Case study: Rethinking rugged subsystem computing design to accommodate military budget cuts
More now than ever, rugged computers must meet a myriad of DoD demands to provide more power and flexibility – all on a tight budget. These requirements have pushed designers of rugged military computers to rethink the traditional rugged subsystem design. The following case study examines how engineers recently designed a rugged computing subsystem to check every box on the military's growing list of requirements.
Recent news of DoD budget cuts has greatly impacted the military electronics industry as designers of rugged computing systems must deliver solutions that can not only survive the battlefield, but the DoD’s pocketbook. Leaders of the U.S. Department of Defense (DoD) propose spending approximately 10 percent less from current-year budgets, which were already further reduced the previous year. As a result, the U.S. government is placing greater emphasis on standard products (Commercial Off-the-Shelf) and tightening budgets for custom Non-Recurring Engineering (NRE).
Consequently, rugged COTS preintegrated computing subsystems are starting to replace what have been traditionally custom MIL-SPEC systems. However, many military programs are requiring higher-performance processing and memory architectures such as those offered by Intel’s Core i7 processor. In addition, military customers are seeking greater support for high-bandwidth data buses (for example, PCI Express, Ethernet, and so on) and modular I/O expansion, plus scalable integration of subsystem functionality – eliminating the stand-alone “bolt-on” system integration paradigm that has added unnecessary weight, complexity, heat, volume, and power draw to vehicle platforms. C4ISR/EW initiatives are particularly interested in interoperable networked-based solutions where mission computer, IP routers, and switches – along with other application functions – can be elegantly integrated and optimized for Size, Weight, and Power (SWaP).
The following case study reveals how Parvus engineers overcame the design challenges involved in developing a rugged computing subsystem for unmanned and manned aircraft, ground vehicles, and maritime platforms to meet the DoD’s requirements for a flexible, cost-effective computing solution.
More power, more heat, more solutions
In light of the aforestated paradigm, engineers recently set out to design a rugged computing subsystem, combining the in-demand Intel Core i7’s powerful graphics and multicore processing capabilities with ultra-reliable mechanical robustness for extreme environmental conditions per MIL-STD-810G (thermal, shock, vibration, exposure to dust, water, and humidity) and EMI conditions per MIL-STD-461F. However, as the unit’s Core i7-2655LE-based PCIe104 Single Board Computer (SBC) can generate as much as 45 watts of power in its embedded 2.2 GHz dual-core variant, managing the thermal dissipation without increasing the size of the enclosure presented the largest challenge when designing this system.
In rugged designs, thermal dissipation is heavily dictated by the surface area of a chassis; therefore, the use of that surface area becomes very important when considering thermal management. When designing this system, there were several Integrated Circuits (ICs) in the system that were reaching over 100 °C. After some analysis and imaging with a thermal camera, engineers made a connection from these hot spots to the chassis wall using an aluminum heat sink at strategic locations of the chassis that were 2 to 3 degrees cooler (Figure 1). With this change, engineers saw the temperature come down on the ICs by over 10 °C, greatly reducing the risk of overheating the components. Although the external chassis only had a 2 to 3 degree temperature change, engineers were able to channel this temperature difference from the external surface of the chassis to dramatically lower the temperature of the ICs on the electronics.
Another IC in the system had a thermal protection mechanism that would turn the device off when it reached 100 °C. This meant that in a 71 °C ambient environment – the upper operating temperature of the system – engineers needed to keep this IC’s heat sink below 100 °C for the device to continue full operation. Engineers improved the conduction path from the IC’s heat sink to the system chassis using additional aluminum heat sinks. This produced similar results and as with the other components, lowered the temperature by approximately 10 °C for the IC, with minimal chassis temperature changes.
Although both of the examples illustrated here were solved with simple aluminum heat sinks, other materials were used in the system to solve more complex thermal management issues. For example, combination copper and aluminum heat sinks, where copper is press fit into aluminum parts, effectively doubled the subsystem’s transfer rates. (Aluminum has a thermal conductivity of approximately 170 W/mK, and copper has a thermal conductivity of approximately 390 W/mK.) In other areas of the design, pyrolytic graphite, with a thermal conductivity as high as 1,700 W/mK, was used to efficiently distribute the heat across the surface area of the chassis.
