With the process of digital design of motorcycle products, the establishment of CAD models for motorcycle products has become the basis for the design of new motorcycle products. Among them, CAD modeling of complex surfaces is the focus and difficulty of motorcycle modeling. This paper takes a newly designed motorcycle fuel tank as an example to explore the law of motorcycle surface modeling.
First, riding motorcycle fuel tank structure
While riding the fuel storage function, the motorcycle fuel tank has always been an important part of the overall design of the vehicle, and has high requirements on its surface quality and appearance. The main structure of the fuel tank is composed of a fuel tank body, a fuel tank cap, an oil level sensor and a fuel switch. Among them, the structure of the oil tank body is shown in Figure 1. The 3D modeling of the upper tank body and the lower tank body is the main content of the fuel tank modeling.
Figure 1 tank structure
Second, the fuel tank upper body 3D modeling
The data on the upper body of the tank is derived from the sludge model. Using the Atos measurement tank to get a lot of point cloud data, the amount of data is large, the arrangement of data points does not imply any data organization, and there are many background noises, which need to use Surfacer software for data preprocessing, including data sparse, Layered placement, cross-section processing, removal of noise points, etc. Figure 2 shows the point cloud data of the motorcycle fuel tank that was imported into the UG software after Surfacer processing.
Figure 2 fuel tank point cloud data
Since the UG software is based on the NURBS technology for surface modeling, it is necessary to use block reconstruction for the complex curved surface of the fuel tank to construct a method for satisfying the G1 continuous transition surface. How to reasonably block the complex surface, judging the continuous boundary of the surface is the key to the modeling, often requires multiple attempts and adjustments to complete.
Surface segmentation
In the modeling process of motorcycle fuel tank, it is often necessary to design the main profile first, and the intersection line between the profiles constitutes the shape feature line, which is the key part of the design. The intersection line is then chamfered to form a smooth curved surface. Therefore, for complex surface segmentation, planning the surface reconstruction process is a trace of the modeling process of the sludge model. When dividing the surface of a complex surface, the key is to grasp the feature, and at the same time to balance two factors: on the one hand, the single surface is as simple as possible to facilitate the construction of the model; at the same time, the total number of surfaces is in the premise of reflecting the characteristics. Keep it as little as possible to avoid the trivial surface of the model being built.
In the main profile and characteristic line of the tank surface shown in Fig. 3, the curved surfaces A~L are the main profiles, except for the characteristic lines L1~L5, the other characteristic lines can be directly chamfered. L1~L5 expresses the fact that more and more curved chamfers used by motorcycles in the modeling process gradually disappear, so that there is no obvious transition surface between the curved surfaces A, B, C and D, which is the entire surface reconstruction. Difficulties.
Figure 3 Tank surface main profile and feature line
Most of the main profiles can be described by a single basic surface. However, for surfaces A~D and I, it needs to be further divided into basic surfaces and connected surfaces, and the dotted line shows the block boundaries. After the block is determined, the modeling of the basic surface and the joined surface can be performed.
2. Establishment of basic surface model
The basic surface model refers to a surface that does not need to be established by boundary constraints. Generally, a simple surface description is used, such as a ruled surface, a stretched surface, a rotating surface, a scanning surface, and the like. For this motorcycle fuel tank, most of them are complex free-form surfaces. It is necessary to construct a curved surface with a grid curve, determine the U and V directions of the surface according to the characteristic direction of the curved surface, and construct a quadrilateral curved surface.
Firstly, the surface model is coarsely constructed, and the point cloud data of the block is directly used, and the surface is generated by controlling the U and V orders of the surface and the deviation from the measured data. The primary surface generally only reflects the characteristics and trends of the surface of the model. Further adjustment and improvement are required from the precision, smoothness and size of the patch. The primary surface can be controlled by directly adjusting the surface to control the vertices and further adjust to improve the surface quality to some extent. In this process, it is often necessary to complete the expansion of the surface. For the surface to be expanded, it should be expanded according to the curvature variation law of the surface. Although the curved surface of the extended part has no data points that can be compared, it must be handled carefully because it determines the variation of the intersection line between the surfaces.
Then modify the surface model. Modifying the surface model is the most important process in surface reconstruction. It is an effective and accurate method to fine-tune the surface by establishing the curve and surface model of the surface and controlling the surface quality by adjusting the control curve. The curve can be obtained by extracting and projecting on the rough surface, and then adjusting the position, length adjustment and smoothing of the curve. The following principles should be observed when building a curve model:
(1) The curve satisfies the accuracy requirement;
(2) The curve is constructed according to the characteristic direction of the curved surface, and the purpose of using the least curve and the curvature change of the curve is the most gentle. Curves in the same direction should have similar curvature trends;
(3) The density of the curve distributed on the curved surface should be reduced as much as possible within the allowable deviation range, and the adjustability of the curved surface is increased. The curvature of the surface changes greatly, and the curve is arranged densely. The curvature of the surface changes gently, and the curve is arranged less;
(4) The curve is mainly described by the spline. Under the premise of ensuring the accuracy, the number of segments and the order of the spline should be reduced as much as possible. The curve in the same direction should have the same number of segments and order. It is generally recommended to generate 3 or 4 Step curve
(5) The smoothness adjustment of the curve is very important, and the adjustment is performed by using the relevant functional modules of the software. After the curve is smoothed, it is mathematically guaranteed that C2 is continuous, there is no extra inflection point, and the curvature changes evenly;
(6) The curve curvature is greater than the curvature value of the transition surface.
