Abstract

This paper describes the need for standardized representations of product information which are suitable for electronic communication between engineering and manufacturing functions. It then describes how a single international standard, ISO 10303 -- Product data representation and exchange, has been designed to meet this industrial need. This standard is more commonly referred to as STEP, STandard for the Exchange of Product model data. The benefits of STEP and the scope of the standard are presented. The standardized product data representations all use a rich object description language called EXPRESS to define the content of the data to be communicated. The actual communications interfaces, a message/file format and an application programing interface, are independent of any particular implementation technology. A high level technical description is given; this includes a description of the segments of the standard that can be used by readers to determine if the standard satisfies any of their company=s needs for product data exchange. Key players in the development and implementation of STEP along with sources of additional information are provided.

Problem

Data interchange capabilities among Computer-Aided Design and Manufacturing (CAD/CAM) and Product Data Management (PDM) software systems are severely limited by their lack of interoperablity. This is a major barrier to U.S. manufacturers= efforts to improve their engineering and production processes, and ultimately their competitiveness in the global marketplace. While U.S. CAD/CAM and PDM systems dominate the market and U.S. manufacturers lead the world in employing this technology, the product designs and engineering specifications produced by using this technology remain locked within the systems= proprietary structures. Point-to-point translators between pairs of systems have been used to alleviate this problem, but this strategy has not succeeded for a number of reasons. Each system has somewhat different capabilities. Their data representation contains concepts that are slightly different for similar systems, making exact translation impossible. As the number of systems used increases, the need for translators grows tremendously. Vendor release improved versions of their systems regularly which require changes to the translators; it is very difficult to keep the translators synchronized with the production versions of systems. Finally, the market for translators is not sufficient to support the quality of translators that industry needs. So, manufacturers are not able to use technology that they have paid for to its fullest potential.

Importance of Open Product Data Exchange Standards

The need to develop standardized representations is well understood [1]. Standards have the advantage of providing a published and stable representation; and standards have a change process that is based on consensus. Proprietary exchange formats can and do change at the discretion of the companies that developed them. Further, manufacturing is increasingly global. To be most effective, the standards for exchanging product information must gain world-wide acceptance.

This recognition has led to the development of a standard with capabilities which far exceed those of other standards, open specifications, or proprietary exchange formats. A single international standard for Product Data Representation and Exchange, also known as STEP -- STandard for the Exchange of Product model data, addresses the basic representation of product information. Product information includes the totality of related information needed to completely define and describe a product over its expected life-cycle [2]. STEP facilitates improved communication of this information by electronic exchange and shared access via standardized interfaces.

The world-wide interest in STEP is motivated by the following benefits:

a) Improved time to market of products as a result of improved communication and access to product representations. This facilitates increased simultaneous development for tasks such as design, production planning, manufacturing, and maintenance.

b) Improved ability of multiple companies to work together on new and improved product developments.

c) Reduced product costs due to the elimination of non-value added activities, increasing the efficiency of making revisions and upgrades to the design of a product.

d) Improved ability to manage upgrades and the introduction of new software capabilities.

The Scope of STEP

The initial standard supports the system independent representation of shape, presentation information, product structure and configuration information, engineering changes and approvals, and the application context of the generating system [3]. The initial version of the standard contains an exchange interface for CAD applications using various forms of common CAD model representations (e.g., wireframe, surface, and boundary representations), drafting applications, and PDM applications (e.g., configuration/approval information). Many additional application interface specifications for the downstream applications of the mechanical, electrical, and civil engineering disciplines are active work items.

All of the data specifications use the EXPRESS language to define the content of the information to be communicated. EXPRESS is an object-like conceptual data definition language [4]. A file exchange format defines the actual communications interface[5]; it uses the EXPRESS to structure the occurances of product data. The application programing interface, ISO 10303-22 [6], which is suitable for accessing product data from an application which is stored in either a file structure or a database is a draft international standard. This access interface, although not an approved standard, has received considerable validation by prototype implementation of the emerging standard [7,8]. STEP does not couple the ability to specify process and data representations which is useful for process optimization; this requirement is being addressed within the EXPRESS 2nd edition project within SC4.

Technical Approach

STEP (ISO 10303) is a large, complex standard that was designed to support the needs of many different industries and disciplines to exchange product data. The component of the STEP standard that addresses the specific needs for product data exchange of a particular industry sector, engineering discipline, or CAD/CAM application technology area is called an Application Protocol (AP). Each Application Protocol defines the basis for implementing interfaces for the exchange of product data between similar applications from different software vendors. Further, the AP defines a basis for assessing these interfaces for conformance to the requirements of the standard. Each AP is documented in a same manner [9]. There are four major components of an Application Protocol:

• Scope and an activity model to tell you what the Application Protocol covers and what it excludes (in clause 1 and annex F of an AP respectively).
• Information requirements to describe the types of data to be exchanged (in clause 4)
• Standardized data specification to govern the data exchange (i.e., data structures, relationships, semantics and constraints) which contains the fully specified EXPRESS data model to be used (in clause 5).
• Conformance requirements to define the implementable subsets of the AP for exchange purposes (in clause 6).