Even with all these specialized internal heat sinks, engineers needed to rework the external surfaces of the chassis (Figure 2). To improve the chassis, Parvus engineers designed nontraditional aluminum grooves on the exterior of the chassis, which not only minimized weight, but significantly increased the amount of surface area while reducing the vulnerability of more traditional fins. In comparison, most other chassis are designed with large fins that are mechanically weak and prone to bending and breaking. By increasing the surface area in this way, the chassis allowed for a thicker rib that not only provided increased surface area for heat dissipation, but also improved structural integrity.
The thermal design solutions implemented in this i7-based system have proven very effective. In past rugged subsystem designs, the internal air temperatures reached or exceeded 15 °C to 25 °C above the ambient external temperature. However, with these latest thermal management solutions in place, engineers have cut those numbers almost in half with a rise in internal air temperature of only 10 °C to 15 °C above the external ambient temperatures.
Modularity a must for cost savings
To make rugged systems viable for today’s military requirements, system architectures must be scalable and flexible to change with the military’s varying demands. To create a modular rugged computing subsystem, engineers needed to design a system that could support a stackable PCI Express bus architecture for I/O card expansion and a modular mechanical design that could grow based on I/O requirements. Offering a computing subsystem that allows this flexibility extends the life and usability of a system – a must for budget-conscious DoD programs.
One of the significant engineering challenges in building a modular card stack was the large number of signals – about 160 – that needed to be interconnected for external use in less than 1" of space, and designed in a way that is still easy to manufacture. To accomplish this task, board-to-board interconnections were used to eliminate bulky cable-runs as 160 wires would be nearly impossible to fit in such a small space (approximately 5"L x 2"W x 1"H). Another large challenge was the interconnection between chassis modules. With other modular systems, the sections are secured with a single bolt running the length of the system at each corner. This causes some problems with structural integrity of the system, as well as difficulties in sealing the unit against ingress protection.
To maintain structural integrity, as well as ingress protection, engineers included strengthening ribs on the enclosure. The ingress protection was provided with a proprietary o-ring groove. A rib was included on each interconnecting module, which upon mating, was inserted into the o-ring groove. This increases the pressure on the gasket enough to allow only four bolts to be used at each interconnection point, rather than having bolts every few inches as is common in other designs.
From the electrical side, the intermodule communication and power interfaces use board-to-board connections to improve structural integrity in heavy vibration and shock environments, improve reliability by eliminating cable failures, and to allow ease of manufacturing in such tight spaces. Dual 2.5" form factor Serial ATA (SATA) flash disks slide in and out on mounting trays behind a sealed, hinged door on the rear panel (Figure 3). Engineers also created the electrical interconnects with PCIe-like connectors and connectors with tight pitch and forgiving alignment tolerances. Selecting these specific parts allowed for normal machining of mechanical parts and standard PCB tolerances to control costs.
With this building-block design, high-wattage and low-wattage cards could be more easily integrated in the DuraCOR 80-40’s card stack. Typically with many rugged computing systems, each PC104 I/O card should not exceed 8 watts or risk overheating. This wattage limitation created problems as many military applications require high-wattage cards. By creating a flexible card stack, higher-wattage cards can now be attached to heat sinks to allow operation in hot environments or high-altitude situations where there is less air to dissipate the heat. The flexibility of this card stack design allows the military to adapt the rugged computing system to meet changing requirements.
Rugged computing meets future challenges
Rugged electronic design for the military is faced with more challenges today than ever before. Tasked with creating computing systems that not only have to endure the world’s harshest environments, today’s military electronics manufacturers must also deliver products that will endure military requirements for years to come. By employing a variety of techniques to improve thermal performance to accommodate increased processing power and improve structural integrity for a modular subsystem, engineers are proving that their designs are equipped to improve battlefield success while keeping budgets in check.
Parvus Corporation 801-483-1533 www.parvus.com