Transferring solid model data.
SOLID MODELS ARE defined by the information that describes them in their database and are independent of the display methods used to show them to the engineer. Since the benefits of solid modeling in mechanical engineering often are not fully manifest until the geometry is passed to downstream applications, such as finite element analysis or numerical control (NC) machining, an important aspect of a solid-based design strategy is data transfer. Currently, there are limits as to what can and cannot be done with solid model data.
Nearly every major Unix-based CAD system and many Intel- and Macintosh-based PC packages have incorporated solid modeling technology. Some vendors, such as Intergraph Corp. of Huntsville, Ala., and Dassault Systemes of Paris, have had solid modeling technology in their design systems since the mid-1980s. Parametric Technology Corp. of Waltham, Mass., inaugurated a new generation of feature-based solid modeling systems. Others, such as Autodesk Inc. of Sausalito, Calif., and Manufacturing and Consulting Services Inc. (MCS) of Scottsdale, Ariz., have adopted or developed new solid modelers in the past year.
Some solid modeling system on the market today, such as those based on the ACIS Geometric Modeler developed by Spatial Technology Inc. of Boulder, Colo., can pass the geometry element of a solid modeling file openly to other compatible systems. Other vendors, such as Matra Datavision of Tewksbury, Mass., rely on dedicated links with the applications of third-party partners to pass complete files. Standards bodies such as ACIS Open and PDES Inc., the former being a loose association of vendors and the latter an industry/government consortium, are working to improve data communication between disparate systems. However, people in the industry note that open exchange of solid modeling data has not been attained.
"It's the Gulliver syndrome; lots of little guys can block a giant," said Jean-Jacques Auffret, Dassault's director of Catia marketing and services. He indicated that consensus on solid modeling standards is thwarted by the inevitable conflicts of technology and marketing strategies among participating vendors. For this reason, engineers selecting a solid modeling system as part of an integrated CAD/CAM/CAE environment are likely to be restricted to products from their vendor and its partners for some time.
While much of the attention surrounding solid modeling systems has focused on display graphics, user interfaces, and construction techniques, the geometry itself provides engineering benefits that do not exist in wireframe and surface representations. Descriptions of solid geometry in a database contain topology data relating to features throughout the model. The topological data can convey information about the model's mass properties to an application, such as inertia to a kinematics analysis program. A solid model also contains volume data, which are required for partitioning the model into solid blocks, as happens when it is meshed in preparation for finite element analysis.
The earliest solid modeling systems for mechanical design were based on the boundary representation (B-rep) approach. This method, which arose from surface modeling techniques, defines a solid as an arrangement of surfaces. The volume data are derived by calculating the area enclosed by the model's skin. According to Dan Staples, manager for mechanical design products at Intergraph, B-rep modelers allow users to take a more free-form approach to design as surfaces are relatively easy to draw. Since surfaces and solids can coexist in a B-rep database, surfaces can be used to shape solids into complex forms.
The constructive solid geometry (CSG) approach defines solids by combining primitives with Boolean and related operations. Primitives are basic geometric shapes, such as cylinders and rectangles. These can be combined and otherwise crafted using the Boolean operations of union, intersection, and difference. More complex shapes can be defined as primitives, and additional operations that permit more refined construction techniques have been developed.
Solid modeling systems currently on the market draw on elements of one or both of these techniques. In addition, vendors tend to add value to their systems by providing features that allow users to add more descriptive data to their models than just its geometry, such as construction history and dimensions. Some systems have modeling techniques that use these data for more advanced operations, such as associative and dimension-driven modeling. Parametric Technology built its user base and its reputation--on this type of solid modeling when it introduced Pro/Engineer, its solid modeling system.
Michel Vrinat, vice president for strategic marketing and new business development at Matra Datavision in Paris, said basic solid modeling technology is just the foundation of a comprehensive system for mechanical design. Both B-rep and CSG methods are incomplete because they store only lists of mathematical definitions of surfaces or primitives and do not convey design intent very well. "By storing data on how objects were constructed in addition to the objects themselves, an engineer can communicate why a part was designed the way it was," Vrinat said. This is important if the feature is to be duplicated or modified in the future.
Matra's Euclid 3 CAD/CAM system includes a solid modeler that features a history-tree approach to storing data. Users can construct and edit solid models using B-rep and CSG operations interchangeably. The Euclid database stores the geometry and the commands that were used to create the geometry. Users may access the database and consult the history tree to determine what operations were used to create the model.
With a history tree, users can define relationships between features in the model, which is an important part of Matra's adaptive modeling technology. Adaptive modeling enables users to establish explicit parameters during any part of the design process. These parameters enforce design constraints and simplify the process of altering features because dependent features automatically adapt themselves to changes in the model.
