A quantitative analysis of parametric CAD model complexity and its relationship to perceived modeling complexity

Digital product data quality and reusability has been proven a critical aspect of the Model-Based Enterprise to enable the efficient design and redesign of products. The extent to which a history-based parametric CAD model can be edited or reused depends on the geometric complexity of the part and the procedure employed to build it. As a prerequisite for defining metrics that can quantify the quality of the modeling process, it is necessary to have CAD datasets that are sorted and ranked according to the complexity of the modeling process. In this paper, we examine the concept of perceived CAD modeling complexity, defined as the degree to which a parametric CAD model is perceived as difficult to create, use, and/or modify by expert CAD designers. We present a novel method to integrate pair-wise comparisons of CAD modeling complexity made by experts into a single metric that can be used as ground truth. Next, we discuss a comprehensive study of quantitative metrics which are derived primarily from the geometric characteristics of the models and the graph structure that represents the parent/child relationships between features. Our results show that the perceived CAD modeling complexity metric derived from experts’ assessment correlates particularly strongly with graph-based metrics. The Spearman coefficients for five of these metrics suggest that they can be effectively used to study the parameters that influence the reusability of models and as a basis to implement effective personalized learning strategies in online CAD training scenarios.     

计算机辅助结构设计与分析的集成框架研究

针对计算机辅助结构设计和分析的集成问题,设计了一种基于统一模型库 方式的组件式CAD/CAE 集成框架,以结构CAD/CAE 集成模型库为框架底层,以几何造型、 可视化交互、第三方CAD、第三方CAE 等组件为中间组件层,以空间结构设计子系统、空 间结构分析子系统为上层应用层,提供空间实体建模、工程模型管理、多种有限元模型分析 与计算等功能,并应用于港口码头的结构设计及有限元分析。     

Detecting design intent in approximate CAD models using symmetry

Finding design intent embodied as high-level geometric relations between a CAD model’s sub-parts facilitates various tasks such as model editing and analysis. This is especially important for boundary-representation models arising from, e.g., reverse engineering or CAD data transfer. These lack explicit information about design intent, and often the intended geometric relations are only approximately present. A novel solution to this problem is presented based on detecting approximate local incomplete symmetries, in a hierarchical decomposition of the model into simpler, more symmetric sub-parts. Design intent is detected as congruencies, symmetries and symmetric arrangements of the leaf-parts in this decomposition. All elementary 3D symmetry types and common symmetric arrangements are considered. They may be present only locally in subsets of the leaf-parts, and may also be incomplete, i.e. not all elements required for a symmetry need be present. Adaptive tolerance intervals are detected automatically for matching inter-point distances, enabling efficient, robust and consistent detection of approximate symmetries. Doing so avoids finding many spurious relations, reliably resolves ambiguities between relations, and reduces inconsistencies. Experiments show that detected relations reveal significant design intent.     

Behavior modeling in mechanical engineering — A modular approach

Many engineering activities are confronted with the relation between shape and behavior. Modern state-of-the-art CAD systems can support a dynamic design process with flexible and umambiguous geometric modeling of artifacts. The Finite Element (FE) method is a general method to model and simulate physical behavior. CAD and FE integration enables numerical prediction of the physical behavior of artifacts. To deal with the complexity of modern products and the dynamic character of the design process, the integration must be addressed from both a systems and a process point of view. The models must thus be flexible, scalable, and reusable. A modularized modeling approach based on four stages is proposed, and exemplified with behavior modeling of a turn-key grinding machine.     

A database system of mechanical components based on geometric and topological similarity. Part I: representation

A database of mechanical components is an important issue for some manufacturing activities such as cost estimation, process planning, and design by case-based reasoning. In this paper, we give the representation scheme of the CAD model in such a database. Components are represented using attributed graphs in which the nodes correspond to the surfaces of the component and the links correspond to the edges of the component. The graph is based on the standard for the exchange of product information (STEP) physical file of the component. STEP file should be unique for a single component regardless of the underlying CAD system. The process of creating the graph of a component constitutes two sub-tasks: (i) importing the CAD model from the CAD system in STEP format and (ii) transforming the STEP data into attributed graph-based representation. The graph and its attributes describe the topology of the component completely together with some geometric data that are not dependent on any coordinate system such as surface type and curve type. These geometric data are helpful in the retrieval and matching processes in the database.     

