Options for helping CAD systems communicate

DN Staff

March 20, 2000

15 Min Read
Options for helping CAD systems communicate

One billion dollars-that's the yearly cost interoperability problems exact from the US automotive industry. So estimates a recent study commissioned by the U.S. Department of Commerce's National Institute of Standards and Technology (NIST). The study cites one U.S. manufacturer that claims as many as 435,000 product data exchanges each year within its company, and among its company and suppliers. Another calculates the electronic exchange of CAD data alone occurs at least 7,000 times per month, and can reach 16,000 per month during peak operation.

As complex industrial products are increasingly designed using collaborative methods, and as designers and design teams become spread throughout an extended enterprise, the need for better CAD interoperability is a critical issue. Translating CAD files from one application to another, however, remains a difficult problem.

There are currently a variety of modeling techniques used in CAD systems: CSG (Constructive Solids Geometry) or B-Rep (boundary representation), surface modeling techniques, and newer "feature-oriented" modelers to describe parts. To optimize design efficiency, some broad-spectrum applications will offer several modeling modes (parametric or explicit, surface or solids, exact or "mock-up," etc.) Systems can differ in terms of functionality, they may address different applications, (mechanical, electrical, etc.), and may be either 2D or 3D. Today's 3D modeling technology carries both geometry and topology information, since topology is required to maintain the completeness and integrity in a solid model.

To complicate things further, many earlier CAD systems represented objects as wireframe, so the surface geometry is incomplete, or the topology is missing. Unless a converter/translator can reconstruct accurate geometry or recreate topology from the geometry, called healing, these model transfers will fail. Efficient translation of these files requires the checking and possibly the reconstruction of the topological relations.

Most CAD applications will claim to be open, but to what degree and in which direction? The "big four," (Dassault, PTC, SDRC, and Unigraphics Solutions (UGS)), allow varying levels of accessibility to the internal workings of their applications, with only one, UGS, having licensed access to its kernel, Parasolid. SDRC has made a step toward better interoperability by allowing developers to have complete access to the I-DEAS database. While this doesn't give access to the inner workings of SDRC's proprietary kernel, it does make it an easier job to develop interfaces. The open access to the database also means that HTML files can be automatically generated for visualization.

Other CAD companies, such as think3 and CADKEY, have made major investments into their ability to accept and heal files from other CAD systems, making them more "open" to accepting, but not necessarily to conversion to other formats. In another example, Microcadam uses the Designbase kernel licensed from Ricoh Co., and has focused on converting the 2D AutoCAD user to 3D, with Helix Capture, which can automatically create solid models in Helix modeling from 2D AutoCAD drawings, including corrections.

Making the conversion

Due to the widely varying types of CAD systems, the conversion of a file from one system to another can mean approximations and algorithms that imply a reduction of accuracy and sometimes data losses. In the field, users often pick translators based on the predominant geometry created.

"It's a judgment call that needs a lot of research and expert interpretation," says Ken Versprille, program manager and senior analyst for MCAE, CAD/CAM at the consulting group D.H. Brown Assoc. Inc. (Port Chester, New York). "We could stand here and theoretically define the best conversion mapping from product A to product B. Of the ten vendors that might produce this software, however, some will do a very good job, and some a poor job-it's all in the implementation of the code."

The ease of surface model modification is very much dependent on the software capabilities. "The toughest thing is maintaining geometric relationships (continuity, tangency, curvature) between adjacent surfaces," says Marc Evrard, an independent consultant based in Paris. "Once the file is imported, the system may only permit local modifications on single surfaces, while others allow for global modification on a set of surfaces. There are also wide differences in accuracy between systems, so when converting from one system to the other, two adjacent surfaces that are "sewed" together can become "unsewed," and further operations may crash. This is why the "healing" function of data conversion systems is so important."