UG software provides a large number of curve analysis and adjustment tools. Mastering and using these tools is the basic guarantee for quickly reconstructing surfaces. The most commonly used UG software curve adjustment is the "Edit Spline/Edit pole" option. Turn on the Curvature Comb to check the curvature change of the curve in real time. Figure 4 shows the curves and surface models of the panels C, F, I, G of Figure 3.
Figure 4 Tank part surface and curve model
For symmetric surfaces such as A and B in the fuel tank, half of the surface is constructed based on one side measurement data. When the surface is adjusted to a certain precision, the grid lines of the constructed surface are mirrored by the plane of symmetry, the curves in the direction perpendicular to the plane of symmetry are reconnected, smoothed, a symmetrical grid is formed, and then a complete structure is constructed. Surface.
3. Establishment of the connected surface model
In the shape of a motorcycle, C1 can meet the needs of the shape in a row. There are two ways to create a universal connection surface:
(1) Chamfer method: Use the intersection of the two curved surfaces that need to be connected as the guide line to perform equal-radius chamfering or variable-radius chamfering. It is not necessary to predetermine the tangent edge, and the surface shape change is adjusted by adjusting the chamfer radius value to approximate the measurement data. The quality of the generated surface of the chamfered surface can be effectively controlled by controlling the smoothness and tendency of the intersecting line. This is a relatively easy way to implement;
(2) Direct modeling method: establish a joint surface with boundary constraints. When achieving a smooth connection between the curved sheets, first ensure that there is a common edge between the connecting patches. More importantly, it is necessary to ensure that the control lines of the patches are smooth, which is necessary to ensure the smooth connection of the patches. condition. At this point, you can modify the start and end constraints of the control line so that the curvature or tangent is consistent at the joint. This method requires repeated adjustments to the continuous boundary and control lines, which is not easy to implement.
Both methods are often used simultaneously during the modeling process. Figure 5 shows the wireframe display of the surface model. From this, the generation order of the surface and the generation method of the connected surface can be easily seen. Except that the curved surfaces C1 to C6 directly generate the continuous curved surface with the boundary constraint, the other connecting surfaces are directly generated by the chamfering.
Figure 5 fuel tank surface model wireframe display
The surface of the motorcycle fuel tank has high requirements on the surface quality. Therefore, in the surface modeling process, the surface quality inspection is always running. The smoothness of the surface is usually checked by the Gaussian curvature method and the reflection stripe method. Figure 6 shows the smoothness check of the upper surface model of the tank. In order to correctly reflect the modeling characteristics of the surface, the deviation between the surface and the data is generally controlled to about ±1 mm.
Figure 6 Tank surface reflection stripe check
After the upper surface of the fuel tank is completed, it is thickened by the UG function, converted into a solid model, and then designed with the fuel tank cap to design the oil filling port structure. Since the top surface of the fuel tank cap is flush with the upper body of the fuel tank, a water pipe is specially designed to discharge the water in the fuel tank cap. Figure 7 shows a three-dimensional model of the upper tank.
Figure 7 Three-dimensional solid model of the upper tank
Third, the fuel tank lower body 3D modeling
The surface modeling of the lower tank of the fuel tank is relatively easy. Referring to the structure of the similar model, it satisfies the need to install the fuel tank on the frame and the oil level sensor, the front tank of the fuel tank and the oil outlet on the fuel tank, and design the curved surface according to the direction of the mold. The shape is usually completed by chamfering the extruded surface. The welding flange of the upper body of the fuel tank and the lower body of the fuel tank needs to be designed. Figure 8 shows a three-dimensional model of the lower tank.
Figure 8 3D solid model of the lower tank
After the 3D model of all components in the fuel tank is completed, a virtual assembly inspection is performed, as shown in FIG. The model after the examination can be directly transmitted to the mold factory for mold processing.
Figure 9 three-dimensional model of the fuel tank
Fourth, the conclusion
At this stage, the combination of general CAD/CAM software and reverse design software is an effective method for 3D modeling of complex surfaces based on point cloud data. Among them, for the reasonable segmentation of complex surfaces, judging the continuous boundary of the surface is the key to modeling. The quality of the surface depends on the quality of the curve. Adjusting the curve is the main work of surface modeling.
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