A feature-based approach to structural design

Despite the continuing improvements in computeraided design (CAD) systems and improvements in geometric modeling, most CAD systems are used as advanced drafting and drawing management tools by structural designers. A computer model of a structural design usually is generated by creating a detailed geometric model of the primitive components of the design and then attaching attributes, such as physical properties and loading conditions, to the various geometric components to reveal the structural characteristics of those components. Thisbottom-up approach has been inherited from early drafting techniques and contrasts sharply with a structural designer's natural way of thinking and reasoning about the design. Geometric features, on the other hand, provide high-level abstractions of design information and can be tailored to a designer's specific engineering needs. In this paper the advantages of using feature-based techniques in structural CAD systems are discussed. These techniques provide better modeling primitives for users and superior data models for CAD systems for reasoning about the geometry, topology, and engineering properties of a structure.     

基于哈希的二维工程 CAD 图纸检索技术

针对企业在设计创新过程中大量采用已有计算机辅助设计(CAD)图纸进行设计重 用的情况,提出了一种基于哈希的二维工程 CAD 图纸检索方法。首先基于环形分割算法提取 工程 CAD 图纸中每个组件对象几何特征;基于传统 LBP 算子提出了一种局部拓扑矢量量化模 式(T-LVQP),实现对各个组件拓扑特征的提取;然后基于协方差描述符融合几何特征和拓扑特 征,通过 LBG 算法将所有组件按照几何特征分组后得到工程 CAD 图纸的特征向量表达;最后 通过迭代量化哈希算法生成图纸的哈希序列。实验结果表明,该算法检索速度快、准确度高, 对于二维工程 CAD 图纸具有较好的检索效果。     

Adaptive CAD model (re-)construction with THB-splines

Computer Aided Design (CAD) software libraries rely on the tensor-product NURBS model as standard spline technology. However, in applications of industrial complexity, this mathematical model does not provide sufficient flexibility as an effective geometric modeling option. In particular, the multivariate tensor-product construction precludes the design of adaptive spline representations that support local refinements. Consequently, many patches and trimming operations are needed in challenging applications. The investigation of generalizations of tensor-product splines that support adaptive refinement has recently gained significant momentum due to the advent of Isogeometric Analysis (IgA) [2], where adaptivity is needed for performing local refinement in numerical simulations. Moreover, traditional CAD models containing many small (and possibly trimmed) patches are not directly usable for IgA. Truncated hierarchical B-splines (THB-splines) provide the possibility of introducing different levels of resolution in an adaptive framework, while simultaneously preserving the main properties of standard B-splines. We demonstrate that surface fitting schemes based on THB-spline representations may lead to significant improvements for the geometric (re-)construction of critical turbine blade parts. Furthermore, the local THB-spline evaluation in terms of B-spline patches can be properly combined with commercial geometric modeling kernels in order to convert the multilevel spline representation into an equivalent – namely, exact – CAD geometry. This software interface fully integrates the adaptive modeling tool into CAD systems that comply with the current NURBS standard. It also paves the way for the introduction of isogeometric simulations into complex real world applications.     

Computer modeling approach for microsphere-packed bone scaffold

A computer modeling approach for constructing a three-dimensional microsphere-packed bone graft structure is presented. The modeling approach consists of both geometric and CAD-based computer modeling. The geometric model uses two extreme microsphere packing models (minimum-density packing and maximum-density packing) and a statistical packing model to determine the number of microspheres packed in a synthesized bone graft. The pore size of the packed internal porous structure is predicted, and a parametric study of the effect of microsphere diameter on the number of microspheres and pore size is conducted. Based on the results obtained from the geometric model, a CAD modeling approach for designing randomly microsphere-packed three-dimensional bone grafts was developed. The hierarchy of the CAD model and the steps for constructing a bone graft model are described, and application of the CAD-based bone graft model in internal structural examination, visualization, prediction and comparison with in vitro bone ingrowth is presented.     