For solids, the transfer can turn "intelligent" geometry (a form that uses either CSG or parametrics to describe the process of creating the form) into "dumb" geometry, (no history or parametrics, just B-Rep geometry). "File transfer using the standards (IGES, STEP) cannot maintain either the parametrics or the history (the CSG) of the geometry," says Evrard. "As a consequence, most solids that are transferred into parametric systems will be very difficult to modify, because the parametric definition of the design is not included in the transfer."

Certain editors have developed direct converters from one system to another with the history, usually for the bigger systems on the market. For example, SDRC has a direct converter from CATIA to I-DEAS and PTC has a direct translator for Pro/E/CATIA, and Pro/E/CADDS5. While these converters are only useful for very specific situations, the fact that they include the construction history means that modifications to the geometry should be much easier.

If only B-Rep geometry is transferred, modifications to existing geometry are difficult, since only some limited additions/subtractions can be made, (i.e. one can't modify the dimension of the hole, but one can add a new hole). Some CAD systems, such as think3, allow you to import B-Rep geometry as a collection of B-Rep geometry "entities," which does facilitate the modification process. The ability to change the file from a solid to a surface and back will also open more possibilities for modification.

  • Data Exchange costs the U.S. automotive industry over $1 billion/year.

  • An OEM estimates the company performs 435,000 product data exchanges/year.

  • Another OEM estimates electronic exchanges of CAD data occurs 7,000 times/month with peak periods reaching 16,000/month.

  • As much as 70% of the time spent during Finite Element Analysis is for reworking or recreating CAD models; downstream applications such as rapid prototyping demand as much as 20-50% of the time for reworking CAD files (from Datamation, vol. 7 no. 6).

Turning to the specialists

Third party translators (i.e. by companies specialized in translation applications), can be quite mixed. Some applications are in fact IGES translators with added healing, but others will include a proprietary conversion engine for more advanced translation. These applications are often designed without the CAD vendors APIs (application protocol interfaces) to avoid the necessity of a CAD system license to make the transfer. Developing a converter without using the API is more difficult, but is a significant savings to the end-user.

For example, Coretech International's Trans 3D uses its own geometric modeling conversion engine that has dedicated surface geometry recognition and topology reconstruction algorithms for healing. Founded by one of the creators of Matra Datavision's Euclid software, Coretech provides a range of direct converters built on top of this conversion engine. Trans 3D also offers an add-on module called Trans-CSG, which can convert both geometry and history from Euclid to CATIA (Dassault Systemes), and the company's R&D team is working on broadening this capability towards feature recognition.

Another leading translation software company, Theorem Solutions, has developed a set of Generic Cad Objects, (G.C.O.) for its direct converters and also offers STEP-based converters. The company's product line includes CADhealer to fix problems.

In an original bid to move into translation specialization, Spatial Inc. (the license holder of the ACIS kernel), has launched Web-based translation and model-healing services. Spatial's ACIS 3D Toolkit is one of two standard modeling kernels, used in over 200 CAD applications. Translation services from 3Dmodelserver.com allow end users to upload any IGES or ACIS-based file to translate and heal the model into IGES or ACIS file format.

Launched at the end of 1999, the company is still developing interfaces for other CAD systems, as well as STEP. Spatial is offering a money-back guarantee, (the user only pays for the translated files that are accepted), and users are charged per healed megabyte on the file. Spatial estimates that 3Dmodelserver.com will cost users 1/10 or less of the cost to manually fix 3D models by rebuilding the models in their current CAD system, the most common method.

No simple answer

In the end, choosing the best conversion solution comes down to evaluating the specific situation, with sometimes surprising solutions. "Even different groups within the same company will decide on different converters to have a solution that is best for their kind of parts," says Versprille. "When we asked automotive suppliers what approach they had in selecting converters, everyone said, 'we use whatever works best-and there is no rhyme or reason to it.' One supplier found the best translation from Unigraphics to IDEAS for his types of parts was to convert Unigraphics to Pro/E, and the Pro/E to I-DEAS-they used TWO converters!"

