Not all military and avionic electronics need to be ruggedized, but unprotected systems would last only minutes in many of these environments. Thus, solutions such as rugged cases, conduction cooling, and highreliability connectors become mandatory design elements.
Elma’s air transport rack (ATR) 6U VPX is a good example of the starting point for military rugged designs (Fig. 1). The conductioncooled case has machined walls that act as integrated card guides. Its wedge locks transfer heat from the cards to the walls of the case. Designed for environments with temperature ranges of –40°C to 70°C, it meets a host of MIL-STD and ARINC specifications as well.
Another sealed ATR system, Mercury Computer Systems’ PowerBlock 50, is available with Intel and PowerPC processors and Xilinx FPGAs (Fig. 2). It supports x4 PCI Express switch fabric and can incorporate an internal hard or solid-state disk. Liquid cooling is an option.
The VPX and PowerBlock 50 highlight two key issues surrounding rugged designs—military, avionics, or otherwise. The first is environmental isolation, keeping sand and surf away from the electrical components. This is usually the easier of the two issues. The second is heat dissipation, which tends to involve heat redistribution away from the source (typically the electronics or its power source).
For example, the airframe houses the electronics in unmanned aerial vehicles (UAVs) like the Predator from General Atomics Aeronautical Systems, but containing heat only makes the temperature rise. Conduction systems provide one way to move heat to an area where it can be dissipated. However, this approach often requires heat movement to different places around an aircraft.
Liquid cooling is one alternative to conduction cooling that promises high heat dissipation, providing a mechanism to move heat via a fluid to almost any point within a system. The challenge with liquid cooling is the infrastructure required. Normally, the cooling liquid is enclosed, and specially designed heatsinks transfer heat from hotspots such as the processor or GPUs to the cooling liquid. Also, pumps are required to move the liquid.
SPRAY COOLING One alternative to liquid cooling is spray cooling from Spray- Cool, which uses liquid to cool a system without an enclosed fluid system like conventional water cooling. Instead, it uses a non-conductive dielectric liquid that’s distributed using a misting system. The mist condenses into a liquid and is drawn out using a series of drains. The system then acts like a conventional liquid cooling system with a pump that moves the liquid to a heat exchanger.SprayCool offers a number of standard rack systems (Fig. 3). The company also can apply the technology to custom systems. The approach can be applied to an entire system with multiple boards. It also can be targeted like a conventional liquid cooling system, in which case it typically cools hot chips such as the main processor or a GPU.
Part of a design includes the spray system and the intakes for the condensed liquid. The number of intakes and their position depend on the operating environment. For example, an aircraft system where inverted operation is a possibility would normally have intakes all around the case interior, whereas a system that would be fixed could get by with a couple of intakes on the bottom where liquid would collect.
The SprayCool system can operate from –65°C to 71°C and altitudes up to 70,000 feet. As with other liquid-cooled systems, it can reduce overall weight by providing a more efficient cooling system. It’s a sealed system that can provide protection in hazardous environments where sand and water are the enemy. Finally, liquid cooling systems can handle significantly more heat dissipation than convection or conduction systems.
One advantage of the SprayCool approach versus conventional liquid cooling is that SprayCool can handle most hardware without modification. There’s no need for conformal coating or other special protection for the boards. SprayCool systems are finding homes in UAVs such as the Predator and the Northrop Grumman Global Hawk.
MORE RUGGED TECHNOLOGIES Sealing and ruggedizing other system components is equally important. It doesn’t do much good if the electronics continue to run if the I/O systems no longer work. One example of a ruggedized display system is Eurotech’s 6.5-in. thin-film-transistor (TFT) LCD DuraVIS 4300 multifunction display (MFD) subsystem (Fig. 4).The VGA-resolution, LED backlit display is readable in sunlight and can be dimmed down to 4%. It has 18 backlit keys. The MIL-STD-810F-qualified 4300 also has room for a pair of PC/104+ boards and comes with a 1-GHz Celeron M-based motherboard that’s preloaded with Linux or Windows XP Embedded.
