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Miltope Corp. had only 3 months to go from design to production on its
mortar fire-control computer system for the Army. The extremely tight
schedule meant almost no time for building physical prototypes to
measure temperatures as a check on heat buildup inside the
system electronics.
The sealed, ruggedized system, known as the Commander's Interface Computer, drew 26 W
steady-state, spiking to 67 W when two removable batteries were charging. Operating ambient
temperatures ranged from -25 deg. F to +125 deg. F and the unit was simultaneously exposed
to ballistic shocks of 125 g.
Severe shock and vibration coupled with very high reliability requirements meant fans
were not a cooling option; they wouldn't withstand the environment. So Miltope enlisted Coolit
to develop a rugged, passive cooling design, while confidently eliminating extensive prototyping
of thermal solutions, thereby shaving 25% off the development cycle.
The best cooling approach appeared to be conduction cooling of the processor module
components through the chassis. Initially, only the processor and video chips were targeted
for conduction cooling. But Coolit simulations revealed that a greater number of chips than
originally thought would require heat sinking to maintain their temperatures below their maximum
specifications when exposed to 125 degrees F ambient air temperature.
Using Coolit, Miltope designed a custom aluminum heat sink and obtained answers to
what-if scenarios, including conduction sinking of different chips, benefits of thermal pads
with different thicknesses and conductivities, and the impact of various ambient temperatures.
Coolit verified that the heat sink design would maintain all components--power supply and motherboard
components--within specifications and that the touch-temperature of the chassis would remain
within acceptable limits. When actual component temperatures were later measured on
pre-production hardware, the thermal model predictions proved to be accurate to within about 5%.
Interestingly, the heat sink worked so well that engineering ran into an unexpected -25 deg F
cold startup problem. The chips would not heat up fast enough so a small heater had to be added
to the plate to speed low temperature startup.
Coolit also provided solar radiation simulations. Results indicated that the keyboard, closed over the face of the
computer, shielded the unit even when ambient temperatures rose to 125 deg. F. The Coolit design
successfully passed formal solar radiation testing.
By eliminating physical prototyping, the Coolit analyses shaved 3-4 weeks off the
12 week development cycle. Production units sailed through formal environmental qualification
testing, as well as recent early field trials aboard mortar fire vehicles near Yuma, AZ, where
the ambient air temperature outside the vehicles reached 125 F.
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