浅谈实体建模：历史、现状与未来

实体建模技术是 CAD 软件的“功能心脏”,相关基础理论与算法是 CAD 发展历史上最关键的成 果之一,成功回答了为使计算机能够辅助产品设计与制造,需在计算机中存什么几何信息以及怎么存的问题。 本文对实体建模的主要历史发展脉络做了简要介绍,同时对各发展阶段的关键问题以及研究现状进行了讨论, 最后选取了 3 个方向对实体建模的未来做出展望, 重点关注从 Computer-Aided Design 到 Computer-Automated Design 的发展趋势。     

大规模城市可计算模型快速构建方法研究

为了给数值模拟计算提供大规模城市的可计算模型,需要具备面向大空间尺度、海量数据、频繁更新的城市的快速可计算建模能力。但是,由于数据采集过程中存在的误差,以及模型处理所做的必要的简化操作,导致直接生成的数字城市模型中存在大量非闭合、相接、相交、悬空等CAD模型缺陷,使得城市模型不可用于数值模拟。而采用商用软件直接修复这些缺陷的代价过高。为此,提出面向大规模城市可计算模型的快速构建方法。通过分析LoD1城市模型中所涉及的CAD模型缺陷,提出了需满足的可计算条件和对应的处理方法。 提出的方法能够直接生成不存在CAD模型缺陷的城市可计算模型。为了提高可计算处理的效率,还提出了面向大规模城市的图形快速检索算法。实验表明,构建710 km2、30余万栋建筑、217万面片的可计算模型仅需约25 min,满足了在大规模城市上进行数值模拟的需求。     

基于T-Spline的全自动几何拓扑修复方法

从高质量曲面网格生成的需求出发,提出了一种基于T-Spline的全自动几何拓扑修复方法.本文方法创新性主要可归纳为:1）对原有计算机辅助设计（Computer aided design,CAD）几何模型不进行任何修改保留其本真,自动识别CAD几何模型中常见不必要的几何特征,成功解决了CAD几何模型中存在的几何瑕疵,如短边、窄面、退化边、退化面、非连续光滑边界及尖锐特征等,利用新生成的"虚边"、"虚面"处理几何瑕疵,同时通过虚拓扑重构CAD几何模型的B-Rep;2）开发了一套CAD/CAE集成系统,统一了几何模型与计算分析模型,实现计算机辅助工程（Computer aided engineering,CAE）与CAD两者的无缝集成,所有拓扑修复操作及后续CAE分析计算均在同一环境下进行,避免了几何模型在CAE与CAD系统间进行转换时造成的数据丢失.该方法能够对复杂实体实现全自动几何拓扑修复及网格生成,实验表明,在保证不失真的前提下,修复后的几何模型能够生成质量良好的网格且能降低网格的生成规模,验证了本文方法的实用性和有效性,以满足工程实际分析的需要.     

一种面向方案设计的装配建模表示方法

ＣＡＤ系统对方案设计支持的关键是建立独立于几何细节而又表达的功构关系的装配模型,本文试图运用逻辑装配单元的定义来实现产品功能需求和结构表现间的有效关联,分别讨论了方案设计中功能描述和几何构形表示方式,形式化表达了方案设计的逻辑造型和装配建模过程,给出了支持自顶向下方案设计的装配模板定义,为方案设计的计算机辅助实现作了有益的尝试。     

基于ParaSolid的CAD系统中螺旋实体的建模

螺旋弹簧及其它螺旋类实体的建模一直是CAD系统中的一个难题。该文基于Parasolid几何核心系统,给出了变节距圆柱螺旋弹簧创建的具体步骤及其他类型螺旋实体的创建方法,解决了通用CAD系统难以创建变节距弹簧实体的问题。该方法在造型设计中已得到了应用。     

CAD支撑系统构件—软总线模型

在分析开放系统的特点和CAD支撑系统的具体需求的基础上,提出了CAD支撑系统构件-软总线模型（CSB模型）,在该模型中,图形功能模块和应用程序都被封装成为彼此独立且便于复用的构件,由软件总线实现集成,解释型和编译型两种类型程序语言开发的应用构件在系统中共存,并可在运行态动态增加/删除;同时,由总线适配器、解释器和应用服务代理组成的应用服务界面代理了构件之间复杂的通信过程,完成异构数据向同构数据的转换,使构件更专注于功能实现,根据CSB模型实现的一个开放的CAD支撑系统OpenDesign∧TM已经成功实现了产品化,应用实践证明,基于CSB模型的CAD支撑系统效率较高,具备比较好的互操作性和伸缩性,系统开放程度增加。     

Logic-based 2D geometric modeling in a CAD environment

This paper examines the use of predicate logic for deductive construction of geometric models. The process of logical inference and its use for geometric modeling is examined. The logic-based definition of a retaining wall is presented, the implementation of a logic-based CAD system using a commercial software is briefly discussed and its use is illustrated.