One of the Holy Grails in globalization is the exchange of product data between different computer-aided design (CAD) systems.
Toward that goal, first came the Initial Graphics Exchange Specification (IGES). Then came the STandard for the Exchange of Product model data (STEP). IGES has been around since 1982, but really didn't become trusted (read: accurate, effective, and popular) until about 1990. STEP has been officially around since 1995, but really didn't become trustworthy until last year.
Today? "IGES has never been more popular," chuckles Martin Hardwick, president of STEP Tools, Inc. (Troy, New York).
|Standard for the Exchange of Product model data (STEP) provides "a complete, unabmiguous, computer-interpretable definition of the physical and functional characteristics of a product throughout its life cycle.|
This popularity should come as no surprise. Plenty of time has gone by for IGES translators to be developed, debugged, and tweaked. Both CAD vendors and users have by now designed their CAD system around IGES. And much of the CAD-based design going on is still two-dimensional drawing, which works just fine in IGES.
That's about to change. Tests show that STEP not only does more than IGES, but STEP works well, too.
Some History About IGES and STEP
Over the years, IGES got diluted. First, it started out as an American standard, but standards organizations in other countries have created their own IGES-like product data exchange standards, such as SET in France and VDAFS in Germany. Second, all IGES translators seem to deviate from the IGES standard in some way. None seem to work the same and they require a lot of manual intervention to accurately exchange data between different CAD and computer-aided manufacturing (CAM) systems. Third, IGES was initially designed for exchanging strictly the two-dimensional geometric representation of a part design. Eventually, IGES could handle 3D data.
Six versions of IGES have been released. The latest versions include most of the entities used in major CAD systems. Most IGES processors only support IGES Version 3. High-end CAD systems and the more popular PC-based CAD systems typically support IGES Versions 5 to 5.2. Later versions of IGES are backwards compatible; early versions often simply ignore the newer entities included in the later IGES versions. Or, the processors based on early IGES versions will crash.
In 1984, Product Description Exchange Specification (PDES) was born as a single international standard to cover all aspects of a neutral format for product data exchange and to supersede existing national and de facto standards. The resulting standard, now called STEP, is ISO 10303. Within that standard are several industry-specific data models, called Application Protocols (APs), which are also international standards. Two APs of note:
•AP203 defines the geometry, topology, and configuration management data of solid models for mechanical parts and assemblies. Some consider AP203 to be the first true release of STEP. It has been used in the aerospace industry with considerable success.
•AP214, designed for the automotive industry, defines the core data for the automotive mechanical design process, specifically the car body, powertrain, chassis, and the interior parts of a car. AP214 goes well beyond AP203, providing a far more comprehensive model for automotive applications and covering the lifecycle of a design from engineering through manufacturing. It also covers such design issues such as colors and layers, geometric dimensioning and tolerancing, and design intent. AP214 is considered an extension of AP203. Spearheading AP214 is ProSTEP, the leading European STEP Center, headquartered in Germany. (ProSTEP GmbH is a joint subsidiary of several European automotive OEMs and suppliers, and the ProSTEP Association.)
As with any international standard, vendor acceptance has been slow going. Just this past April, Dassault Systèmes and IBM were the first to announce a STEP-certified product: CATIA Release 4.2.0 with CATIA STEP AP203 Interface, running on an IBM RS/6000 under AIX and using an IBM-developed STEP toolset.
AP203 Interoperability Testing
|STEP in Action|
Boeing, the aircraft manufacturing giant, is in production using STEP. But as early as 1997, Delphi Delco Electronics Systems (Kokomo, IN) was using STEP to exchange solid model data from EDS Corp. Unigraphics to Dassault Systèmes' CATIA when producing climate control systems for Chrysler and Saturn Corporations. Delphi uses the Unigraphics system; the two automotive companies use CATIA.
Previously, Delphi engineers used IGES to translate design geometry. Typically, the solids and surfaces had to be regenerated because of poor accuracy and the large number of untrimmed surfaces translated. Such clean-up work took an average of 24 to 32 hours per part.
Now, over 95% of the models are successfully transferred between the two CAD systems using STEP. Those files imported into CATIA that did have problems, required about 30 minutes of rework to remove unwanted reference geometry. Even with this rework, using STEP has dramatically reduced the design cycle time and costs, and increased the accuracy and quality of the geometry input into CATIA. For example, 12 files transferred using STEP saved approximately 50 hours of manual intervention.
