4 lation and optimization. These are essential constituents of the iterative process, leading to a feasible and, one hopes, optimal design. 1.3 Content of the Book In Chapter 2 we present briefly the history of CAD. The main components of CAD systems are identified, and their principal functions described. Economi cal and interdisciplinary aspects are discussed. Chapter 3 starts with a systems analysis of the design process. The notion of a process is introduced as a fundamental tool to describe activities like design as a whole, computer-aided design, program executions, terminal sessions etc. The environment and the resources which the environment must supply for the successful execution of any process are discussed. The problem of modelling the design objects in an abstract schema and the interrelation between the schema and the planning of the individual step in the design are analysed. Chapter 4 concentrates on the interfaces among the components of a CAD system, including the human operator. The problem of mapping an abstract schema onto the capabilities of various programming, command, or data de scription languages is described in detail. Emphasis is laid upon the resource aspect and its influence on the design of CAD systems. The concept of a CAD software machine is introduced, and rules for designing such machines are given.

Product Data Technology encompasses the information related to all stages in the product life cycle from product design via production planning, production processes, production control, etc. through the delivery and operational stages of the technical product. Product Data Technology takes a coherent, unified view of the information captured in this whole life cycle and provides methodologies to support this integrated perspective. The Product Data Technology Advisory Group (pDTAG), is a Special Interest Group supported by the CIME Division within DG m of the European Commission. Founded in 1991, the PDTAG has encouraged the formation of a new subseries on Product Data Technology within the existing series of Research Reports ESPRIT. This subseries will provide a depository for the important contributions made by ESPRIT projects to the evolving area of Product Data Technology, particularly also those based on the STEP (ISO 10303). PDTAG is grateful to Springer Publishers for establishing this subseries which will serve to report recent international developments in Product Data Technology.

Principal authors: U. Kroszynski, B. Palstr9Sm 1.1 The evolution of concepts and specifications for CAD data exchange The CAD/CAM community has witnessed, during the last decade, the appearance of several specifications as well as proposals for standards which either attempt to cover wider areas or to be more reliable and stable than the others. With the rapid evolution of both hardware and software, the capabilities offered by CAD systems and CAD based application systems are far more advanced than they were only ten years ago, even when they are now based on micro-computers or personal comput ers. The situation with standards, however, is not and cannot be so. In order to be reliable and accepted by a wide community of both vendors and users, a standard has to be sta ble. This implies a life span of at least a decade. This also implies that the standard has to be general and flexible enough to accommodate present as well as expected future developments. 1.1.1 IGES The initial development of concepts for CAD data exchange is strongly influenced by the US Integrated Computer Aided Manufacturing (ICAM) programme, that dealt with the development of methods for data exchange. In September 1979, a subgroup was estab lished with participation of the National Bureau of Standards, the General Electric Com pany, and the Boeing Company. The result of this effort was the Initial Graphics Exchange Specification (IGES) that was published as a NBS report [61] in 1980.

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.

Edited for Working Group 2 (WG 2): Cisigraph, Cranfield Institute of Technology, Danmarks Tekniske Hojskole, Kernforschungszentrum Karlsruhe GmbH, NEH Consulting Engineers ApS, Universitat Karlsruhe