Leslie Gordon Senior Editor
Imagine you work for a smaller manufacturer that decides to digitize its product-development process to make it more efficient. To keep costs down, the company wants to use a standard, open language that anyone can edit instead of expensive proprietary interfaces. The facility has been keeping track of products it makes with a relational database containing a lot of tables with records of each part. Records include individual fields that might hold important design and manufacturing data such as Dept., ID, Name, CAD Title, Material, and Size.
Yes, you could put the database tables into HTML for viewing the product information on a browser. The problem is, HTML basically just tells content how to display. Thus, you might get nice-looking tables, but you lose all the relationships between fields that were established and enforced by the database structure. This would hinder attempts to automate processes because computers would have no way to efficiently search for information.
But there is a way to digitize this information and maintain the relationships: use XML. It provides a highly defined way to automate the transfer of BOMs, assembly instructions, engineering definitions, manufacturing information, and so on, downstream. The use of XML is transparent to most designers. However, it can be helpful to understand how XML portrays CAD data in digital product pipelines. One interesting question that arises is exactly how granular XML gets when handling CAD files.
“CAD files process engineering information and are highly compressed, while XML is intended for exchanging text and being descriptive,” says Peter Velikin, director product marketing, PTC, Needham, Mass. “CAD data structures could be broken down into XML, but this would be inefficient. For one thing, tags would take up too much space. And because CAD files are binary, XML would force software to process them in a trial-and-error way that would be too slow. A CAD program such as Pro/Engineer, instead, directly specifies, or links to, binary-file offsets, so it quickly jumps to the byte where the next step would be.”
Velikin says a good use of XML is in the exchange of complete CAD files. For example, XML might carry structural, property, tabular, and manufacturing data about an application component (part, drawing, or assembly) as well as information such as version number, recipient, and department. A combination of XML data and tessellated geometry (for the display of 3D components) or vector data (for the display of 2D data) in a container such as a ZIP file completes the CAD-file definition.
Velikin says PTC believes strongly that XML is great at automating how design information gets represented to end users, important because CAD files alone are sometimes not sufficient to understand design intent. So, the developer’s Arbortext dynamic-publishing package lets users associate documents to product design, database, and business systems containing records for products, parts, and assemblies. Users can create or reuse existing text, illustrations, and animations derived from, and linked to, CAD files. XML ensures that documents get automatically configured and published in the right format and language, on the media of choice (Web, CD, print).
Dynamic linking to databases ensures documents automatically update when relevant data changes, says Velikin. Thus, the correct design, manufacturing, operating, and service information gets delivered to all intended recipients, regardless of geographic location or level of expertise. When it comes to manufacturing, Velikin says there is currently an initiative called STEPml, a collection of XML specifications based on product-data schemas from the widely known STEP standard. In addition to manufacturing, STEP schemas control processes such as analysis, engineering, and supply- chain management.
An advantage of using XML over STEP or IGES is it allows efficient exchange over the Web, says Velikin. “Product teams need not have a particular CAD program to view files. Another advantage is XML enforces strict adherence to the standard and so provides an efficient communication mechanism. It uses what are called document-type definitions (DTD), sort of like “blueprints” that define and impose constraints. A movement is afoot in the publishing community to standardize how companies that customize their XML implementations would refer to a common source. I’m not aware this has yet happened for data exchange, but I’m sure it eventually will.”
Most packages predate a lot of the efforts in XML, so CAD kernels are not set-up to store information that way, says John O’Connor, director of product and market strategy, Vistagy Inc., Waltham, Mass. The company provides specialized engineering software for the design and manufacture of airframes, transportation seating, and components made from composites. He says where XML shines is in letting users add more details and data to geometric models and then share this information to anyone on the team in any format.
“For example, our EnCapta program ties into existing PDA and PLM databases while working inside CAD packages such as Catia, Pro/E, and Unigraphics. Users can author applications in XML to capture information and store it in models that thereby provide a truly complete 3D product definition. The software houses this information in XML and shares it downstream in any format, or for any PLM, PDM, ERP system,” says O’Connor. “The software even lets users associate nongeometric data that’s in XML to a particular portion of geometry (for example, a point or a line) in CAD models.”
He says XML’s capabilities come in handy for composite design because a lot of nongeometric information must be captured to fully and accurately describe parts. It also supports new forms of manufacturing such as forming and stamping composites.
“A designer of military jets uses an EnCapta application to deliver model-based definitions to the shop floor that contain all data in a 3D environment,” says O’Connor. “This reduces the need for 2D drawings, while the structured nature of XML allows the automated generation of 3D supplier bid-packages. This has saved the company a lot of money.”
A GUI on top of XML
Unlike previous programs mentioned, Seemage software from Seemage, Newton, Mass., is not CAD. Basically it is a GUI on top of XML that uses existing CAD PLM data to generate 3D documents and animations for collaboration, markup, training, assembly instructions, and the like.
“Packages that don’t use XML provide proprietary interfaces for associativity,” says CEO Chris Williams. “This means companies wanting to combine data from different PDM and CAD systems and maintain associativity between them bear the burden of integration. But what makes for a difficult problem in proprietary systems becomes simple in XML-based ones.”
According to Williams, files in the software contain named objects and links to geometries, for example on a server at particular addresses. Using everything with a different geometry is just a matter of changing the link because of the open nature of XML.
“Users can get geometry and metadata separately, or merged together in what we call an SMG file,” says Williams. “The software includes converters that translate, say, Pro/E files to SMGs. These are stored in an XML fashion so users can programmatically edit them.”
He explains the software is fundamentally a property manager and includes about 500 properties including color, material, revision, version, name, position, and piece of geometry. The authoring program lets users store these properties in an XML document type definition.
“A manufacturer might use the software to generate 3D geometry characterizations for 3D search,” says Williams. “Firms often want to link to STEP files for manufacturing. Aerospace concerns such as Airbus also use a format called VML for technical illustration, which is an offshoot of XML 1.0. It defines a format for encoding vector information together with additional markup to describe how that information displays.
Comparing XML with HTML
In HTML, special code called an element comprises markup tags that contain content and tell a browser how to display it. For example, consider a database table to be put into HTML. Data about each product type (say, antennas) is contained in rows made up of individual fields such as ID, Name, Material, and Size. HTML turns the relationships between these enforced by the database structure into just raw pieces of text with formatting instructions around it. The tags <tr> </tr> and <td> </td> merely tell the content to display as columns and rows in a table:
In contrast, XML uses tags that name the content they display. They use the same field names as found in the database and thus keeps the relational structure intact:
A PDF of a digital plybook page used in the manufacture of a composite aircraft part shows design details including ply number, material, and orientation. The details were stored in Vistagy’s FiberSIM software, which automated their export into the PDF document via XML. The language also allowed sharing the associated CAD model to improve engineering collaboration and reduce errors in manufacturing.
Seemage Publisher uses XML to manage all information about the assembly.
A “dynamic” document created with PTCs Arbortext XML authoring and publishing software links to a database or business system containing the records for the product, part, or assembly. Documents thus update every time design data, manufacturing data, or CAD files are changed.