scanning based measurement and feature-based modeling. This paper discusses the advantages of hybrid modeling method, and explains how to use it to build a native, parameterized and complex impeller model, which can be used to make casting and production molds

advantages of combination

traditional CAD models are established by using the processes determined by two-dimensional and three-dimensional entities (features). CAD designers use their skills and experience to complete the new design, that is, mandatory modeling, by controlling the solid forming through the specified solid parameters. The completed parametric model has great flexibility and can generate a variety of variants, which is convenient for repeated design and rapid experiments with various forms and functions, as shown in Figure 1

Figure 1 is an example of challenging object modeling used in traditional CAD modeling

(picture provided by Geomagic)

when modeling from scratch, the traditional CAD method is good, but when users face the challenge of reconstructing a complex surface, it has defects. It takes a lot of time and energy, and it is not guaranteed to be an accurate model. In some cases, it is almost impossible to reconstruct the surface using function based methods, because it is difficult to determine and quantify the parameters used to control the shape of the object

hybrid modeling provides a solution to the deficiency of traditional CAD in reconstructing complex surfaces. Basic geometric parameters such as datum, curve and original features can be measured and extracted from 3D scanning data. It is easy to switch between the real source SP software of CAD and DS, so we can choose which program is better

capture the physical part

the first step in this process is to capture the existing tangible part. The impeller is scanned by the "white light" scanning system, that is, two high-resolution cameras are used to capture the surface map projected by the surface of the component. These surface maps on the part are analyzed with millions of points of triangles, and a point cloud is generated

due to the gloss of the part surface, powder coating can be used to reduce the interference of reflection on the projected pattern. Due to its complex shape, it needs to be scanned many times from different positions to provide the light diagram of all surfaces of the impeller, as shown in Figure 2

Figure 2 prepare to scan a impeller for registration

arrange a group of scanning targets on the impeller to assist in adjusting multiple scans. Using photogrammetry technology, the key target in the center of the impeller will be automatically detected by a high-resolution camera. Collect visible light points from all directions and provide enough information to correct each scan

more than 20 scans, scanner capture points. The scan is corrected and merged to create a polygonal model, as shown in Figure 3

Figure 3 original STL data from the scanning system

cleaning and repair

after inputting the scanning data to establish an STL model, the next step is to use an automated process to clean and repair the data in the DSSP software. Typical cleaning procedures include deleting redundant data, reducing defects, extracting data to reduce file size, filling loopholes, and repairing intersections. In this process, the whole polygon model is repaired, but in many cases, only some models need to be cleaned up in the process of parameter reconstruction, as shown in Figure 4

Figure 4 DSSP software is used to clean and complete polygon models

extract curves and datum planes

after creating the model with DSSP software, we begin to reconstruct the central shape by detecting the central axis of the impeller, determine the contour of the center with the extracted curve, and then rotate the curve around the axis to generate a rotating surface

the central axis can be found by selecting the circumference of the impeller surface. This aspect should be designed to be completely cylindrical and produce a stable reference axis. This method is also used to select the central surface to calculate the rotation axis

the next step is to create a side curve that will define the center shape. Here, due to the interference of the blade on the central cross-section, the simple plane cross-section curve cannot be extracted. Fortunately, we can learn from the tools in our CAD system and use another method: variable section scanning. We can extrude the semicircle passing through the central axis to produce a face, and use the non planar curve from the central plane to control the diameter of the semicircle, as shown in Figure 5

on the left side of Figure 5, create curves in DSSP software. In the middle and right, input the curve to CAD and create a variable scanning surface

the bottom surface of the impeller is not subject to any obstacles of the blade, so a simple plane curve can be extracted and used to create a rotating surface. To create additional parameters to control the shape of the surface, we can consider using the orthogonal boundary line of the scanning surface to create a new and idealized curve

combined with free-form plane cutting

blade is the most complex part of impeller design. These surfaces are difficult to measure or define with basic parameters. However, using NURBS plane cutting in DSSP software, blades can emerge quickly and can be input as IGES or step features, as shown in Figure 6

on the left side of Figure 6, NURBS plane cutting in DSSP software. In the middle and right, input NURBS surface to CAD and mapping

after a simple blade is input, it can be copied (or imitated) around the input datum axis to create multiple blades on the central plane. The whole input and imitation process can be repeated with the second separated blade. Instead of directly measuring the angular distance to define the space between blades, it is better to use the total number of blades as a parameter to define the angular distance space. This will allow us to easily change the number of blades while retaining the appropriate blade spacing

after merging the blade surface into the solid model, the parameter radius is defined at the surface intersection. This radius can be adjusted at any time in the future. We can also use the rich tools in CAD system to define other common parameters, including variable radius and rotating ball

trim and fusion

after all blades are positioned, the rest along the performance is stable and reliable, and the datum plane and curve of the outer surface of the impeller can begin to be well determined. Thirdly, use the non planar contour curve and variable section scanning in DSSP software to generate a surface to trim the outer surface of the blade, as shown in Figure 7

on the left side of Figure 7, determine the curve in DSSP software. In the middle and right, trim the periphery with a variable cross-section scanning surface in CAD

and then generate a cylinder surface to trim the entire outer surface of the impeller. This cylinder is centered on the extracted datum axis, and its diameter can be determined by the 3-D feature cylinder of the constructed polygonal surface. After trimming the periphery of the cylinder, we can produce a perfect centrally symmetrical impeller

complete the parametric model in one day

finally, the model can meet all design parameters and coincide with the entity. Using computer-aided inspection software, we can compare the final CAD model with the original scanning data to verify its accuracy, as shown in Figure 8

on the left side of figure 8, computer-aided inspection software is used to verify the accuracy of the model. In the middle and right,

use CAD to modify the number of parameterized blades, and prepare the molding of the model

hybrid modeling method, providing complete parameters to control the surface shape of the object, the impeller radius, and the number and spacing of blades. The whole process, including scanning, trimming and modeling, can be completed in less than one day

with some changes, the 3D model generated by hybrid modeling can be used to create fast modeling. The model can also be used to directly drive the rapid prototyping system called rapid casting to accelerate mold production

benefits

hybrid modeling provides several key benefits

(1) it makes use of the existing 3D investment. CAD manufacturers have invested thousands of people to create highly evolved digital design and creation systems. Hybrid modeling utilizes existing modeling systems, that is, the existing skills of CAD users and experts, and enhances them with 3D measurement and rapid prototyping capabilities

(2) it enables new designs to be created in a shorter time. Compared with traditional CAD technology, the measurement and modeling method of hybrid modeling can greatly reduce the time required to copy an existing design. In some cases, the hybrid modeling method can complete tasks that conventional technology can only complete in days or even weeks in a few hours

(3) generate native parametric CAD geometry. Using the advantages of CAD system, all parties in the design A. adopt PID control strategy to realize the closed-loop control surface of load, which can be controlled by numerical value or other system parameters, such as dense assembly. When users create the core geometry in CAD system, many I/O translation problems can be avoided

(4) it can produce accurate results. Point clouds including millions of sampling points can be used to generate free-form surfaces. The secret of this technology replication is that the surface structure will not be lost. Although the data obtained by the operator through scanning is less than the typical first test, the process is highly repeatable. (end)

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