It’s essentially a complete ruggedized PC, including the display. Another trend is the move toward solid-state storage, such as Elma’s ACT/Technico flash-memory storage. Its SecurStor and RAIDStor come with magnetic rotating disks or flash drives (see “ACT/Technico Sports CompactPCI And PMC Disk Storage”). Flash is more expensive, but it has no moving parts.
BACKPLANES FOR RUGGED SYSTEMSThough many rugged systems are highly customized, they’re often based on standards such as VME. These backplane systems, which usually come in 3U and 6U form factors, are used with a range of approaches from convection cooling to spray cooling. Typically, more than half of the backplanes used in these commercial off-the-shelf (COTS) environments are customized versions of standard products. This is primarily due to the custom I/O on the backplane. These systems also usually have custom I/O out front as well.
VME is out in front with products like Kontron’s VM6250 multiprocessor board, which incorporates dual- or single-core Freescale MPC8640/8641 processors with Altivec vector processing support (Fig. 5). This 6U VME VITA 57-compliant board, which only uses 27 W, is available in convection- and conductioncooled versions. It supports the faster 320-Mbyte/s VME 2eSST bus speeds and has a pair of mezzanine sockets for PMC or highspeed serial XMC cards. It also features an FMC slot.
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CompactPCI is a competing board standard. Adlink’s 6U cPCI- 6880 blade sports a 2.53-GHz T9400 Core2 Duo processor (Fig. 6). It’s optionally available with an extended operating temperature range of –20°C to 70°C, and it typically only uses 40 W.
Also, CompactPCI can use multiple high-density pin and socket connectors like VME. A 3U board has a single 220-pin connector to support a 64-bit bus. The 6U has an additional connector. A smaller, 32-bit bus version uses a 110-pin connector. As with VME, it uses a parallel bus. In this case, it’s PCI.
VME- and CompactPCI-bus-oriented systems have been used in thousands of projects and will likely continue even with new emerging standards. Part of the reason is that many projects don’t even push the VME or CompactPCI performance envelope.
The PCI Industrial Computer Manufacturers Group (PICMG) is the standards source for CompactPCI. VITA is the source for VME and a range of other board standards, such as VPX and VXS. It hosts standards such as VME, XMC VITA 42, and FMC VITA 57 as well.
VPX is the high-speed serial alternative to VME. It retains the ruggedized standards associated with VME, but uses point-topoint interfaces such as PCI Express and Serial RapidIO to link boards. For example, Curtiss-Wright Embedded Computing’s VPX6-187 6U VPX-based single-board computer hosts an eightcore 1.5-GHz Freescale QorIQ P4080 chip (Fig. 7). The six serial ports can be PCI Express or Serial RapidIO. The board features a PMC/XMC slot and another XMC slot.
VPX’s seven-row high-speed connectors handle up to 6.25 Gbits/s. The 112-pin P1 connector targets the serial fabrics, while the P2 and optional P3 through P6 112-pin connectors can be used for I/O. In the connectors’ “chiclet” style design, the contacts on the connector are actually small circuit boards mounted at right angles to the main board, versus VME’s pin-grid approach.
The VXS VITA 41 blends the VPX high-speed serial connections with the VME parallel bus. The Pentek 4207 supports a dual-core MPC8641D PowerPC processor with Altivec vector processing support (Fig. 8). It can also be equipped with a Xilinx Vertex-4 FX FPGA. The 6U board has a pair of PMC/XMC sites and a fabric-transparent gigabit high-speed serial crossbar switch. VXS can bridge VME to a range of high-speed serial standards from InfiniBand to Aurora, including PCI Express, 10 Gigabit Ethernet, and Serial RapidIO.
VITA doesn’t have an edge with high-speed serial. Compact- PCI Express is an alternative to CompactPCI. Among other factors, the PICMG 2.16 version of CompactPCI slowed the adoption of CompactPCI Express. It added a Gigabit Ethernet switch fabric to the backplane along with network support while retaining the PCI interface for peripheral access.
