车身覆盖件网格自动生成中的数据预处理技术研究
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摘要
汽车覆盖件冲压成形是一种非常重要的汽车零部件加工方法，而目前我国多数汽车生成厂家在汽车覆盖件成形模具的设计制造中仍然采用传统方法，工艺分析及模具设计主要依靠经验，制造模具需要反复调试，生产效率低，成本高，周期长，而且产品质量难以保证。本文结合吉林大学汽车车身与模具研究所板材冲压成形与模具设计CAD/CAE/CAM一体化系统的开发工作，对车身覆盖件冲压成形工艺仿真商品化软件KMAS的有限元网格生成器KMAS/Auto-Meshing中的若干数据预处理技术进行了深入研究。
有限元网格生成是有限元分析前处理的核心部分，网格生成器是CAE分析系统中不可缺少的一部分，这方面的研究开发对于整个系统的完整性非常重要。网格生成器的数据入口是CAD设计数据，所以对网格生成器的开发离不开对CAD的研究。网格生成器课题组对CAD中性文件IGES和STEP做了深入的分析，基于边界表示法（B-REP）构建了网格生成器的框架和数据结构。该框架的基础是几何体的拓扑结构，内部明确地界定了曲面、曲面包围盒、曲面边界、曲线、单元、单元边、节点之间的拓扑关系，有机地将CAD几何数据与CAE网格数据衔接起来。在此基础上，根据课题组对IGES和STEP格式进行的研究，本文开发了KMAS网格生成器的数据读入接口。该接口可以有效地将CAD信息读入网格生成器，并首次提出了读入过程中的数据修复模块，能够在读入过程中对文件进行分析，找出语法错误和相关的数据破损信息，并可以在模型转换过程中自动修正。
由于种种原因，读入的CAD数据常常会出现曲面叠加或者各种缝隙的情况。如不进行必要的数据修复直接进行CAE分析，即使能够分析，也会对结果的准确性有很大影响。工程师们往往会手动修复由CAD数据生成的网格以便获得准确的分析结果，基于网格的数据修复也是非常困难和耗时的过程。本文从CAD角度深入研究了板材网格生成中的数据修复问题，并依据模块化的
设计思想，开发了网格生成器数据修复模块。该模块包括初始化、前处理、几何体简化、缝合、几何体修复重构、后处理等子块，对CAD数据的修复做了比较详细的分析和处理。由于不同CAD系统间的数据转换，形成了由公差导致的数据破损。本文对公差问题进行了深入研究，并全面地规定了可变的修复模块的公差，这也是数据修复的基础，而建立在对读入几何体的精确分析基础上的几何体简化、缝合和重构是数据修复的核心。这种全面的模块化数据修复方法并非单一的研究型程序，而是适用于工程应用的研究成果，因此模块也很好地集成到了KMAS系统之中。
本文还面向车身覆盖件的冲压分析实际，对网格生成中的法线一致性问题、工艺孔消除问题和T形问题进行了深入的研究。首次提出了基于CAD数据的曲面法线快速自动一致性调整算法、自动消除工艺孔算法和T形问题处理算法。以上三个方法可以大幅提高车身冲压件的整体网格质量，并提高了KMAS冲压分析前处理的效率。
对网格的法线进行一致性调整，虽然算法简单，但是效率和精度低是其致命缺点，因此本文将调整提前到网格划分之前。算法中对相邻曲面的公共点处的法线一致性进行判断，从而决定相邻曲面的法线方向。
本文自动消除工艺孔方法的基础是拓扑结构中对构成面的内外环的分类定义。由于单独依靠内外环的准则不足以确定特征是否为孔，所以增加了判断准则：判断内环所在面是否在环处与其它面相邻，并共用此环或其一部分。如果不是，在内环为孔。消除孔特征后划分的网格在原孔特征处质量明显提高，符合冲压分析需求。
T形问题会导致有限元计算结构的精度降低，本文对消除T形不相容边界方法进行了研究。对存在T形问题的曲面边界处，不修改CAD数据，而是重新分配离散节点。这样直接向网格划分程序提供信息，改善了不相容边界处的网格质量。
查找相邻曲面是本文中频繁涉及的问题，数据修复、法线一致性调整、孔特征的消除和T形问题都与此密切相关。在数据结构中我们定义了实体和曲面的包围盒类，对每一片曲面我们都可以得到给定坐标系下的一个包围它的空间盒，再根据误差范围，和一个可调参数将该空间盒放大。将包围盒相交作为
曲面相邻的充分条件，这对于提高计算效率有很大贡献。
文中另一个频繁使用的方法是计算点到曲线的距离。曲线间距离、曲线与曲面间距离的计算最终都落实到点到曲线的距离。本文采用了Newton-Raphson方法，用NURBS表示曲线，将距离问题转化为求一个最优问题的极值。
本文立足于汽车覆盖件冲压分析软件KMAS自主开发的基础上，对于有限元网格生成中的若干数据预处理技术进行了研究，为整个软件的完整性打下了一定的基础。
Automobile panel stamping is an extremely important process for manufacture of automobile parts. Nowadays, traditional methods are still in used in panel die designing and manufacturing in domestic automobile industry. Process planning and die designing depend mainly on the experience of designers and a trial-and-error procedure is often adopted to get qualified dies. This method is inefficient, costly, time consuming, and the quality of dies can not be ensured. In this thesis, some preprocessing techniques of finite element mesh generation in automobile panel are investigated and then the independently developed CAE commercial code KMAS/Auto-meshing for sheet metal forming is moreover perfected. The researches belong to the development of CAD/CAE/CAM integration system for sheet metal stamping and die design at Institute of Automobile Body and Die Engineering, Jilin University.
