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CAD meets CAE

From interface to database, software suites get design and analysis speaking the same language.

By Jean Thilmany, Associate Editor of Mechanical Engineering magazine

Only two years ago, one of the biggest stumbling blocks that engineers faced in using a design and analysis software package was the lack of integration between programs. The need to translate, clean up, and further process design data for use in analysis applications limited the effectiveness of linked computer-aided design and computer-aided engineering software. With the rapid growth of 3-D CAD installations, integrating CAE capabilities has become a top priority among engineers and software developers.

Over the past few years, software vendors have been moving to tightly couple CAD and CAE software programs by tying them into suites using a shared database and a single user interface. The common database means that engineers no longer have to translate CAD programs to international graphics exchange specification (IGES) or standard for the exchange of product model data (STEP) files so they can be read by the analysis software they may be using. What's more, sharing the database allows updates in one system to be immediately reflected in the other. CAD and CAE programs that are fully integrated also share the same user interface, making it easier for a user to switch from one program to the other.

CAD/CAE software suites are making their way to the forefront of current design practices, because they contain capabilities that allow designers to build an initial prototype that has already been analyzed by the CAE system for design flaws. The engineers can identify and correct for flaws on the computer. This capability allows engineers to investigate more design possibilities than they could have if they had to build a prototype for each proposed change or export each altered design for outside analysis.

The effort to package design and analysis capabilities has been made several times. But only recently have vendors introduced products that have met with success in the marketplace, according to Bruce Jenkins, vice president of Daratech, a Cambridge, Mass., research firm. "Bringing CAD and CAE together is well under way in the solids modeling community, but the second initiative of the CAD and CAE software providers is to package modeling and analysis in ways that make them easy and safe to use by ordinary rank and file design engineers," he said.

Traditionally, analysis was done much later in the design process by specialized analysts. When analysts found flaws, they returned the design to the engineer for further refinement. Often, the analysts would build a prototype, find it wasn't successful, and the design process would begin again.

INTEGRATION WITHOUT TRANSLATION

Movement toward complete integration without translation has been made possible by what Jenkins termed "the explosion of midpriced CAD systems." As mechanical engineers turn from two-dimensional to solid modeling, they seek the ability to analyze as they design to reach a workable solution earlier in the process.

The successful products are available at costs comparable to those of midpriced CAD systems and, more importantly, require a much shorter learning curve than traditional CAE programs have needed. This ease of use comes about because of the shared interface. Because both programs run on the same desktop, the user doesn't have to move between two separate software programs. The CAE software looks and operates like the CAD software with which designers are already familiar.

Translation, which can introduce error, is no longer necessary, eliminating one step of the process and thereby simplifying design and analysis. Data, for instance, is sometimes lost in translation and has to be retrieved from the CAD system. Also, with nonintegrated systems, a designer who makes a design change in the CAD program won't find that change automatically reflected on the CAE side. In an integrated system with a shared database, any change a designer makes to the CAD model is immediately reflected in the CAE system.

"There's been quite a continuum on this integration project as vendors attack it from different ways," said Dave Williams, manager of CAE product development at Unigraphics Solutions in St. Louis. "In the '70s and '80s, CAE was a separate discipline and no one had an integrated system. CAE was done in a different system than the design work and was done much later in the process.

"One of the objectives has been to enable the analysis and simulation to be done as early as possible in the design process," he added. "Instead of CAE being the last thing you do, CAE is now one of the first things you do to make sure you have the best design possible."

Unigraphics markets its own suite of design and analysis software. Designers are able to tweak the models they create and immediately see those changes reflected in analysis.

Other software providers, such as SolidWorks in Concord, Mass., bring in outside CAE vendors for integration with CAD software. In SolidWorks' case, the CAE program Cosmos/Works from Structural Research and Analysis Corp. of Los Angeles is one of the programs used for full integration. Companies can choose to couple other CAE programs with the SolidWorks CAD software, but they will lose a level of integration.

The idea, said Scott Harris, SolidWorks' director of product engineering, is to let the user choose a level of integration. The CAD/CAE software isn't sold as a package, though Cosmos/Works offers complete integration. "Companies can use whatever software they may currently be using, or they can shop around and buy the best application to integrate with their particular needs," Harris said. "A designer of thin shells may choose a special CAE software and another designer might choose software better suited to assemblies. The idea is, there are different levels of integration."

Companies are finding that the easy-to-learn nature of these CAD/CAE programs allows them to put analysis capability in the hands of design engineers for the first time. For example, Pratt & Whitney of West Palm Beach, Fla., has trained design engineers in the use of Unigraphics Solutions' CAE program, Scenario, which is integrated with Solid Edge.

The company hopes to use the pool of analysts' time and expertise more effectively as it moves to provide design engineers across the board with an easily understood analysis tool. The engineers, who design parts for the gas turbine engines that Pratt & Whitney manufactures for use in military aircraft, are charged with working nearly all structural problems from the part as they design it.

Their designs are then sent to a specialized analysis group where advanced computer testing studies failure, vibration, and fatigue characteristics, said Dominic D'Abate, a Pratt & Whitney senior project analyst who trains design engineers to use Scenario. The analysis group formerly carried out all structural testing, including work that design engineers now handle. By giving engineers a CAE tool, the company is seeking to save analysis costs by ensuring that analysts receive designs that are as clean as possible.

