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Über dieses Buch

ESPRIT Project 322, "CAD Interfaces", has been established to define the most important interfaces in CAD/CAM systems for data exchange, data base, finite element analysis, experimental analysis, and advanced modeling. The definitions of these interfaces are being elaborated in harmony with international standardization efforts in this field. One principal goal of the project is to develop techniques for the exchange of CAD information between CAD systems, and from the CAD domain to CAA (Computer Aided Analysis) and CAM (Computer Aided Manufacturing). This volume presents a proposal for a neutral file format for CAD data: curves, surfaces, and solids. The specification is based on a reference schema for CAD data bases and is defined informally with respect to its semantics and formally with respect to its syntax. This volume is a revised edition of "Specification of a CAD*I Neutral File for Solids" Version 3.2. The revision reflects the enhancements which result from the implementation of communication processors in eight different CAD systems and from the practical exchange of solid models between these systems. Due to the close interaction between the CAD*I project and the coming-up international standard STEP this specification also serves as an introduction to the geometry model that will be included in the future international standard.

Inhaltsverzeichnis

Frontmatter

Introduction To The Specification

Frontmatter

1. Introduction

Abstract
The purpose of this paper is to document the results of Working Group 1 (wireframes), Working Group 2(solids), and Working Group 3 (surfaces) of ESPRIT project 322 CAD*I (CAD Interface). The goal of these working groups is:
  • 1. Develop a neutral file format for transfer of CAD data (curves, surfaces, and solid models) between CAD systems, and from the CAD domain to CAA (computer aided analysis) and CAM (computer aided manufacturing).
  • 2. Develop pre- and post-processors with a number of representative CAD systems for this neutral file format.
  • 3. Develop representative test model files and perform cycle tests and inter-system tests for CAD model transfer.
  • 4. Contribute to the standardization activities in the national standardi-zation bodies and in ISO for the establishment of a neutral file format for CAD data.
E. G. Schlechtendahl

2. Approach

Abstract
The purpose of this chapter is to provide an overview of the needs and requirements, the environment, and assumptions used in the development of this specification.
E. G. Schlechtendahl

3. Basic Principles

Abstract
This chapter describes the overall concepts underlying the specification of a neutral file for exchange of CAD models between CAD systems or a CAD system and other application areas.
E. G. Schlechtendahl

Reference Manual

Frontmatter

4. The High Level Data Specification Language HDSL

Abstract
With reference to the algebraic specification of data types, the specification of a type T, defines a set (usually infinite) of constansts (T-constants). Besides, certain T-specific properties (T-Axioms) Are valid for these constansts (semantic of T). For example, the type NATURAL defines the set N=(0, 1, 2, 3, 4, 5 ...) of NATURAL constants, where the relation 2=Suc(Suc(0)) is one of the T-Axioms (Suc denotes the successor function /Suc(X):=X+1).
E. G. Schlechtendahl

5. The CAD*I Reference Model Specification

Abstract
This specification is set up such that for each entity, property, or attribute we define in sequence
E. G. Schlechtendahl

6. Levels of a Schema Implementations

Abstract
As CAD systems provide different levels of capabilities one cannot expect all systems to support the full schema in their pre- and post processors. On the other hand, it must not be left to the choice of the implementer of processors which subset of entities and attribute he wants to support. Thus, this specification includes a definition of allowable are of the complete subsets of the complete subsets schema. These allowable subsets characterized by a set of numbers, called levels. Such levels are used to identify
  • 1. the geometric modeling capabilities (lg)
  • 2. the capabilities of defining assembly structures (la)
  • 3. the capabilities for parametric models and macros (lp)
  • 4. the capabilities of references (lr)
E. G. Schlechtendahl

7. The Physical Layer

Abstract
The physical layer of the CAD*I neutral file for solids describes how the CAD system data structures are mapped onto a sequential file. There are several levels of regarding the same sequential file:
  • 1. the physical level,
  • 2. the metafile level,
  • 3. the alphabet level,
  • 4. the token level,
  • 5. the statement level, and
  • 6. the file structure level.
E. G. Schlechtendahl

Implementation Guidelines

Frontmatter

8. Definition of the Finite State Machine Post-Processor

Abstract
This chapter defines the neural file defining the effect of each statement on the file in terms of the changes which will result in the post-processor data structure. This technique is borrowed from the the GSK standard. The post-processor data structure consists of a number of “state list” defined more precisely in “9. State list” on page 175.
E. G. Schlechtendahl

9. State Lists

Abstract
All state list are divided into three sections:
  • 1. control section
    The control section is a storage for information which is relevant only as long as an entity is open. This section will not survive when the list is closed. Essential items in this section are the enclosing entity and its state. Also the name which was used on the neutral file to identify the entity is part of this section.
  • 2. directory section
    The directory section contains a directory of all named entities received within the open entity from the neutral file. The directory contains the names as given on the file together with references to the entities after the storage in the open entity.
  • 3. value section
    The value section will store the result of actions which have been carried out to define the open entity. Upon closure of an open entity the value will become one entry in the closing_entity_list. When the open world is closed, its value is copied into the receiving CAD system data base (either on the world level or on an assembly level defending on the directives to the post-processor).
E. G. Schlechtendahl

10. Relationships Between Operating Systems and CAD

Abstract
We define the correspondence between computer operating systems CAD systems by introducing special terms for the CAD model and by defining their meaning in conventional operating system terminology.
E. G. Schlechtendahl

11. Programming Rules for Cad*I Processors

Abstract
The CAD*I environment consists of a set of internal CAD*I processing programs interfacing external products such as CAD systems, FEM computational programs. All interface CAD*I software should be written in a common FORTRAN 77 subset.
E. G. Schlechtendahl

12. Programming Interface

Abstract
It is planned to define a FORTRAN subroutine package consistent with the schema defined in“ 5. The CAD*I reference model specification ” on page 43. This package should allow reading and writing from and to neutral files. The GKS FORTRAN binding is to be used as a reference for this approach.
E. G. Schlechtendahl

13. Comments on Pre-Processors

Abstract
The task of the pre-processor is to neutral file from the information in a CAD system data base. The data base is assumed to be consistent with the schema described in “5. The CAD*I reference model specification ” on page 43. Even though the implementation of the pre-processor will have to depend significantly on the individual CAD system, a basic algorithm for the translation process from CAD system data base to the neutral file can be formulated: The main purpose of this algorithm is to ensure the neutral file is sequential.
E. G. Schlechtendahl

14. Comments on Post-Processors

Abstract
The post-processor has the task to interpret the neutral file and to build an internal representation of the transferred model in the receiving CAD system. The post-processor consists of the following modules:
  • 1. scanner
  • 2. parser
  • 3. semantics analysis and data structure generation.
E. G. Schlechtendahl

15. The Interface of the CAD*I Parser

Abstract
The CAD*I project has a developed a scanner/parser which is suitable for post-processing the neutral file language. This software was developed for speeding up the generation of post-processors for different systems. The advantage of using this scanner/parser is that post-processor development for another CAD system can now be restricted to the analysis of semantics and building up the data structure. This chapter describes the theoretical basis for the parser and its interface to later phases of post-processing.
E. G. Schlechtendahl

Backmatter

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