Finite element mesh generation is the kernel of pre-process. Mesh generator is the necessary part of CAE, it is very important for the integrality of whole system to do work in this area. The development of mesh generator can not do well without the research of CAD, because the CAD design is the input of mesh generator. According to the deep analysis of IGES and STEP, mesh generator team designed framework and data structure of mesh generator based on B-Rep. The foundation of this framework is the topology relation of geometry, including surface, surface surround box, surface border, curve, element, element edge and node. As a result, this framework can link CAD data and CAE mesh data into an organic whole. Basing on the research of IGES and STEP, the data reading interface of mesh generator in KMAS has
been developed. This interface can read CAD data credibly into the mesh generator, and including data repair module in reading process. By using this module, mesh generator can analyze CAD file, find semantic error and failure information, and repair them while transform the CAD model.
Some bad complexions will come out in read CAD data, for instance, furl in border, edge repeat, and different aperture and so on. The veracity of result will be blocked deeply by CAD data, even if the direct CAE analysis can be performed without necessary data repair. It is necessary for engineers to repair the mesh of CAD design in order to get the correct result sometimes. Mesh repair is a very difficult and time-consume process. In this paper, CAD design repair in sheet metal mesh generation is researched from CAD point of view. The CAD data repair module of mesh generator is developed according to modularization idea, including initialization, pre-process, geometry simplification, stitching, geometry building, and post-process. The data disrepair led by tolerance will occur when the data is transformed between different CAD systems. In this paper, tolerance problem is investigated in-depth, alterable tolerance of data repair module is defined in the round. The tolerance regulation is the foundation of data repair, and geometry simplification, stitching and building which base on the geometry analysis are the kernel. The modularized data repair method is the research for engineering application, not simple program; as a result, it is integrated in the KMAS system well.
In this thesis, three important problems are investigated aiming at automobile panel stamping analysis: consistency of surface normal direction, erasion of processing hole and T-connectivity. Three kinds of CAD-based automatic treatment arithmetic are put forward for the first time. By using these three methods, the whole quality of automobile panel mesh is enhanced greatly; the efficiency of KMAS pre-process is improved.