The ability to share information among related applications is provided through the reuse of a series of EXPRESS models that are defined to be used as building blocks by the application protocols. These models are termed Integrated Resources. These Integrated Resource models are general in nature and describe concepts which have been found to recur across many design and engineering disciplines. The Integrated Resources are extended whenever requirements from emerging application protocols identify the need for concepts which are more than specific cases of an existing concept. The high level technical organization of these data models is illustrated in figure 1. A more complete illustration of the STEP data architecture is given in figure 2. A simple example of some alternatives for populating a shape representation is shown in figure 3.

Contents of the STEP Building Blocks

The initial standard contains building block models for describing:

• shape properties of products, as mathematically defined by geometry and topology;
• how shape is represented in a CAD model, such as wireframe, surface, or boundary representation;
• product structure and assembly relationships;
• presentation, annotation, and drafting conventions (to render shape and other properties into drawings or views);

• product identification and engineering change management including approvals; and
• the application context and administrative information.

A major extension to these capabilities was recently sent to ISO for final review prior to publication as of the international. These building blocks contain:

• material and material properties with uncertainty specifications;
• shape tolerances and dimensions;
• process structures; and
• kinematic representation.

The definition of a parametric representation is still under development.

Implementation Results

The approval of STEP as an international standard in 1994 was met with guarded optimism by industry. Adopting STEP requires a significant investment by vendors and business process improvements by U.S. manufacturers to realize the vision of STEP. However, serious efforts to implement the first STEP application protocol, ISO 10303-203 -- Configuration managed design for mechanical products, by members of the CAD industry have produced a series of high quality translators. A handful of these systems have been sufficiently interoperable to allow some corporations to put STEP technology into production use. By the fall of 1996, there were two outstanding issues which were impeding wide-spread adoption⁴, neither is related to any fundamental flaw in the standard. The first of these is the differences in the numerical accuracies used within CAD systems for mathematically representing the shape of a product [10]. The second is the need for industry-wide practices for CAD model creation. A longer term issue is the interoperability of data interchange between the originating and downstream software systems. An industry consortium, PDES, Inc., has taken the lead on this issue and is receiving input from various AP development projects world-wide.

Communication of practical implementation experience, critical success factors, and concrete benefits is needed to spread the adoption of this emerging standard. Some results are available [11,12]. Inserting STEP technology into products or production is still currently beyond the resources of small CAD/CAM vendors or users but the agreements to adopt STEP by large segments of the aerospace and automotive industries will change this [13,14]. Most of the companies that have been successful in early adoption of this technology have leveraged relationships with their suppliers/customers and the industry groups contributing to the STEP standard.

Key Players in the Development of STEP

There are a number of communities that are contributing to STEP. These include recognized standards making bodies and industry consortia.

Standards Bodies:

1. Industrial Automation Systems and Integration - Subcommittee on Industrial Data (ISO TC184/SC4)

This group within the International Organization for Standardization (ISO) is developing international standards for the exchange of product models and specifications. ISO 10303 - Product Data Representation and Exchange is a series of 14 documents which have achieved international standard status as of September 1996 and over 30 active application protocol projects which are extending these capabilities. ISO 13584 - Parts Library is defining specifications for the exchange and access of digital part catalogs; this work leverages building block capabilities of ISO 10303. There are three Parts Library specifications which have achieved draft international standard status. ISO 14959 - Parametrics has decided to propose extensions to some of the existing STEP Integrated resources as a very near term solution and is also developing a longer term solution. Manufacturing Management Data (MANDATE) is the least mature of the standards under development within SC4.

NIST maintains an on-line repository of proposed and approved SC4 standards, position papers, issues and resolutions, project management information, and software for use by experts who are contributing to or reviewing these standards.⁵ See http://www.nist.gov/sc4 for access to the contents of this repository.

2. IGES/PDES Organization (IPO)

The Product Data Exchange using STEP (PDES) is the project within this American National Standards Institute (ANSI) accredited standards making body which represents U.S. industries= technical interests in SC4 standards. The PDES project has been a key contributor to the STEP (ISO 10303) standard. This organization also developed and maintains the Initial Graphics Exchange Specification (IGES). A description of activities and future meetings can be found on the Web at: http://www.scra.org/uspro.

3. ISO TC184/SC4 Implementors Forum

This activity provides a valuable discussion forum for implementors and standards developers to resolve issues that arise during the implementing of the STEP standard. This activity also brings together the results from industry consortia roundtables and determines if industry practices (outside the scope of the standard) or enhancements to the standard are needed.

