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A STEP to Improved CAD PUBLIC ACCESS

Using a Standard that Enables Complete Product Model Data to be Transmitted Digitally, A Company can Build Open Systems to make Information Available on Many Platforms throughout its Operations.

[+] Author Notes

Henry Baumgartner is a contributing editor for Mechanical Engineering magazine.

Mechanical Engineering 120(02), 84-85 (Feb 01, 1998) (2 pages) doi:10.1115/1.1998-FEB-5

This article highlights that by using a standard that enables complete product model data to be transmitted digitally, a company can build open systems to make information available on many platforms throughout its operations. The world of CAD/CAM has viewed the International Graphics Exchange Standard (IGES) as its translation standard for years, using the system to move two-dimensional models from one program to another. While IGES does, in fact, do a good job of transmitting basic geometry, another translator—the Standard for the Exchange of Product Data (STEP) —has been gaining on IGES in popularity. STEP goes considerably further than just transmitting geometry; it provides users with the ability to express and exchange digitally useful product information throughout a product's life cycle, including design, analysis, manufacturing, and support. In short, IGES transmits two-dimensional drawings, while STEP transmits complete product models. The updated schema, consisting of an ASCII file written in EXPRESS, is simply run through a utility that merges the extensions that have been added by Unigraphics with the standard ST-Developer libraries. Updating the translator to incorporate an upgraded version of the libraries is just as easy.

The world of CAD/CAM has viewed the International Graphics Exchange Standard (IGES) as its translation standard for years, using the system to move two-dimensional models from one program to another. While IGES does, in fact, do a good job of transmitting basic geometry, another translator-the Standard for the Exchange of Product Data (STEP)-has been gaining on IGES in popularity. STEP goes considerably further than just transmitting geometry; it provides users with the ability to express and exchange digitally useful product information throughout a product's life cycle, including design, analysis, manufacturing, and support. In short, IGES transmits two-dimensional drawings, while STEP transmits complete product models.

The key advantage of STEP is its much broader reach compared with previous translators, as it covers the entire engineering process from design to manufacturing. This can make it the keystone for integrating a company's engineering processes, according to Dave Loffredo, vice president for product development at STEP Tools in Troy, N.Y., which provides computer-aided software engineering tools that simplify the process of implementing STEP. And since STEP has the imprimatur of the International Organization for Standardization, it should be compatible with whatever comes along next.

Loffredo noted that combining STEP with World Wide 'Web technology can enable a company to build an open system and make engineering information available throughout its operations. During the last couple of years, businesses have used the Web and intranets-private networks based on the Internet-to open up their operations. STEP provides a common standard representation for engineering data, so now companies can do the same with their engineering processes.

So far, Loffredo said, all major CAD vendors have incorporated STEP into their systems, so users of off-the-shelf CAD tools should have ready access to the new standard. Companies that make extensive use of in-house software systems, such as Boeing and General Motors, have already integrated STEP into their operations. Early use of the translator focused on geometry, but product-data-management vendors are also now building STEP into their programs. As a result, customers won't have to reinvent the wheel when implementing STEP in their operations.

Even so, understanding the process of incorporating STEP within a CAD/CAM program can help engineering managers determine what's involved in adopting STEP in their operations. That's because STEP simplifies the integration of diverse engineering systems, making it easier for engineers to assemble large product models using component models created in different CAD programs. STEP can also improve the process of creating large models for analysis or manufacturing simulation using a combination of commercial and proprietary codes.

Stepping Stones for Developers

When EDS Unigraphics in Maryland Heights, Mo., resolved to provide advanced STEP functions in its latest software release, it cut months from the time required for development by making use of pre-existing tools, according to Jim Mohan, product-line manager for data exchange at Unigraphics. The company's release 12 provides full support for conformance classes 1, 2, 4, 5, and 6 under the AP214 and AP203 protocols. Constructing an implementation from scratch would have taken many months of additional development and almost invariably would have involved false starts and shakeouts during the development process. STEP implementation was stream lined by using a development environment that provides prebuilt and easily modifiable libraries of functions to create, destroy, access, and update STEP data and other useful functions.