Because the adjustment of mesh normal direction has a low efficiency and
precision, the treatment is put in front of mesh generation in this paper. The normal directions of adjacent surfaces are decided by the adjustment of normal directions in the common points of adjacent surfaces.
The basic of automatic removing holes is the classified definition of
有限元网格生成是有限元分析前处理的核心部分，网格生成器是CAE分析系统中不可缺少的一部分，这方面的研究开发对于整个系统的完整性非常重要。网格生成器的数据入口是CAD设计数据，所以对网格生成器的开发离不开对CAD的研究。网格生成器课题组对CAD中性文件IGES和STEP做了深入的分析，基于边界表示法（B-REP）构建了网格生成器的框架和数据结构。该框架的基础是几何体的拓扑结构，内部明确地界定了曲面、曲面包围盒、曲面边界、曲线、单元、单元边、节点之间的拓扑关系，有机地将CAD几何数据与CAE网格数据衔接起来。在此基础上，根据课题组对IGES和STEP格式进行的研究，本文开发了KMAS网格生成器的数据读入接口。该接口可以有效地将CAD信息读入网格生成器，并首次提出了读入过程中的数据修复模块，能够在读入过程中对文件进行分析，找出语法错误和相关的数据破损信息，并可以在模型转换过程中自动修正。
由于种种原因，读入的CAD数据常常会出现曲面叠加或者各种缝隙的情况。如不进行必要的数据修复直接进行CAE分析，即使能够分析，也会对结果的准确性有很大影响。工程师们往往会手动修复由CAD数据生成的网格以便获得准确的分析结果，基于网格的数据修复也是非常困难和耗时的过程。本文从CAD角度深入研究了板材网格生成中的数据修复问题，并依据模块化的
设计思想，开发了网格生成器数据修复模块。该模块包括初始化、前处理、几何体简化、缝合、几何体修复重构、后处理等子块，对CAD数据的修复做了比较详细的分析和处理。由于不同CAD系统间的数据转换，形成了由公差导致的数据破损。本文对公差问题进行了深入研究，并全面地规定了可变的修复模块的公差，这也是数据修复的基础，而建立在对读入几何体的精确分析基础上的几何体简化、缝合和重构是数据修复的核心。这种全面的模块化数据修复方法并非单一的研究型程序，而是适用于工程应用的研究成果，因此模块也很好地集成到了KMAS系统之中。
本文还面向车身覆盖件的冲压分析实际，对网格生成中的法线一致性问题、工艺孔消除问题和T形问题进行了深入的研究。首次提出了基于CAD数据的曲面法线快速自动一致性调整算法、自动消除工艺孔算法和T形问题处理算法。以上三个方法可以大幅提高车身冲压件的整体网格质量，并提高了KMAS冲压分析前处理的效率。
对网格的法线进行一致性调整，虽然算法简单，但是效率和精度低是其致命缺点，因此本文将调整提前到网格划分之前。算法中对相邻曲面的公共点处的法线一致性进行判断，从而决定相邻曲面的法线方向。
本文自动消除工艺孔方法的基础是拓扑结构中对构成面的内外环的分类定义。由于单独依靠内外环的准则不足以确定特征是否为孔，所以增加了判断准则：判断内环所在面是否在环处与其它面相邻，并共用此环或其一部分。如果不是，在内环为孔。消除孔特征后划分的网格在原孔特征处质量明显提高，符合冲压分析需求。
T形问题会导致有限元计算结构的精度降低，本文对消除T形不相容边界方法进行了研究。对存在T形问题的曲面边界处，不修改CAD数据，而是重新分配离散节点。这样直接向网格划分程序提供信息，改善了不相容边界处的网格质量。
查找相邻曲面是本文中频繁涉及的问题，数据修复、法线一致性调整、孔特征的消除和T形问题都与此密切相关。在数据结构中我们定义了实体和曲面的包围盒类，对每一片曲面我们都可以得到给定坐标系下的一个包围它的空间盒，再根据误差范围，和一个可调参数将该空间盒放大。将包围盒相交作为
曲面相邻的充分条件，这对于提高计算效率有很大贡献。
文中另一个频繁使用的方法是计算点到曲线的距离。曲线间距离、曲线与曲面间距离的计算最终都落实到点到曲线的距离。本文采用了Newton-Raphson方法，用NURBS表示曲线，将距离问题转化为求一个最优问题的极值。
本文立足于汽车覆盖件冲压分析软件KMAS自主开发的基础上，对于有限元网格生成中的若干数据预处理技术进行了研究，为整个软件的完整性打下了一定的基础。
Automobile panel stamping is an extremely important process for manufacture of automobile parts. Nowadays, traditional methods are still in used in panel die designing and manufacturing in domestic automobile industry. Process planning and die designing depend mainly on the experience of designers and a trial-and-error procedure is often adopted to get qualified dies. This method is inefficient, costly, time consuming, and the quality of dies can not be ensured. In this thesis, some preprocessing techniques of finite element mesh generation in automobile panel are investigated and then the independently developed CAE commercial code KMAS/Auto-meshing for sheet metal forming is moreover perfected. The researches belong to the development of CAD/CAE/CAM integration system for sheet metal stamping and die design at Institute of Automobile Body and Die Engineering, Jilin University.
