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The Esprit Project, CAD*I, started in November 1984, with its main goal the specification and development of software interfaces between various CAD systems, and between CAD and Finite Element (FE) analysis systems. Debbie Thomas and Jan van Maanen were the main people from RAL involved. They were also involved in the resulting ISO standardisation activities. Brian Colyer was also involved in the standards work as part of the BSI delegation.
The project was originally for 3 years but got extended for a further 2 years. Twelve industrial and academic organisations from six member states of the European Community were involved. Early on the specification work was completed followed by interfacing software between systems.
By 1987, the work on CAD*I consisted of finalising the reference model and software, and the writing of the final report. The reference model was able to describe finite element modelling, analysis and results. The programs enabled the transfer of data from a geometry neutral file to FAMbuild, from FAMbuild to neutral file, from neutral file to Nastran, and from neutral file to FAMview. Non-exclusive licences were granted to a partner in CAD*I (GfS, a company based at Aachen in West Germany) to incorporate some of the software into their commercial products. A successful demonstration of the CAD*I project occurred in Copenhagen in October 1988.
CAD*I influenced the development of the ISO STEP standard (STandard for Exchange of Product Information). A compiler for the data modelling language EXPRESS (adopted by STEP) was developed at RAL by Mike Mead.
The final report on product analysis data exchange was written during 1989 and published by Springer- Verlag as a book in a series describing the whole project. The RAL staff acted as editors, collating contributions from other European partners, in addition to making major contributions of their own.
Abstracting from the introduction:
The purpose of this document is to document the results of WG1, WG2 and WG3 on wireframes, solids and surfaces in:
- Developing a neutral file format for transfer of CAD data between CAD systems, and from the CAD domain to CAA (computer aided analysis) and CAM (computer aided manufacture).
- Developing pre- and post-processors with a number of representative CAD systems for this neutral file format.
- Developing representative test model files and performing cycle tests and inter-system testing for CAD model transfer.
- Contribute to the standardisation activities within ISO.
ISO was developing a Standard for the Transfer of Product Model Data (STEP) in ISO/TC184/SC4/WG1. The aim was that CAD*I and STEP should either be identical or have a one-to-one mapping between them.
The basic need was a way of representing CAD data in a compact, machine-understandable form which captures the essence of the data without major loss of information or detail. A prerequisite is a knowledge of structures used in current CAD systems. The methodology must be such that generation and interpretation of the neutral file form is performed in a consistent way. The aim is not to capture just the graphical information but the complete representation of a product as stored in a database.
The user's view is of a pair of pre and post processors performing the translation between the product data representation in a CAD system and the neutral file. As specified, there is a need to define the grammar of the neutral file (a set of BNF productions) and its semantical interpretation (defined as a finite state machine) in order to build a data structure compatible with the reference model specified.
The logical definition of data structures is defined by a data specification language.
The neutral file must be able to handle high level representation schemes such as those used in Constructive Solid Geometry (CSG) and also low-level graphical formats.
In CAD*I, the basic data structure was called an entity which had a structured collection of attributes. The attributes could be predefined or composite ones made from existing attributes.
A constraint put on the neutral file was that it had to be strictly sequential with no forward references so that CAD data structure could easily be created in a single pass.