STEP enhancements in version 12 are among the CAD software's most important new features . The STEP standards are organized into application protocols (APs), which are unique sets of entities chosen for a specific product, process, or industry. For example, APs have been tailored to the automotive, aerospace, and shipbuilding industries; one has also been made for the sheet-metal die-design process. Each AP is a formal document that contains a description of a portion of the life cycle of a product, the application activity model; the pieces of product information that are needed for these activities, known as the application reference model; and an information model that describes everything in the reference model using a library of pre-existing definitions. This information model is written in the EXPRESS language.

Version 12 provides extensive geometry support for AP203, "Configuration-Controlled Design," and AP214, "Core Data for the Automotive Mechanical Design Process." Within these two APs, version 12 supports configuration data management without shape, geometrically bounded wire-frame or surface models, manifold surfaces with topology, faceted B-Rep solid models, and advanced B-Rep solid models. The release has also increased the number of platforms that the STEP translator runs on; these now include Hewlett-Packard, Sun , Silicon Graphics, and RS/6000, as well as 32-bit Intel and Digital Equipment Corp. Alpha and VMS platforms.

When Unigraphics began STEP implementation, Mohan said, managers faced the question of whether to construct translators from scratch or purchase a comn1ercial development tool. Starting from scratch would have required a significant time outlay for developing support software, even before the actual task has begun. The amount of work involved in setting up a physical file reader/writer, creating all classes by hand, designing necessary routines to handle traversal and memory management, and so on would also have been large.

Accordingly, Unigraphics developers looked for a mature commercial environment that could serve as the basis of their effort. The company was committed to object-oriented development, and it was an important requirement that the development environment be object-oriented as well. (Object-oriented codes help software developers create "modular" programs that can be used as building blocks in the creation of new software.)

Unigraphics decided to work with STEP Tools, which had already assisted many CAD/CAM software developers and large users in building STEP interfaces, testing new APs, and constructing STEP data bases. Perhaps the most important reason for selecting STEP Tools' ST Developer package was the open architecture that it provides for extending its support of STEP objects. While ST -Developer contains a full-function library, the objects and methods provided by the development environment can easily be extended to provide additional methods and options using conventional object-oriented techniques. Unigraphics developers took full advantage of these capabilities to enhance methods that operate on the data model embodied in their routines.

Mohan said Unigraphics developers were able to use pre-existing libraries to capture STEP data and make them available to applications as C++ objects that, following the object-oriented paradigm, contain both data and methods for operating on the data. Another set of libraries is used to provide functions to convert the object model to the Unigraphics proprietary database format. The core coding work on the part of Unigraphics developers was to extend these libraries. Some of these extensions simply involved adding methods that reformat and reposition data in the appropriate manner. Other extensions apply modeling rules to the data to incorporate key proprietary functions of the Unigraphics database.

This object-oriented approach means that whenever the STEP schema, or data model, changes, it can be updated with little or no progran1l1ung effort. The updated schema, consisting of an ASCII file written in EXPRESS, is simply run through a utility that merges the extensions that have been added by Unigraphics with the standard ST-Developer libraries. Updating the translator to incorporate an upgraded version of the libraries is just as easy.

Overall, the development process proceeded on a straight-line path with very little wasted effort, Mohan said. The entire project took several man-years, but the development tools saved several man-months. The use of pre-existing libraries also made it possible for Unigraphics developers to focus on the functions of the translator rather than on syntax. As a result, Mohan noted, Unigraphics is now on the leading edge of STEP implementation.

The STEP translator from EDS Unigraphics enables engineers to build massive assemblies, like this car, using sub component models created with many CAD systems.

Grahic Jump LocationThe STEP translator from EDS Unigraphics enables engineers to build massive assemblies, like this car, using sub component models created with many CAD systems.

Copyright © 1998 by ASME
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