Finite element mesh generation is the kernel of pre-process. Mesh generator is the necessary part of CAE, it is very important for the integrality of whole system to do work in this area. The development of mesh generator can not do well without the research of CAD, because the CAD design is the input of mesh generator. According to the deep analysis of IGES and STEP, mesh generator team designed framework and data structure of mesh generator based on B-Rep. The foundation of this framework is the topology relation of geometry, including surface, surface surround box, surface border, curve, element, element edge and node. As a result, this framework can link CAD data and CAE mesh data into an organic whole. Basing on the research of IGES and STEP, the data reading interface of mesh generator in KMAS has
been developed. This interface can read CAD data credibly into the mesh generator, and including data repair module in reading process. By using this module, mesh generator can analyze CAD file, find semantic error and failure information, and repair them while transform the CAD model.
Some bad complexions will come out in read CAD data, for instance, furl in border, edge repeat, and different aperture and so on. The veracity of result will be blocked deeply by CAD data, even if the direct CAE analysis can be performed without necessary data repair. It is necessary for engineers to repair the mesh of CAD design in order to get the correct result sometimes. Mesh repair is a very difficult and time-consume process. In this paper, CAD design repair in sheet metal mesh generation is researched from CAD point of view. The CAD data repair module of mesh generator is developed according to modularization idea, including initialization, pre-process, geometry simplification, stitching, geometry building, and post-process. The data disrepair led by tolerance will occur when the data is transformed between different CAD systems. In this paper, tolerance problem is investigated in-depth, alterable tolerance of data repair module is defined in the round. The tolerance regulation is the foundation of data repair, and geometry simplification, stitching and building which base on the geometry analysis are the kernel. The modularized data repair method is the research for engineering application, not simple program; as a result, it is integrated in the KMAS system well.
In this thesis, three important problems are investigated aiming at automobile panel stamping analysis: consistency of surface normal direction, erasion of processing hole and T-connectivity. Three kinds of CAD-based automatic treatment arithmetic are put forward for the first time. By using these three methods, the whole quality of automobile panel mesh is enhanced greatly; the efficiency of KMAS pre-process is improved.
Because the adjustment of mesh normal direction has a low efficiency and
precision, the treatment is put in front of mesh generation in this paper. The normal directions of adjacent surfaces are decided by the adjustment of normal directions in the common points of adjacent surfaces.
The basic of automatic removing holes is the classified definition of
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