"There's also knowledge to be transferred-that's the 21st century chal- lenge," says Ram D. Sriram, NIST, U.S. Dept. of Commerce, Technology Administration. "The 21st century will have increasingly knowledge-based CAD systems. With Web-enabled design and ordering that includes just-in-time inventory, interoperability is a necessary requirement to be able to deliver quality products in a cost effective and timely manner. Standards are a key to achieving this interoperability."

Translation pros and cons

Translation method

Direct proprietary translators: Either from the CAD developers themselves, or from companies specialized in translation software, these programs are designed to move files from one specific system to another.

Neutral file format (STEP, IGES): CAD vendors have developed standard interface modules that suit their own native format, which is why there are a number of different "flavors" of IGES, (CATIA-IGES, CADDS-IGES, etc.). They are in fact "half-translators", meaning that transfer to a neutral file format is only half the translation.

Standard kernel modelers (ACIS from Spatial Technology and Parasolid from Unigraphics).

APIs (Application Programming interfaces): these are really targeted at translation application vendors, or end-users that have proprietary applications that need to be converted.

Visualization tools: Either Web-based or not, these tools use the tessellated data available in all CAD programs for neutral format visualization.

The visionaries choose visualization

Ken Versprille recently presented a seminar on possibilities of using visualization software to alleviate some of the translation headaches. He spoke with Global Design News on some issues to consider:

"Faced with a constant increase in the number of file conversions that are necessary in today's industry, end-users need to really look at why they are transferring the data. Many people think that getting all the data all the time is more "comfortable", but if it's never used, it is a waste of time, effort, and money. If their use of the file is limited to a certain set of functions that don't include editing the geometry, (such as mark-up and interference checking), then these users don't need all the exact geometry, and can instead use tessellated data in a visualization application.

"Every CAD program already has a tessellation algorithm because tessellation is used for shading. By outputting this data, it's very easy to create assemblies by merging triangles from all the different vendors into a visualization product. Suddenly you have en-visualization-the ability to merge an assembly with components coming from all different CAD systems. Yes, it's true that each of the visualization vendors today has a proprietary database for this tessellated data, but I think in the future we will see standards for this.

"Can we really host all these applications on top of tessellated data? Well, not all of them, but I know we can do quite a bit. We used to think that you absolutely needed exact data for NC machining, and today, virtually everybody's now doing it with tessellated data. This dispels the myth for me. If you can have very good NC machining software based on tessellated data, this data is finely granular enough to describe shapes.

"We recommend that users look at their downstream usage and map those needs to the methods that best suit it. Don't exchange data you don't need! In today's technology, and in the foreseeable future, if you are really going to be editing and creating geometry downstream, then you should really just avoid data exchange. If you can't use one CAD system but need to design in the context of another component in an assembly or reference certain geometry of the other component, you will need to face conversion. If you are doing data inspection, however, start looking now at visualization technology because there are very interesting developments in this area. We're seeing a real ground-swell, and users should put themselves in a position to take advantage of that. In the end, it's a choice between conversion with its inherent problems, or visualization with its inherent limitations-but keep an eye out because things are going to change."

Risks of one-system demands

In an effort to avoid interoperability problems, some larger manufacturers insist that their suppliers use the same system they use. This sounds like a simple solution, but most suppliers work with a number of different companies, each of whom requires a different CAD system. Maintaining multiple systems creates significant extra cost for suppliers, and weakens their ability to respond to manufacturers' demands. Here are some problems:

  • Less than optimal use of the systems in place, since they are not used all the time, (and sometimes only used to transmit data to a specific customer)

  • Decreased proficiency of CAD users in each of the multiple systems maintained and a resulting decrease in the flexibility in the use of the engineering staff

  • Increased system maintenance and staff training costs

Even when a single CAD system is used, data exchange can be problematic, as when there are different versions of the system being used, or when the original CAD file was not constructed properly for all the downstream functions, such as rapid prototyping, finite element analysis or CNC programming.

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