| 0. PrefaceThe development of digital automobile engineering is the development trend of global automobile industry, which is also the core of building premium brands and improving market competitiveness for domestic car. As competition in global automobile industry is getting tough, launching novel models constantly to the market according to consumers'individualized needs, replacing the older generation of products by new ones and occupying the auto market rapidly are all the powerful means for increasing international competitiveness. Changing and remodeling based on the older versions of vehicles while preserving some important features of the high-cost parts as far as possible can diminish the cost of production, reduce the cycles for design and development, and make more economic efficiency for automobile manufacturing enterprises. Therefore, constrained morphing design aiming at digital models has gradually become a hot spot of the research in the field of digital automobile design.Supported by the project of National Programs for High-Tech Research and Development (863 Program, No. 2009AA04Z101), study and research has been carried out on the key technology of constrained morphing modeling for auto body part, an unfolding algorithm with constraints and a new method of three dimensional parametric modeling based on one-step inverse forming FEM are proposed, on that basis, unconstrained and constrained parametric morphing technologies for auto body part are studied intensively.1. Unfolding algorithm with constraints for auto body part based on one-step inverse forming FEMIn the simulation of sheet metal forming, the one-step inverse forming FEM only considers the initial state and the final state without considering the medium state, which makes it more adaptable to the application of the preliminary design stage for automotive products. Combining the one-step inverse FEM and the approximate developing theory of non-developable ruled surface, unfolding the final shape or modifying the initial blank can be concluded as a constrained optimization problem according to the feature constraints. In this paper, an unfolding algorithm with constraints based on one-step inverse forming FEM for auto body part is proposed, it is abbreviated as UAC. On that basis, in order to improve the FE meshes'quality of the unfolded auto body part, an improvement is also made for the unfolding error objective function.Since the L-BFGS-B algorithm which is suitable for large nonlinear optimization problems is employed for the calculation of UAC, its process not only occupies less EMS memory, but also has relatively higher efficiency and precision. The UAC algorithm can solve both unconstrained and constrained problems for unfolding the final shape or modifying the initial blank. Some typical experiments are given in this paper, the experimental results verify that the UAC algorithm can solve the problems such as overlap elements and shape distortion caused by flanging areas on the precondition of preserving important features, the general applicability and validity of UAC is also demonstrated.2. Parametric morphing technology for auto body part based on one-step inverse forming FEMParametric design is the major method in the design field of automotive industry. A new design result can be obtained by adjusting the parameters of the parametric models, in this way, the geometry shapes of the products can be modified and controlled at any moment. To reduce the design cycle for the automotive products and attract more consumers, the automobile manufacturing enterprises would like to design the new products based on the existing products according to customers'individual needs. Aiming at the data of FE meshes, the directly manipulated FFD method is applied to the unconstrained morphing design for auto body part by dragging one single point, the morphing technology based on the grid is studied in this paper.Considering the difficulties in setting constraints for the data of FE meshes, the CAD data using parametric surfaces to describe the parts is more adaptable to the manipulation of morphing. But sometimes there are some problems caused by operating multiple surfaces, such as gaps between surfaces. To solve these problems, a three dimensional parametric modeling method based on the unfolding algorithm of one-step inverse forming FEM is proposed in this paper. It reconstructs the multiple parametric surfaces with one single NURBS surface. On the basis of those studies above, the parametric morphing technology for auto body part based on one-step inverse forming FEM is proposed and applied to the examples of unconstrained morphing.3. Feature-preserved morphing design method for auto bodyIn the development process of new versions of vehicles, in terms of saving developing cost and improving economic returns, the automobile manufacturing enterprises would prefer the older versions to carry on the work. Some important features, such as the sizes and shapes of the side doors of a motor vehicle or the front and rear windshield, are required to be preserved because their production cost is too high. To meet the rapid parametric design requirements for remodeling the auto body described above, a Feature-Preserved Morphing Design Method for auto body, abbreviated as FPMDM, is proposed aming at the CAD model. Firstly, a NURBS volume is constructed as the parametric solid in the way that the parametric domain is identical with the object's bounding box, so the parametric coordinates of the object need not to be calculated. Then, operate the morphing design without rigid constraints, the"Minimum Rigid Body Displacement Principle"is defined based on this unconstrained morphing result to set constraints for the features which are required to be kept unchanged, and the feature-preserved part can smoothly transit to the rest part at the same time.The contrast experiments of manipulating the feature-preserved morphing and unconstrained FFD design are given, the correlative experimental results demonstrate the efficiency and validity of FPMDM.4. An improved feature-preserved morphing design methodAccording to the FPMDM algorithm described before, an Improved Feature-Preserved Morphing Design Method, abbreviated as IFPMDM, is proposed. It uses the 3D parametric modeling method based on the unfolding algorithm of one-step inverse forming FEM to reparameterize the multiple surfaces in CAD data for auto body part, so many problems caused by multiple surfaces such as gaps between surfaces can be avoided. The"New Minimum Rigid Body Displacement Principle"is defined to set the constraints. The unconstrained morphing design method and IFPMDM are manipulated separately, the experimental results demonstrate that IFPMDM can do the morphing with the important features preserved simultaneously.
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