“I have not failed. I have just found 10,000 ways that won’t work,” said Thomas Edison of his attempts to invent the first incandescent light bulb.
Edison’s evident exaggeration doesn’t diminish his central point: It takes a lot of trial and error to come up with a great design, something to which BittWare, a supplier of hybrid signal processing boards, can attest.
At BittWare, the tool of choice for design iterations early in the product development cycle is CFdesign upfront CFD software from Blue Ridge Numerics. The software enables BittWare to experiment with design changes to find the right balance of power, conductivity, weight, size and ruggedness.
Early experimentation using upfront CFD played an integral role in developing BittWare’s ruggedized 3U cPCI processing board, known as the GT3U. The GT3U is the only conduction-cooled card that combines Altera Stratix II GX field-programmable gate arrays (FPGAs) with TigerSHARC digital signal processors (DSPs) from Analog Devices. ?According to BittWare, the GT3U is designed to “provide flexibility and processing power for commercial and extreme environment applications.” The first commercial shipment was made last year to Northrop Grumman for a project yet to be announced.
From the mechanical side, the principal challenge with the GT3U was cooling a 10-watt FPGA chip underneath the mezzanine of the frame, where there was only a small amount of space to provide cooling. BittWare also had to select the best thermal materials to maximize conductivity for the gap pad positioned between metal and non-metal surfaces.
“I started with a default design that represented the simplest, cheapest solution,” says Andy Buonviri, lead mechanical engineer for the GT3U. “It was obvious immediately that there wasn’t enough room underneath the mezzanine for the usual amount of metal, and that I’d have to add features for better cooling.”
Integration And Auto-Mesh
Two factors were central to maintaining the integrity of BittWare’s CFD model and speeding design iterations: integration between CFdesign and SolidWorks, the company’s 3-D CAD/CAM software, and automatic meshing.
CAD integration within CFdesign allows the software to import a SolidWorks model without any translation or data loss. Volumes, void-filling, boundary conditions and material properties are assigned automatically, and associativity of all geometry is maintained as the part or assembly moves between SolidWorks and CFdesign.
“CAD integration is a big advantage for us,” says Buonviri. “CFdesign takes the SolidWorks model natively without oversimplifying it. There’s no translation needed and we see all the elements of the real model and temperatures in 3-D.”
Automatic meshing in CFdesign eliminates the need to fine-tune the mesh to get all the elements correct, a process that in traditional CFD takes hours of time even with highly trained specialists.
Test, Then Verify
The first visualization from CFdesign quickly revealed to Buonviri where temperature rises were the worst. It was then a series of back and forth between SolidWorks and CFdesign to optimize the design. Buonviri estimates that BittWare did about 20 iterations changing the model, and another 10 or so changing material properties, adjusting power dissipation of components, making power supply changes, and working to get the best gap-pad material.?
The solution that Buonviri arrived at was to have the frame wrap around the edges of the card so it made contact with the chassis cold wall on both the top and bottom surfaces, instead of just at the top as in the default design. Because the biggest rise in temperatures occurred in the gap pad, he selected the best-performing thermal materials for the pad. The aluminum frame had high conductivity, and didn’t require any material changes.
Upfront CFD Paradox
Buonviri says that upfront CFD gives BittWare the ability to accomplish two things that seem paradoxical: do a lot more design experimentation and shorten time-to-market.
“It saves time because once you go through the iterations, you are sure the product will work as designed,” he says. “You don’t have to do all the physical testing in the late stages.”
For a company such as BittWare, complexity and design trade-offs are inevitable. In upfront CFD, the company has found an ideal tool to accommodate this reality. Complexity is reduced by easy-to-understand visualizations that show the impact of design changes on thermal efficiency. Results from design changes can be assessed quickly, eliminating the pain that used to accompany multiple iterations.
As Edison surely realized, the faster you can work through the failures, the faster you’ll see the light at the end of the project.
Bob Cramblitt is principal of Cramblitt & Company, a communications firm based in Cary, NC. He writes about technologies and processes that make major differences in the way people work, live and play.
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