"We're asking the designer to do a little more up-front analysis," D'Abate said. "Before, the design was sent to the analyst and that person looked at a cross section and said, "This won't work, this won't work, this won't be able to take the loads,' and then threw it back to the designer. Now, we're asking the designers to use this software to keep the structural side in mind to come up with something that will work before the analyst sees it."

The goal, said D'Abate, is for a design that reaches the analysts to be what he called 75 percent ready. That means, after analysis, it needs only one-quarter of the design changes a design done without the help of a CAE program would need. A second Pratt & Whitney goal is to have designers carry out up-front analysis without the time-consuming need to learn a complicated analysis program or spend time translating documents to IGES or STEP files.

The designers' use of CAD/CAE software has enabled Pratt & Whitney to shrink the number of analysts through attrition. Often, analysts who leave no longer need to be replaced, D'Abate said.

"With the reduction in workforce that all companies are going through, we've come up with a document of standard work that says what a designer's job is, and it includes this structural analysis. It's really up to them what tool they want to learn, but Scenario is straightforward and intuitive," D'Abate said.

WHAT-IF MODELING

As these systems have evolved over the past few years, they include capabilities that weren't available in earlier models. These new capabilites, such as simulation modeling, guide engineers as they move to reconfigure the part.

The integrated systems from some vendors let designers ask the software a number of what-if questions to elicit answers intended to help solve for the design flaws the CAE software has found. For instance, Parametric Technology Corp. of Waltham, Mass., offers Pro/Mechanica, which can be used with its Pro/Engineer designs to automate technical aspects of simulation. The software automatically builds a simulation based on the design geometry.

"Say you want to make a table that holds a computer," said Dayne Turbitt, Parametric's product line manager for simulation projects. "You start off by designing a table with one leg, because that saves on material. But you also knock off designs for tables with three legs and four legs. You run the analysis and find the table with one leg does as well as the table with four legs. So you start refining that design."

In this case, the engineer has specified that the table hold a computer's weight and that it use as little material as possible. Quick analysis determines that a one-legged table is a practical table. But then the designer goes further, by interrogating the system with a number of what-if scenario questions.

"Now that you have the concept of a one-legged table, you ask it, What if the table is this high, how does that affect load?" Turbitt said. "What if I make the tabletop one inch thick? What if I make it three inches thick?" The analysis program runs those numbers and is able to present the engineer with numerical comparisons. The designer may find that a three-inch table may not be necessary to hold the computer's weight.

"The engineer will have to make these choices anyway," Turbitt said. Better to model the product, test for potential flaws, and remodel on the computer than to build prototype after prototype, he added.

Simulation modeling can also shave design time. For instance, engineers at Cannondale in Bethel, Conn., a manufacturer of lightweight mountain, racing, and specialty bikes, used the integrated Pro/Engineer and Pro/Mechanica programs to shave one year off development time of its Raven frame, which was introduced two years ago and recently upgraded.

The Raven frame doesn't look like the traditional aluminum bike frame, in part because of the PTC software, said Chris Peck, frame design engineer at Cannondale. Engineers used the CAD/CAE programs to design the composite frame, which weighs only 4.7 pounds, by analyzing as they created a design unique to the bicycle industry, and meant to be both lightweight and stiff. The full-suspension frame includes a suspension fork as well as a rear swing arm, which allows both the rear and the front wheels to travel as the cyclist rides over bumps.

MORE COMPETITIVE DESIGN

As the bike industry moves from easily designed aluminum frames to the more complex structures required by mountain bikers and bike racers, design has become more competitive and engineers seek a competitive edge. Cannondale found an edge in the ability to model new frame shapes quickly without the need to build prototypes. "We're moving to more complicated structures for lighter weight and performance, and some of these are made with cast parts and composites," Peck said. "They're designs that have a high tooling expense. It costs us a great deal of money to build the tools, like fixtures, molds, and castings, that build the prototypes."

In what Peck called "the good old days," or about five years ago, bikes were made of an easily designed aluminum tubular structure familiar to nearly all as a basic bicycle. Now, Cannondale engineers build a three- dimensional CAD beam model of a new bicycle frame they seek to design. They construct this simple model, then test it to glean ballpark analysis numbers.

"Once we've done that and we're more confident, we'll model it more completely," Peck said. "In general, we probably spend at least a month and probably a little more doing analysis before we order the tooling to build the prototypes."

Cannondale engineers spent close to one year designing the Raven frame using the PTC software suite. Peck estimated that process would have taken about two years if the engineers hadn't used a packaged CAD/CAE application and instead had built multiple prototypes.

Other companies incorporate integrated software as they move to 3ĞD CAD design. Husky, the Pacific, Mo., manufacturer of gas pump nozzles and nozzle parts, incorporated the Parametric CAD/CAE package at the same time it moved from two-dimensional to 3ĞD design two years ago. Although Tom Mitchell, senior design engineer, has no calculations on time-to-market savings, he said the main benefit of the integrated software program has been the knowledge that a casting won't break as it's machined. Husky machines parts in-house.

"You've got to clamp the castings to machine it, but you don't want to ruin them," Mitchell said. "We know how much force we need to clamp it with and with the analysis we can make sure the casting doesn't deflect. We can make sure we're holding it right, and we know where it can be cut and the different ways to hold it. Prior to using the analysis, we just guessed."

Husky used to produce the casting without thorough prior analysis. "Then we'd see what happened once we machined it," Mitchell said. "If the casting broke, we had to go back and redesign it."

This article is provided by Mechanical Engineering magazine, a publication of the ASME. For information on becoming a member of the ASME, or to learn about how they can assist your career search, click on the icon above.

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