FEA and optimization leads to a more-efficient design process

Oct. 11, 2001
While designing a next-generation city bus to showcase their analysis capability, a design consulting and FEA-software company devised a more-efficient process for design-ing large assemblies.

While designing a next-generation city bus to showcase their analysis capability, a design consulting and FEA-software company devised a more-efficient process for designing large assemblies. The project at Altair Engineering, Troy, Mich., integrated CAD, CAE, industrial design, and design engineering across several disciplines. In creating the process, the project team had assistance from suppliers to the bus industry, several transit authorities, and vehicle maintenance technicians.

"A bus is a good subject because design methods and technology in the industry have not changed since the 1960s," says Jos Timmermans, bus program and engineering manager with Altair. "Using our software on a bus let us control an entire vehicle, which is an optimal test of the software."

The target for Timmermans' team was a standard 40-ft bus, one that carries about 40 passengers. "We found that 40 ft is not an engineering decision. It is the result of transportation-industry rules and regulations. We discovered that a lighter and less-expensive 37.5-ft bus could carry the same number or more passengers than incumbent 40-ft designs.

Traditional bus design starts with a required length, height, width, door locations, axles, and so on, followed by a structural design. Requirements from purchasing agencies dictate additional components from suppliers, such as seats and engines. A bus company then assembles the collection.

However, Timmermans' team began without weight or cost goals. Their focus, instead, was durability. "The highest cost for fleet operators is maintaining buses," he says. "Having a bus out of service calls for the added expense of using another vehicle to meet schedules coupled with maintenance costs on the out-of-service vehicle.

In addition, setting strict design goals has a drawback, Timmermans adds. "The bus could be built at a relatively low 14,500 lb, but this would mean using light brakes, and small tires and axles. Maintenance costs would skyrocket when light-duty components started to wear. Or, we could set a 20% weight reduction for the HVAC system, and we would have hit the target. Rather than calling out specific components or targets upfront, a global system-design approach was used. One payoff is an HVAC system that costs and weighs 50% less than those on conventional buses," he says. "A new level of packaging freedom let us create several processes that are now being patented," he said.

Timmermans' tactic was to start with the number of seats. "We packaged 44 seats in empty space, arranged the suspension, engine, HVAC, other components, and made a few iterations," he says. After adding all necessary components, the space left over was the design space for the structure.

With the components packaged and the suspension concept complete, the team used the company's MotionView and multibody dynamics simulation to tune the bus handling characteristics and to determine dynamic loading. Using this information, the team then turned to the company's topology-optimization program. This software, called OptiStruct, calculates the optimal structural layout within the allowable structural design space for a set of loads and boundary conditions. "The software provides density plot locations for structural components but without size data," says Timmermans. "The resulting structure looks organic, not intuitive at all. The task then is to transfer the locations into a CAD design for the structure."

One durability indicator is a structure's global stiffness. "A high value for the first bending and torsion frequency means the global stiffness is OK, and that global fatigue is not an issue. Experience and structural theory agree on this matter," says Timmermans.

With high values for the first bending and torsional stiffness, Timmermans contends, it is not necessary to study every connection for fatigue. However, fatigue analysis was conducted in several areas, usually where plates attached to the frame.

Benchmarking is another indicator of success. "There is a highfloor bus in Europe with the highest stiffness we know of. Its first bending and modal frequencies became one of the few goals for our low-floor design, which is generally less stiff. We achieved that goal following the first optimization study.

Timmermans says a complete battery of conventional structural analyses were performed on the bus structure, which is mostly a welded tube construction. Typically, only one tube size is selected by manufacturers for the entire structural design. But to design the most efficient structure, the team used its optimization knowledge and technology to define algorithms that would physically locate, within the global structure, optimal tube sizes from commercially available tubing.

To make the process and final design more useful, the team worked on a range of configurations that might be requested from transit authorities. The possibilities included about four different engines and five different transmissions, and hybrid drivelines that might use compressed natural gas or fuel cells. "Most of these are packaged in detail. Others were used for load studies, because we knew they did not violate the package," says Timmermans. "The structure can handle many different arrangements such as fuel tanks on the roof or doors."

Such a large number of possible arrangements can lead to datamanagement problems. Creating a BOM for each of many configurations would be impractical. "But if you don't create them in the drawings, how will you know you have the right information for sales or service?"

To solve the problem, the company's data-management team wrote a configuration builder using Metaphase, product data-management software from SDRC, Milford, Ohio. "We have one BOM, a so-called superbuild, that holds the entire structure and all permutations. Because it is one BOM, it is easy to modify.

Should someone locate an error in a part number, for example, no one has to look through a hundred lists to correct it. You fix it once in one spot. The structure also has only one CAD file, currently in SolidWorks. This lets users ask the PDM system to display, for example, a vehicle with a Detroit Diesel, an Allison transmission, and two doors. The configurator would then display one CAD file on the screen.

The bus electrical wiring system received special attention. It was designed using the company's HarnesLink. The harness software is built on a database that holds all electrical components. Each component carries information on how it connects along with a small CAD file. After a designer locates components, the software creates a wire harness. A routing tool straightens it into neat bundles and creates a wire list. Then changing the design by adding a different engine, turn signals, or driver controls takes just a couple of modifications to the database to generate a new wire harness in minutes.

"Thanks to the new process, the assembly sequence becomes irrelevant. The engine can go in first, then lights, and then the harness. It doesn't matter. That gives manufacturing a lot of freedom when components arrive early or late," adds Timmermans.

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