You’ve probably heard many news stories about the harsh conditions soldiers endure during combat, but have you ever considered the effect a war environment has on sensitive electronic weapons?
This was top of mind in 2002 for the U.S. Department of Defense. Seeking to improve and better shield the Paladin self-propelled Howitzer, U.S. Congress ordered a major upgrade to the entire Howitzer with a special focus on the Paladin Digital Computer Unit (PDCU). The PDCU is an on-board computer that receives input from sensors and accurately directs a projectile as far as 25 miles.
Reliability and accuracy is mission critical. Based on the requirements from the Department of Defense, Sechan determined that it would need to build a computer nearing six times the current processor speed, requiring flawless operation for at least five years. Not too challenging by today’s standards, except that this computer needed to work in a sealed space in order to withstand hot water wash-downs (to counteract chemical exposure) and salt fog, as well as holding up to shock generated by gunfire, and the incredible vibration produced by a tracked vehicle.
Where to start? Extreme temperature introduces the best chance for system failures. In fact, the U.S. Air Force has determined that more than 50 percent of all electronic failures are attributed to shortcomings in temperature control. They conclude that there is reliability degradation of approximately 10 percent for every two degrees Celsius rise in temperature in silicon-based devices.
The old fashioned way. Detailed classical analysis (hand calculations based on simplified planar and rectangular geometry) indicated that 38 percent of the heat transfer from the chassis would be due to natural convection mode and approximately 62 percent would be due to the radiation mode. The calculations suggested that the chassis external temperatures would be in the range of 79 degrees Celsius, 19 degrees above the maximum ambient. Required operating temperature ranged from minus 45 degrees Celsius to 60 degrees Celsius. With this information, Smith was ready to begin work with an analysis package.
CFdesign is the winner. In selecting a CFD analysis package, Smith was firm on his requirements – the package must work directly with the Pro/ENGINEER models, eliminating the need for re-modeling, and must provide a high level of analysis with reliable accuracy (within a couple of degrees Celsius). Finally, the package should be user friendly enough for intermittent use without the need for refresher training.
That’s when Smith was introduced to CFdesign Upfront software from Blue Ridge Numerics.
“CFdesign runs directly from within the model software and features a parametric relationship with component geometry and assemblies built in Pro/ENGINEER,” explains Jim Spann, vice president of marketing at Blue Ridge Numerics. “It automatically applies the optimal mesh required for accurate simulations, which is exactly what Sechan was looking for.”
Smith was impressed with what he saw. “With CFdesign, I was able to see the total product flow and thermal picture. There's no way data from a collection of thermocouples could have given us the same level of knowledge and confidence about our design,” he says. “The CFdesign tool was utilized extensively to optimize the thermal performance of internal heat transfer components and assemblies.”
The assurance of empirical verification. Upon completion of the initial physical units, Smith conducted a thermal test involving fifteen thermocouples in an environmental chamber. “Verification of the CFD simulation demonstrated the accuracy of the model with close correlation of temperature data," Smith reports. For example, one test location indicated a stabilized temperature of 67.3 degrees Celsius at 60 degrees Celsius ambient, while the CFD analysis indicated a temperature of 67.7 degrees Celsius – a difference of only 5.2 percent of the thermal gradient. Other test locations exhibited similar variances between the simulation and test data of 3% to 8%.
Passing of the qualification test. The final step for new military equipment is qualification. “Due to the accuracy of the digital prototypes designed in Pro/ENGINEER and CFdesign, we were able to use the first physical parts for qualification testing,” Smith concludes.
In the end, the pairing of Pro/ENGINEER and CFdesign gave Sechan the opportunity to meet all of the requirements specified by the Department of Defense.
Approximately nine months after passing qualification testing, Sechan Electronics began full production of the PDCU, ultimately putting reliable equipment in the hands of the U.S. Armed Forces.
Sechan Electronics, Inc., based out of Lititz, PA, designs and manufacturers military electronic systems for the Department of Defense, handling everything from circuitry, packaging and thermal analysis to documentation, qualification, and production support.
Blue Ridge Numerics, Inc., based in Charlottesville, VA, specializes in fluid flow, heat transfer, and electronics cooling simulation tools for MCAD-centric product design.
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