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Multifunction subsystems drive innovative thermal management solutions for military projects

Wed, 11/27/2013 - 1:02pm
Juan Zoppetti, Mechanical Engineer, Parvus Corporation

Despite current budget challenges faced by the Department of Defense, the military continues to demand more performance and improved functionality from rugged computers and communications subsystems to fulfill its mission requirements. Many military programs have begun to request subsystems that combine network processing, Ethernet LAN switching, and IP traffic routing into a single box.

Depending on the project, the demand for multifunction subsystems may be motivated by Size, Weight & Power (SWaP) constraints or objectives to simplify systems integration. The U.S. Army’s VICTORY initiative is an excellent example of this trend, as ground vehicle architects aim to trim unnecessary fat and yet leverage modern computing and networking architectures.

Some box designers have tackled the "everything and a kitchen sink" request by simply putting computing and networking boxes together in a bigger enclosure.  However, this solution may not fare well for thermal management.  Combining computing and communications functions goes beyond throwing two boxes into one bigger box - innovative designs must expertly handle increased thermal management.  Engineers must consider chassis design for heat dissipation using materials that conduct heat extremely well to dissipate heat in stacked subsystems.

The challenges of multifunction subsystems
Multifunction subsystems designed for extreme conditions such as those in manned and unmanned military vehicles experience extreme temperatures, shock and vibration, atmospheric pressure, exposure to sand, dust, fresh and sea water, and electromagnetic interference (EMI). These conditions create a unique set of design challenges.  To increase reliability they cannot use any fans or moving parts, and must also be sealed for EMI and water ingress or immersion resistance.  Dissipating heat without fans, holes in the box, or liquid cooling while keeping the computing system working reliably under extreme conditions requires innovative thermal solutions.

Size and weight are the most obvious challenges of designing a thermally sound multifunction subsystem.  When computing functions are combined into a single sealed box the heat from the most thermally-demanding components of the system, such as CPU or graphics processing unit (GPU), may affect the performance of other components that otherwise would not have any thermal problems. The reduction in space available for large internal heat sinks forces the designer to rely more and more on conduction than on internal convection to draw heat away from the system hot spots. Furthermore, as processor speeds continue to increase, power supplies need to efficiently generate enough power without adding an excessive amount of heat to the system. 

Another challenge designers face is future-proofing multifunction designs.  Since military projects often require subsystems that can support I/O expansion over time to extend the life of the project, they must be able to handle the added electrical, mechanical, and thermal pressures that may arise.  If a new module is added in the future, the system will have increased power demands, which in turn will likely create new thermal challenges.  Therefore, designers must create subsystems that are thermally sound, with the capability to manage heat should another function be added or upgraded in the future.

Chassis design
In order to manage heat, multifunction subsystems can be designed as modular interlocking chassis segments with an internal power/control bus to ease the integration of additional functions.  When additional I/O modules are added to the system assembly, the electrical connection happens automatically in conjunction with the mechanical connection.

As shown in Figure 1, the Parvus Intel Core i7-based DuraCOR 80-40 mission computer is an example of a modular, stacked subsystem designed to interconnect with the Parvus DuraMAR 5915 IP router, Ethernet switches and/or other application-specific I/O functions to enable mix-and-match possibilities for multifunction computer/router /switch appliance requirements. 

Each functional segment in this type of stacked assembly integrates an I/O interface board with a power and control bus that connects each chassis segment and gives capabilities for isolated power, Ethernet, serial, and zeroize signals to integrated expansion I/O cards anywhere in the card stack. The subsystem’s CPU board can be internally linked, for example, to an Ethernet port of a network router or Ethernet switch card in the card stack or connect the console port of these network devices.

Since designers have to account for stacking up several mechanical parts, the chassis must be designed to allow for tolerance stackup and dissipate heat very well, preferably with passive cooling devices within the system to transfer heat out of the case efficiently and reliably.  The next step is to move heat from the processor, which in the case of the Figure 1 system lies under the solid state storage devices in the stack, and needs to transfer that heat up to the finned heat sinks on the top of the chassis.

Thermal links
Commercial heat pipes may not be sufficient in a combo functionality subsystem meant for extreme conditions since they are affected by gravity, vibration, and extreme acceleration.  Innovative copper thermal links (See Figure 2) address the challenge and prove to be a more reliable method for particularly taking heat from one part of a subsystem to another when there may be a large gap in between assemblies. Typically, a very hot processor would interface up to a heat sink directly on the outside of the enclosure.  However, in a system such as shown in Figure 2, the heat from the CPU must travel around storage devices before reaching the top of the system. Thermal links enable the DuraCOR 80-40’s PCIe104 Single Board Computer (SBC) to elegantly support PCIe104 bus expansion on the bottom of the card, while integrating two removable 2.5” Solid State Disk Storage Devices (SSD) and addressing thermal interface requirements on top of the card.

 

An innovative solution for this challenge is the use of a flexible thermal link.  A flexible thermal link accounts for movement, tolerance stack-up and normal manufacturing process variation.  With a rigid mount and several parts in a multifunction subsystem, designers may have 0.10” inch of variation when all of the parts are at highest versus lowest tolerance.  In order to account for the tolerance stack-up and assembly itself, flexible thermal links ease the integration while maintaining a reliable thermal conduction path.

Materials selection
The choice and correct use of special materials is key in effective thermal management, especially when heat pipes are not used as part of a thermal management solution.  As shown in Figure 3, some materials can be several times more conductive than others.  The thermal conductivity of aluminum, used to make most traditional heat sinks, is approximately 165 w/(m-k), while copper is more than twice that thermal conductivity.  Copper is however a heavier material, so designers must take that into consideration.

Material

Conductivity

Pyrolytic Graphite

Up to 1700 w/(m-k)

Copper

388 w/(m-k)

Aluminum

167 w/(m-k)

Figure 3. Thermal management materials used in the Parvus DuraCOR 80-40 mission computer

Annealed pyrolytic graphite is more than five times more conductive than aluminum, yet is the mostly costly material of the three. Using it in a demanding multifunction subsystem design, however, can optimize the distribution of heat and prove invaluable.  By choosing materials that will withstand extremely hot and cold temperatures, designers can create a multifunction subsystem that manages heat efficiently and can withstand not only the internal thermal conditions but also the rigorous environment.

Summary
Tight spaces in military platforms such as tanks drive much of the demand for multifunction systems, as they have little room for ever-expanding computers and communications subsystems.  Legacy systems take up too much space, leaving little room for soldiers and their gear.  Modern state-of-the-art equipment offers the DoD more than legacy products—one subsystem can now integrate the capabilities of multiple boxes to satisfy SWaP constraints.

Subsystem designers must design not only for current thermal needs but also prepare the system for increased thermal management needs should military projects add more components to the system.  A well-designed enclosure can dissipate heat to accommodate some interesting combinations of computing functionality without needing forced air or liquid cooling to keep temperatures at bay.

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