A face based mechanism for naming, recording and retrieving topological entities

doi:10.1016/S0010-4485(00)00099-3

Computer-Aided Design

Volume 33, Issue 10, September 2001, Pages 687-698

https://doi.org/10.1016/S0010-4485(00)00099-3 Get rights and content

Abstract

One of the principal tasks of history based parametric modeling is to maintain the design history of parts. When some constrains or definitions of a feature on the part are modified, it should be guaranteed that its design history could be replayed automatically and correctly. Thus the users can get the desired part expediently, quickly and unambiguously. Topological naming mechanism is one of the main important technologies to fulfill this task. In order to solve this problem, this paper first analyzes the design history of parts, then presents a face based mechanism for naming, recording and retrieving topological entity and finally proposes a method named parametric space information to solve ambiguity when recording and retrieving entities. This mechanism, which provides a solid basis for replaying the design history precisely, has been implemented in InteSolid, a history and feature based modeling system.

Introduction

Nowadays, new generation of feature based parametric modeling systems makes it easy to define part model, which can generate various similar parts. This function is realized through flexible editing tools, by which dimension values, constraint relations and feature definitions can be modified. Parametric function makes it possible to change the dimension values of part at any time and drive the part to get a completely new variant through the various information stored in part model. This is one of the main purposes of parametric models [1]. In these systems, designers usually define a part model through dimensions, constraints and features which make it possible to generate variants. When designers want to get a variant of the designed part, they can edit the designed model using editing tools. However, some times designers may not be able to get the satisfying results through this way. The typical reasons are:

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the ability of solving constraints is not strong enough to handle some odd cases;
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the newly set dimension values are not reasonable or engineering oriented; and
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some related entities cannot be retrieved or identified from the information in the part model.

The first two of the above can be solved by applying Dimensional Constraint Manager (DCM), a software component module that manipulates geometric designs to satisfy given dimensions [2], [3], to feature based modeling systems. However, the third is relevant to referencing topological entities. The history of designing part model is a sequential one in feature based parametric system. Usually a base feature is designed first then the other features are attached to it. Every modeling step is a design history stage in which one feature is generated. Every feature contains its specific information, such as parameters, attributes, feature operations and the records of referenced topological entities. These topological entities, that is, faces, edges and vertices, are used mainly for the following three purposes:

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as operation object of a feature, i.e. the removed face of a shelling feature;
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as datum object of a feature, i.e. the datum plane of the sketch of a protrusion feature; and
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as medial object between a feature and its technical information, i.e. the entities in a feature referenced by dimensions.

When some of the dimension values, constraint relations or feature definitions of features in a part are modified, if all the referenced topological entities can be retrieved or identified from the records, it should be reconstructed automatically and unambiguously according to the specific information in every feature. However, in traditional modeling systems the referenced topological entities cannot be retrieved correctly from their records. To solve this problem, a robust mechanism for naming, recording and retrieving topological entities should be applied to feature based modeling systems. The basic functions of this mechanism are how to name the topological entities, how to record the topological entities through names and how to unambiguously retrieve the entities from the topological names when regenerating the part model.

This mechanism is not only what the 3D parametric modeling systems need, but also one of the notable characteristics that differ from traditional modeling systems. Moreover, it can be used to establish feature associations among various applications, such as, the mapping from design features to assembly or manufacturing features. Hence, the development of a mechanism for naming, recording and retrieving topological entities is necessary.

This paper is organized as follows. In Section 2, the related research is reviewed. Section 3 gives an exploration of the design history of parts. How to name topological entities is described in Section 4. Section 5 illustrates how to record and retrieve topological entities. Eliminating ambiguity is discussed in Section 6. Finally a brief summary is given in Section 7.

Section snippets

Review of related research

Among the previous related research on mechanism for naming topological entities, the work of Kripac [4], [5] and Capoyleas [1] is representative. However, the research related in this area is not enough.

Kripac presents a topological ID system, which systematically assigns IDs (names or labels) to topological entities in solid models. The IDs of topological entities in the old version of the solid model are mapped to the IDs of the corresponding topological entities in the new version of the

Explore the design history of parts

Exploring the design history of parts is helpful to understand the idea of setting up the mechanism for naming, recording and retrieving topological entities. In a feature based modeling system it's usual to design a base feature first, and then attach the other features to the previously designed features. There are a part body and an original feature body at every design stage. Generally, with the current feature defined, a design stage, S_i, is defined. PB(S_i) specifies the part body in stage

Attach original name to face

It is natural to attach names only to the faces in the original feature as attributes because:

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the topological edges and topological vertices in part body may be EWOS'es;
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any topological face in part body has a corresponding source in one of the original features. Thus, attaching names to the faces in part body can be implemented by attaching names to faces in original features; and
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during the process of Boolean operations, the topological edges and topological vertices in part body are changed

Record and retrieve topological entity

As discussed above, if a feature in designing references some topological entities as its operation objects, datum objects or medial objects, these referenced topological entities should be recorded as topological names (real names). And while rebuilding the whole part or editing a feature, those referenced topological entities should be retrieved from the real names recorded in the feature. Actually recording and retrieving topological entities are the two aspects of the same problem.

Eliminate ambiguity

In the process of recording and retrieving topological entities, ambiguity will show up from time to time. The usage of component PSI in the expression of a real name is no other than eliminating ambiguity.

Summary

The face based mechanism for naming, recording and retrieving topological entities has been implemented and applied to InteSolid 1.0, a history and feature based product modeling system developed by state key laboratory of CAD in Huazhong University of Science and Technology. The result makes it clear that the mechanism enables the modeling system to correctly and unambiguously replay the design history possible and the editablity of parts is enhanced greatly.

Acknowledgements

This paper is based on the work of developing a feature based product modeling and design system, which is supported by National Subject Fund of People's Republic China.

Wu Junjun is a research fellow at State Key Laboratory of CAD of Huazhong University of Science and Technology (HUST), His research interests lie in feature-based mechanical modeling, design for manufacturing and design for assembly. Wu received his BS, MS and PhD degrees in mechanical engineering from HUST.

References (8)

V. Capoyleas et al.
Generic naming in generative, constraint-based design
CAD
(1996)
The dimensional constraint manager, 2D DCM Manual, Version 3.4, D-Cubed Ltd,...
The dimensional constraint manager, 2D DCM Part Positioning Product Manual, Version 1.8.0, D-Cubed Ltd,...
J. Kripac
A mechanism for persistently naming topological entities in history-based parametric solid models
CAD
(1997)

There are more references available in the full text version of this article.

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Zhang Tianbing is a PhD candidate in mechanical engineering of HUST. His research interests include CAD/CAM and solid modeling. Zhang received his BS and MS degrees in mechanical engineering from Xi'an Jiaotong University.

Zhang Xinfang is a professor of mechanical engineering and chairman of state key laboratory of CAD. His research interests include Concurrent Engineering, CAD/CAM and Engineering Database. Zhang received his BS, MS and PhD degrees in mechanical engineering from HUST.

Zhou Ji is a professor of mechanical engineering and president of HUST. His research interests include optimal design, Intelligent CAD and computer integrated manufacturing system. Zhou received his BS degree in mechanical engineering from TsingHua University and MS degree in mechanical engineering from HUST. He received his PhD degree in mechanical engineering of New York State University at Buffalo.

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