Industry Consortia and Associations:

1. PDES, Inc.

This consortia is focused on building application protocols (the implementable part of the STEP standard) and effecting their implementation by member companies. Much of the technical resources are concentrated on supporting several STEP pilot projects in conjunction with the member companies. The concentration is on mechanical and electrical/electronics domains, developing a neutral data model for exchanging design intent information, and providing STEP support to member organizations.

2. ProSTEP Association

This association is focused on developing application protocols for the European Automotive Industry and effecting their implementation by commercial vendors in the CAD/CAM market. ProSTEP Association has the lead on ISO 10303-214 -- AP: Core data for the automotive design process, and ISO 10303-212 -- AP: Electrotechnical design and installation.

3. Automotive Industry Action Group (AIAG)

This association is working on an ambitious effort to implement STEP in their AutoSTEP pilot. The initial focus was on CAD solids translator interoperability. The Phase Two effort, to end in late 1997, addresses CAD solids with assemblies, introduces surfaces, wire frames, configuration management, associative drafting, and examines product development business processes. The goal is to deploy STEP in production environments across the automotive supply chains of their members.

4. Other STEP Centers

PDES, Inc. and ProSTEP are two of the seven world-wide STEP development centers. The USPro Web site, http://www.scra.org/ uspro/servers.html, provides links to most of the related activities at these centers.

NIST's Contribution

NIST=s contributions have focused on providing infrastructure technologies and mechanisms that are used by industry experts to specify these standards. NIST has also developed STEP specific software toolkits that are used by industry collaborators to accelerate the validation and implementation of the standards, and built reference implementations that are used to test implementations of the standard. NIST facilitates the standardization process by serving in the role of the SC4 Secretariat and providing the technical planning and administration for the U.S. standards efforts in this domain. Further, NIST works with U.S. industry on some pilot implementation activities to facilitate improving the standards base on the resolution of issues from implementation. These pilots also provide the opportunity to measure the results from applying STEP technology in an industrial setting against existing design and manufacturing practices to producing the solid business cases that are needed to obtain full scale industrial commitment to STEP.

References

1 Mason, Howard. The Historical Need for STEP. Australian STEP Conference Proceedings, Sydney, Australia, March 1995.

2 ISO 10303-1. Industrial automation systems and integration B Product data representation and exchange B Part 1: Overview and fundamental principles. ISO, Geneva, Switzerland. December 1994.

3 Owen, Jon. STEP B An Introduction. Information Geometers Limited, 1993 (ISBN 1 874728 04 6).

4 ISO 10303-11. Industrial automation systems and integration B Product data representation and exchange B Part 11: Description methods: EXPRESS reference language manual. ISO, Geneva, Switzerland. December 1994.

5 ISO 10303-21. Industrial automation systems and integration B Product data representation and exchange B Part 21: Implementation methods: Clear text encoding of exchange structure. ISO, Geneva, Switzerland. December 1994 and Technical Corregenda 1, August 1996.

6 ISO/DIS 10303-22. Industrial automation systems and integration B Product data representation and exchange B Part 22: Implementation methods: Standard data access interface. ISO, Geneva, Switzerland. August 1996.

7 Sauder, Dave and Morris, K. C.. ADesign of a C++ Software Library for Implementing EXPRESS: The NIST STEP Class Library,@ Proceedings of the EXPRESS Users Group, July 1996.

8 Denno, Peter, and Morris, KC Paper. "Opportunities and Challenges of the STEP Product Data Facility," Proceedings of the Product Information Management Symposium, Ann Arbor, MI, DH Brown, and Associates, September, 1996.

9 Palmer, Mark, and Gilbert, Mitchell. Guidelines for the Development and Approval of Application Protocols. ISO TC184/SC4 N433, May 1996. URL http://www.nist.gov/sc4/howto/methods /ap/ballot1/sc4n433.ps.

10 Anderson, William. PDES, Inc. Accuracy Team Interim Report, PDES, Inc. Internal report, SCRA, Charleston, SC, August 1996.

11 Frechett, Simon. Interoperability Requirements for CAD Data Transfer in the AutoSTEP Project, NISTIR 5844, Gaithersburg, MD. July 1996.

12 Briggs, David and McKee, Larry. AThe AEROSTEP Project: Exchanging STEP Data Between Boeing and its Engine Suppliers,@ Manufacturing Review, Volume 7, Number 4, December 1994. Related activities are described on URL: http://www.scra.org/pdesinc.html.

13 Memorandeum of Understanding between the Aerospace Industry, PDES, Inc. URL http://www.scra.org/uspro/events/ pdes_inc.html.

14 Letter of Agreement between four World-wide Automotive Associations, STEP Automotive Special Interest Group (SASIG), available from AIAG, Southfield, MI, Decenber, 1995.