Lightweight Formats for Product Model Data Exchange and Preservation

Lightweight Formats for Product Model Data Exchange and Preservation Alexander Ball (1), Lian Ding (2), Manjula Patel (1) (1) UKOLN, University of Bath Claverton Down, Bath BA2 7AY, United Kingdom EMail: [email protected] EMail: [email protected] (2) IdMRC, University of Bath Claverton Down, Bath BA2 7AY, United Kingdom EMail: [email protected] ABSTRACT The designs for engineered products are increasingly defined not by technical drawings but by three-dimensional Computer Aided Design (CAD) models. With rapid turnover of computer hardware and CAD software, these models are in danger of becoming unreadable long before their usefulness has ended. One possible approach is to migrate the models into lightweight formats that are easier to preserve and from which it will be easier to recover information in the future. Such formats also have benefits for design collaboration and dissemination of product model information. Selecting the right lightweight format to use remains a problem, but considering matters of model fidelity, metadata support, security features, file size, software support, and openness, the difference between the formats is not as significant as their common advantages over full-featured, complex models. Product model data, lightweight representations, digital curation INTRODUCTION Since the turn of the millennium, the engineering sector has been undergoing a paradigm shift in the way that products are designed and manufactured or constructed. Formerly, Computer Aided Design (CAD) tools were used simply to generate blueprints and other two dimensional technical drawings, so that the official description of the product could be set down on paper. Increasingly, though, three-dimensional (3D) CAD models are being integrated into the engineering workflow, being used as the basis of finite element analysis, stereolithographic prototyping, numerical control part programmes and product inspections, for example. Thus the 3D CAD models are taking over as the official record of a product’s design. Long-term users of engineering product data — including maintenance engineers, accident investigators and designers working on similar products — face a significant challenge due to the ephemeral nature of CAD file formats and the applications that work with them. One way of dealing with this is to migrate the CAD information as soon as possible into lightweight formats that are easier to preserve and from which it will be easier to recover information in the future. This approach may also have immediate benefits for collaboration and the dissemination of product model information. The complexity of full-featured CAD formats means that the file sizes of the models can be too large for comfortable transmission over the Internet, making distributed design work much harder. Lightweight representations, by contrast, can have much smaller file sizes. Furthermore, lightweight formats often have free viewers, enabling models to be disseminated, accessed and re-used (for example, in marketing documents) much more widely. Probably because of this, lightweight formats typically also have some intellectual property (IP) protection mechanism, whether by approximating the original model 1 or restricting access to exact model data, allowing models to be shared with partner organizations without risking IP assets. The possibilities afforded by lightweight representations are not lost on industrial technologists and software vendors, and in recent times a number of new lightweight formats have been developed, each com- peting for acceptance as a common exchange or dissemination format. These formats are not all equivalent in the information they communicate, and so the question is: which formats are most suitable for product model data exchange and preservation? DESIDERATA FOR A LIGHTWEIGHT REPRESENTATION There are a number of possible uses for lightweight representations of CAD models: communicating design information within an organization, communicating design information with partner organizations, promoting designs to customers, generating maintenance instructions, and preserving design information for future reference and re-use. Each of these uses puts a different set of demands on the format chosen to encode the representation. The most pertinent aspects of the formats to consider are: model fidelity, metadata support, security features, file size, software support, and openness. Model fidelity Modern CAD software uses a combination of different techniques for representing models [6]: boundary representations (B-Rep) that represent shapes using connecting faces, edges and vertices [2]; non-uniform rational B-splines (NURBS) or Bezier surfaces, or similar mathematical surface descriptions; and ‘features’ — generic parts (with known engineering significance) that can be adapted to fit a particular need through the specification of certain parameters, such as physical dimensions or the number of holes in the part. In scenarios involving design re-use, the ideal would be a lightweight format that could handle (a version of) all these different techniques, allowing a model to be converted into and back out of that format without changing the geometry or losing any of the engineering significance. In contrast, in use cases involving dissemination outside the organization, such fidelity would be dangerous. In such cases, it is more advan- tageous for the lightweight format to encode geometry in an approximate way, using polygon meshes or simplified surfaces for example. Metadata support Most CAD software has the capability of recording more than just the geometry of a model: materials, fin- ishes, tolerances, recommended machining techniques and so on. In use cases where such non-geometric information is important, it would be useful if a lightweight representation had a way either of embedding this information directly, or of providing links to allow the information to be stored in a separate file yet still related to the model. In certain intra-organizational cases such as in-service maintenance, it may be useful for engineers to be able to mark up a copy of the lightweight representation with annotations, and use this as a way of feeding back field experience into the design. This is something a lightweight format may be more or less amenable to, but it is more obviously a software issue than a format issue. From a preservation perspective it would be useful to record provenance information as a means of authen- ticating the model and checking its lineage. 2 Security features There are two main approaches to protecting the design data within a lightweight representation. The more conservative approach is to encrypt the model data and either remove direct access to it entirely or introduce some password mechanism to defend it. The more destructive approach is to withhold some or all of the exact design data from the representation — perhaps by using tessellated polygons instead of exact geometry, by scaling the dimensions to use an arbitrary, unknown unit of measurement, or by removing any detail not required for the purpose in hand. In cases where the exact detail of the full design is not needed — customer review, promotional materials, reference components for routine maintenance — the destructive approach is an unproblematic method of securing the IP of the design. A password-protected, exact representation would seem a sensible approach in cases where the design needs to be passed to a trusted third party, such as a regulatory body or a repository used for escrow-type deposits. The most significant drawback of the conservative approach is that it is only as secure as the encryption algorithm (or the password) used, but pragmatically the model data only needs to remain secure as long as the design data remains relevant to current design activity. It should be noted that within design teams, the need for security is less, and security measures may hamper the ability of distributed design teams to collaborate. File size The primary advantage of representations with small file sizes is that they are inherently easier to move around computer networks and transfer over the Internet. This benefits distributed design teams in particular, as it enables them to transfer their designs between sites that much quicker. Smaller file sizes also make it more practical to view the designs on smaller devices such as PDAs, which would be of particular use for maintenance engineers and inspectors. From a curatorial perspective, smaller and simpler models are likely to be preserved more easily and more successfully than full-featured, complex models. Software support One of the benefits of using lightweight formats is that it allows people in the wider enterprise to view the design without the aid of expensive CAD packages. The availability of low cost viewing and annotation software for a format makes that format more economical to implement across the enterprise. Similarly, the more software that is able to support a format, the lesser the likelihood of interoperability failures and, in the short term at least, obsolescence of the format. Openness There is no commonly accepted definition of an open format, but at the very least it means that a complete specification of the format has been published [8], and may also imply that the specification may be read and implemented at zero or nominal cost, and/or that the format is democratically controlled by a group of representatives of interested parties [5, pp. 1-3]. The more open a format is, the easier (and cheaper) it is to write

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