The defense and aerospace rugged-systems market demands a wide range of computing capabilities at extreme environmental conditions. Across the board, applications need more processing power; inevitably, thermal management becomes more challenging as the amount of processing power grows.
Users who need custom electronics enclosures typically specify low volume runs (quantities between one and 15 units) with production delivery in approximately 12-20 weeks; this aggressive delivery schedule leaves no time for prototyping or testing. The makers of these custom enclosures rely completely on engineering experience, along with structural and thermal simulation, to meet client requirements. State-of-the-art simulation tools enable enclosure makers to iterate multiple scenarios in a short period of time; this allows optimization for not only thermal performance, but also weight reduction, noise reduction, cost, and schedule.
Often what is supplied is a metal enclosure (typically brazed aluminum), a high-speed backplane, and a power supply designed to meet specific user requirements. The designers then populate the enclosure with their own suite of electronics, or "payload."Often, however, the enclosure team is provided very little information on the design or end function of the payload. Typically, the team knows only the specifications for the subsystem's overall sizes, power levels, ambient conditions, and the temperature requirement for the electronics-card mounting. From this limited amount of system detail, the designers must craft a solution that is capable of meeting the required temperature in all environmental conditions at the given power levels.
Another challenge design teams often face is that in many cases, the enclosures are sold to the end customer by a third party, with the designed enclosure acting essentially as a component, not a product-level complete system. In these cases, the enclosure designers rarely receive feedback about results.
New designs put strain on enclosures
As applications require more processing power, the trend for new designs of military embedded systems is the transition from VME to OpenVPX standards. These new OpenVPX systems can be configured with an array of new processing elements, driving huge performance leaps relative to the VME systems of just a few years ago. With this transition comes an equally huge leap in power: Typical 6U VME systems will dissipate a maximum of 60 W per slot, whereas a typical 6U VPX system can dissipate as much as 200 W per slot.
This change is an ongoing challenge in the thermal management of enclosures. It is not enough to replace a backplane in an existing chassis and run VPX in an enclosure originally designed for VME, as the cooling system is not going to be adequate to dissipate the higher heat generated. Many systems operating VME often did not even consider thermal analysis or simulations because the power levels were so low. With the advent of VPX and multiple slots, however, the heat to be dissipated in an enclosure can easily top 1 KW. Pair this increase in power with the harsh environmental conditions these systems must operate in, and thermal management becomes one of the highest risk factors in the project. What was originally a nonissue for VME thus becomes critical to the success of VPX-based projects. Therefore, thermal simulation must be an essential part of the design process.
Many factors affect the thermal management of a ruggedized electronics enclosure, but the main factor in an air-cooled system (or a conduction payload system cooled by air) is the operating temperature. Many ground-based platforms are required to operate in extremely high-temperature conditions, as well as harsh sand and dust environments - all adding to the difficulty of thermal management. Fans must be specifically designed to meet not only the high operating temperatures but also take special precautions to deal with the sand and dust. Air-cooled systems must be filtered, with a maintenance schedule provided so that a system does not shut down from over-temperature caused by a clogged filter.
Another important consideration is operating altitude. As altitude is increased, air density decreases, with the cooling capacity of the air mover decreased by the same amount. A 50 percent reduction in air density (approximately 20k ft altitude) will result in a 50 percent decrease in cooling at the same cfm.
Because these enclosures are primarily used in military applications, the environmental conditions can vary greatly and are often extreme. New products are often retrofits for older existing equipment, and the new higher-powered enclosures must be cooled by existing platform cooling systems. Often, the new designs are required to meet specifications that were not required of the system it is replacing, and what worked in the past may or may not be sufficient for the replacement system.
The challenge for design teams, including thermal and mechanical engineers working together, is to meet all of the customer's requirements in a very short time frame. Because the ambient conditions in which the resulting system must perform are harsh and the power levels required are typically high, thermal simulation is critical.
Based on customer specifications and boundary conditions, Curtiss-Wright's Engineered Packaging Group designs and builds powered enclosures that meet the performance requirements as well as the thermal requirements. Based on the cooling system onboard the platform, the group designs enclosures that provide thermal management for air-cooled payload, air-cooled conduction payload, liquid-cooled conduction payload, as well as several hybrid designs; for example, a self-contained, liquid-cooled heat exchanger for a suite of air-cooled payload.
The first step in the overall system design process is to define the system's required functions or capabilities. The next step is to determine the system's (and platform's) physical constraints, and then, based on those constraints, select which existing modules can provide the required functions.
Frequently, the enclosure space is dictated by mechanical constraints that are not fully defined at the quoting stage. This is a result of numerous system features such as I/O connections, cabling, and air plenum allotment that may not yet be determined at the early stage of development. After the results of a preliminary thermal simulation, using Mentor Graphics FloTHERM cooling fin design (CFD) thermal simulation software, indicate that the customer requirements can be met, the mechanical design process begins.
There are often several iterations back and forth between the design-engineering and thermal-engineering teams to reach a final solution. One design aspect that demands this level of attention is CFD optimization. CFD simulation enables quick optimization for pitch, thickness, number of fins, or base thickness. Because each product is customized, there is little opportunity for design reuse. For example, the cooling wall (heatsink) geometry is designed for each particular application to ensure the best design at the lowest cost for each product. While the final solution may be similar to the starting point in the thermal design, it is never exactly the same. Although the time and cost saved by eliminating early prototypes, testing, evaluation, and redesign is hard to measure, when delivery schedules are tight, any and all time savings are crucial.
This article was written by Andrea Schott and published in Military Embedded Systems