How good are the AP203 STEP translators? PDES, Inc. (North Charleston, SC), a joint industry/government consortium for accelerating the development and implementation of STEP, started testing STEP translators for interoperability within CAD systems in February 1995. Initially called "Plugfest," the STEPnet interopera-bility test is now performed by the CAx Implementor Forum, which is jointly run by PDES and ProSTEP. The vendors, says Martha Nicholson, operations manager for PDES, basically beta test the interoperability of their translators based on test cases that the CAx forum provides.
The test trials over the years, three rounds per year, have tested several aspects of AP203 conformance. The latest round focuses on surface model viewing and drawing structure, which involves exchanging the projection of solid model geometry onto planar views. PDES reports the overall results of the trials, versus specifics about particular vendors. Cautions Phil Rosche, PDES' Implementation Support Team leader, users should perform their own tests, using their own production designs, and on the STEP translators associated with the CAD systems they are using or intending to use. Rosche's own experience has been to exchange relatively small assemblies, such as a 54-part steering column, from one CAD system to another. All the pieces were exchanged perfectly, and the surface area and volume was ±0.01% and the center of gravity was "right on the money."
Last year, AIAG completed a STEP pilot program, a program requested by Chrysler, Ford, and General Motors. Called AutoSTEP, the first phase of this pilot "involved exchanging solid models because solids translators were the first implementation of STEP," says Emery Szmrecsanyi, Vehicle Product Data Project Team co-chair at the Automotive Industries Action Group (AIAG) and supervisor of Data Exchange/Model Quality/STEP at DaimlerChrysler (Auburn Hills, MI). The pilot's first phase was completed in March 1996; the second in February 1998.
When AutoSTEP began, Szmrecsanyi reports, "even primitive solid models like cubes, spheres, cones, and tori did not translate successfully between any pair of CAD systems. At the same time, it was generally accepted that the STEP standard definition for solid models, and its representation in the standard file format, were of sufficient quality to enable unambiguous exchange of solid models between disparate CAD systems."
The "most common and intractable problem causing failure" in the STEP data exchanges were the different accuracies in different CAD systems. "However, that's not a STEP problem," points out Szmrecsanyi. "The problem is not one of some CAD vendors having `good' accuracies and others having `bad' or `worse' accuracies. The fact is, different CAD systems measure accuracy to different decimal places."
DaimlerChrysler is a case in point. Szmrecsanyi's German counterparts use a tighter tolerance in CATIA than what's used here in the U.S. When the Daimler and the Chrysler sides of the company work collaboratively or exchange data, they run into problems if their respective CAD systems aren't set to the same accuracy. "And that's going native, CATIA to CATIA," says Szmrecsanyi.
Someone with a sense of humor might consider writing a direct translator from CATIA to CATIA. However, responds Szmrecsanyi, "The accuracy problem is not something that can be easily resolved by translation—direct or indirect. Accuracy is ingrained in the part. That's not an easy task to heal." CAD vendors are looking into this.
|Results of the 4 ProSTEP Benchmark (Example solid model)|
|from STEP-processor||to STEP-processor|
|Benchmark in accordance with STEP AP214/CC1, CC2|
Correct Not correct
|In testing the data exchange of assemblies, the sending systems are listed in the left most column; receiving systems are listed in the top line. The 3-sectioned circles in each spreadsheet cell shows whether the position (left section), structural retention (center section), and part naming (right section) was correct (green) or not correct (red). (Source: ProSTEP GmbH)|
ProSTEP Benchmark Tests
ProSTEP has lately been testing STEP processors conforming to AP214. The latest ProSTEP Benchmark tested the state-of-the-art as of the end of 1998. While its tests "do not represent the entire application spectrum for CAD technology," ProSTEP says that "the proportion of lost data is almost nil. Compared with the previous Benchmarks, a greater uniformity with regard to the quality can be observed. The exchange of solid data has reached a high level of reliability."
As with AIAG, ProSTEP cautions that "not only the capabilities of the receiving system determine the completeness of the transfer, but also the quality of the original model."
For exchanged assemblies, says ProSTEP, "the most important criterion—the placement of the components—functions reliably. However, the transfer of the part names needs improving. This aspect is also of particular significance to CAD/PDM interaction.
"STEP processors have reached a level of maturity which allows them to replace the VDAFS and IGES processors for the exchange of surface and solid data."