Vendors that build customized databases on top of time-proven modeling techniques can offer users unique design capabilities. The solid modeler in Intergraph's I/EMS CAD/CAM package, for example, permits engineers to design many parts on-screen simultaneously. Thus the various components of an assembly can be portrayed in relation to one another, rather like an exploded view with each part being an individual solid model. Intergraph's database allows features on one model to be logically connected to those on another within the same file or for each model to be farmed out to individual files.
In the latest release of Anvil-5000, MCS introduced its Intelligent Modeler, replacing its OmniSolids modeler. The Anvil Intelligent Modeler is a relational editing and solid modeling system that handles wireframe, surface, and solid representations. William Betts, communications manager for MCS, said the new system was designed to permit engineers to use any geometric representation in their models. The Anvil Intelligent Modeler encapsulates any design feature from a point to a solid model within its own data structure. Data structures can be linked in any desired combination to create models with multiple geometric representations.
"Although defining precise blended surfaces and constructing models out of primitives and Boolean operations can be a lot of work, there is a tremendous amount of complexity that the user does not see," Betts said, indicating that insulating an engineer from solid modeling mathematics has not been an easy task for vendors. By contrast, wireframe storage and retrieval have become almost trivial. Surfaces, even complex ones, are mathematically infinitely thin since they are two-dimensional, and thus they are one dimension easier to represent than solids in a database. Furthermore, engineers' growing familiarity with solid modeling systems is fueling demand for more powerful editing capabilities.
"Today we observe that the market has judged that solid modeling technology has come of age and can be used by engineers as their preferred design method," said Dassault's Auffret. Operations such as fillets, draft angle specification, and extrusion of primitives have been added to Catia version 4 to give users more solid creation and editing options. Chamfer operations will be added soon. Greater levels of intelligence can be built on top of the geometry, through the database, such as tools to graphically edit the history tree. These help a designer modify existing solid models, even those designed by someone else.
Catia's associative operators are descriptions of geometry that permit that geometry to be changed in relation to changes elsewhere. For example, an associative operator could cause a fillet to be recomputed whenever one of its adjacent faces is altered. Auffret maintains that vendors who have developed their own modeling systems are best positioned to provide advanced modeling features to their customers. "When we own the system, we can agree on what needs to be done and take action to do it," he said.
A potential downside of this approach is that vendors compete to develop features and capabilities that are particular to their system. Solid modeling technology has been around long enough for there to be a considerable convergence of functionality in the marketplace, much as wireframe CAD systems tend to have a vast stable of functions in common. As solid modeling vendors define themselves by adding value and intelligence to their systems, they reduce points of commonality with other systems. This makes lateral data transfer between different solid modelers more difficult and complicates communications with downstream applications.
Some solid modeling vendors market their own downstream engineering solutions that can use CAD geometry created in their systems. Intergraph has its I/FEA finite element modeling system, I/Flow injection flow analysis system, and I/Maxmill multiaxis CAM software, all of which accept solid models created in I/EMS. Structural Dynamics Research Corp. of Milford, Ohio, has a suite of manufacturing analysis and control software that handles solid models created in its I-Deas CAD package.
Many other vendors, including those with their own alternatives, offer direct links to applications of third-party partners. Parametric Technology has established numerous relationships with vendors in the fields of analysis, manufacturing, and data management. The Pro/Ansys analysis optimization system was developed in cooperation with Swanson Analysis Systems Inc. of Houston, Pa. Many FEA vendors have adapted their systems to use the Pro/Mesh preprocessing system. Work-group Technology Corp. of Lexington, Mass., and Parametric Technology have developed the CMS/Pro product-data-management tool for Pro/Engineer, providing version control, release cycle management, and configuration management capabilities.
Whether the links are internal or external, solid modeling vendors are forced to develop formalized programming tools and application programming interfaces for their systems to be useful in an integrated CAD/CAM/CAE environment. The Catia Application Architecture specifies three levels of compliancy for downstream applications, the appropriate one depends on how closely a third-party developer followed Catia's documented software-development method.
A FIRST STEP
When selecting a solid modeling system, the user is also selecting the range of applications with which his or her CAD files will be able to work. Many vendors, companies, and government organizations are attempting to develop a common framework for handling data exchange. Unfortunately, the complexity of solid models and the features built on them inhibit the adoption of a universally accepted means of transferring solid geometry and design data.
"There really is no standard for transferring solid models as solid models," notes Betts of MCS. Current standards used in the CAD/CAM/CAE industry, such as the Initial Graphics Exchange Specification (IGES), tend to be unsatisfactory because the data that make a solid model a mathematical solid do not survive translation. IGES represents a solid model as a collection of surfaces and does not convey dimensions, design intent, or solids--defining topology.
The Standard for the Exchange of Product Model Data (STEP) is intended to provide a neutral format for exchanging solid models and other CAD data. The standard is being developed and promoted by a number of organizations, including members from the public and private sectors: the IGES/PDES Organization chaired by the National Institute of Standards and Technology in Gaithersburg, Md.; the International Organization for Standardization; the American National Standards Institute; and PDES Inc., an industry/government consortium managed by the South Carolina Research Authority in Charleston.
While many vendors with proprietary solid modeling systems look to STEP as a data-exchange solution and are active in STEP-promoting organizations, nobody expects a useful standard soon. The slow pace of STEP development often leaves the standard behind the latest developments in CAD technology. For example, there is debate within the STEP community over whether parametric modeling should be covered. Furthermore, the standards agreed upon do not always satisfy the requirements of all the participants.
"STEP geometry does not define tolerances, such as the distance at which two points are considered equal," noted Dassault's Auffret. He added that this presents the risk of inaccurate data resulting from exchanges between heterogeneous systems. For this reason, direct links between partners will continue to be an important method for transferring proprietary solid modeling data.
Of course, the problem of data transfer would be solved if everybody adopted the same solid modeling technology. This is the logic behind the ACIS system for representing wireframe, surfaces, and B-rep solid models. ACIS, developed and licensed by Spatial and promoted by ACIS Open, defines the file format that compliant systems must follow. ACIS is a means by which B-rep geometry and topology can be transferred to other applications. While ACIS Open functions as an advisory committee, developers at Spatial are responsible for releasing new versions and features of ACIS. Thus, the ACIS community enjoys a degree of central direction lacking in the STEP organizations.
Graham Rae, director of MSC/Aries product management for the Lowell, Mass.-based Aries unit of MacNeal Schwendler Corp. (MSC) of Los Angeles, likens ACIS to a geometry bus. Although primarily a B-rep modeling system, it also defines five solid primitives, swept and skinned objects, and a number of Boolean-type operations. "ACIS defines the raw constructive capability to build a part. It is up to the vendor to provide distinguishing features for individual ACIS-based systems," Rae said.
Vendors exercise great latitude in the development of their ACIS-based products. Hewlett-Packard Co. (HP) of Palo Alto, Calif., adopted ACIS as the modeling technology for its HP PE/SolidDesigner 3-D CAD system. According to Paul Hamilton, applications specialist with Hewlett-Packard's Software Business Unit in Fort Collins, Colo., HP focused on free-form B-rep modeling in PE/SolidDesigner to make the system easy to work with. There was also an effort to make the solid models generated in the system easy to transfer to other applications.
"We've implemented as pure a B-rep modeler as possible in PE/SolidDesigner," Hamilton said. Changes are made using ACIS constructions, not feature-based operations, he added. "The downstream application receives exactly what the designer sends."
Autodesk selected ACIS as the basic solid modeling technology for its AutoCAD/Designer system. However, unlike HP, Autodesk developers added layers of feature-based modeling capabilities. AutoCAD/Designer enables users to perform parametric operations on solid models. In addition, the system stores part history data and solid geometry in its database.
Since ACIS does not yet represent data other than geometry and topology, any features added to an ACIS model in one system are not carried over to another system, even if it is ACIS-based. Thus, AutoCAD/Designer cannot read features data created by other systems on files imported via ACIS. Other systems receiving solid models created in AutoCAD/Designer via ACIS will not be able to read any features data either. However, Autodesk has developed an applications programming interface that will allow AutoCAD/Designer and third-party applications to exchange data fully, much as non-ACIS solid modeling systems communicate.
According to MSC/Aries' Rae, future ACIS development is likely to focus on representing data other than B-rep geometry and topology. Developers of ACIS-based systems are also actively encouraging third-party vendors to create applications, called husks, for sale to the ACIS community, such as ACIS-compatible rendering husks or hidden-line-removal husks.
When the time comes to manufacture a design, solid models have the virtue of representing the physical object mathematically more descriptively than a wireframe model. Many manufacturing processes, such as milling, drilling, or otherwise removing material from a block, resemble the construction operations design engineers use to create the solid model. For these reasons, users of solid modeling systems may factor manufacturing constraints into their designs with greater ease than users of other types of modeling systems.
This is not to say that solid modeling systems are superior to other representations in all circumstances. For drafting and detail drawing, 2-D and 3-D wireframe construction often is more natural and precise, as it recalls manual drawing methods and can be made accurate to upwards of 16 decimal places. There are widely accepted facilities and standards for transferring wireframe data, such as DXF and IGES. Wireframe systems also tend to be substantially less expensive than solid modelers. Applications for generating complex NC toolpaths, particularly for sculpted surfaces, require very precise surface definition data, more so than is found in many solid modeling systems. It is also easier to transfer 2-D surface geometry than 3-D solids.
However, since an increasing number of engineers are perceiving benefits from solid modeling, CAD vendors are laboring to develop and refine the technology in their product offerings. "Solid modeling brings a new step toward virtual designing," said Dassault's Auffret. "As a concept, solids are closer to reality than any other representation because the geometry captures more of what makes a part real."
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|Date:||Jul 1, 1994|
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