MODULES AND STACKS Self-contained and backplane-based systems aren’t the only way to go for rugged applications. Another VITA standard for modules is VITA-59 RSE (Rugged System-on-module Express).Men Micro’s XM2 runs a 2.26-GHz SP9300 Core 2 Duo processor. The board consumes up to 40 W and hides a connector on the bottom. The side tabs aren’t connectors but rather conduction connections to the metal case that normally encloses the system. It supports conduction or convection cooling within a temperature range of 0°C to 60°C.
Of course, there’s PC/104. Its compact size lends itself to sealing within a rugged case, but there are no specific standards for ruggedized cases or platforms. Still, VersaLogic and other vendors tailor systems such as the VersaLogic Ocelot SUMIT-104 to the designer’s rugged needs.
The Ocelot supports the stackable Small Form Factor SIG SUMIT interface, which combines PCI Express, LPC, SMB, USB, and SPI into a single set of connectors (see “Standard Serial Backplanes Dominate New Designs”). The 104 designation means the board also has a PC/104 ISA connector. This combination allows PC/104 boards to be stacked on one side and SUMIT boards on the other.
CARRIERS AND CONNECTORS Carrier boards with PMC and XMC connections are popular because they allow customization based on standard carrier boards. As a result, intelligent I/O can be placed on the mezzanine card while the carrier board holds standard components such as the processor.GE Fanuc’s Intelligent Platforms ICS- 1556B ADC Module provides high-speed sampling using a four-channel, 400-MHz, 14-bit analog-to-digital converter (ADC). It’s designed for UHF and VHF softwaredefined radios (SDRs). Front-panel coax connectors link the board to the inputs.
Curtiss-Wright Controls Embedded Computing’s XMC-FPGA-05F is an example of a rugged XMC card (Fig. 9). XMC is found in a range of board-level products, including ones based on CompactPCI and its Express version, VME, VPX, and VXS. It sports a Xilinx Virtex-5 FPGA along with four fiber-optic transceivers and four banks of DDR2 SDRAM memory. Also, it has PCI, PCI-X, PCI Express, Aurora, and Serial RapidIO interfaces.
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The four fiber-optic transceiver connections are on the front panel of the XMCFPGA- 05F. This is typical for PMC and XMC cards as well as for many of the boards they plug into. Rear transition modules (RTMs) are available for many boards, permitting connections via the backplane. Likewise, CompactPCI and VME boards, along with their relatives, can take I/O connections via the backplane, but fiber-optic and coax connections are less common and non-standard. This will be changing, at least when it comes to VITA standards.
New connectors and placement standards are in the works at VITA, leading to prototype boards like those from DRS Technologies using new coax connectors (Fig. 10). The approach has key advantages when it comes to rugged designs. It allows for easier board replacement since front-panel connections aren’t required.
VITA 46.14, a variant of VPX, specifies the RF connections. It looks like it will support four and eight coax connections and is suitable even on a 3U board. It essentially takes away from the digital I/O connections, but it’s a reasonable tradeoff.
Similarly, more than half the rugged backplanes are custom, so having a mix of connectors for the various slots is common already. Running connections out of the backplane has significant advantages, including less noise, higher data rates, and less crosstalk versus individual connections via the front panel.
The proposed VITA 46.12 standard addresses fiber connections on the back of VPX boards. The standard specifies three off-the-shelf fiber connectors, including Mechanical Transfer (MT), Expanded Beam (EB), and LuxCis-type.
The MT type is a self-aligned pair of mating shells and pins that maintain the positional accuracy of fibers in the form of a ribbon cable. MT connectors are an established commercial high-connectiondensity standard offering. Unfortunately, this approach doesn’t provide specific alignment of individual fibers, and cleanliness is a requirement. It will likely be used in less rugged environments.
The EB connectors use a spherical lens at the end of each fiber to expand the beam. The connectors bring the two lenses on each side of the connector into close proximity, but without physical contact. These connectors are less sensitive to alignment or contamination and can tolerate many mating cycles without degradation.
The LuxCi s - type connector s ar e designed to qualify for ARINC 801 aerospace applications. They fit into other rugged applications, too. The connectors consist of metal shell pairs with multiple signal connections using ceramic ferrules for fiber alignment. This approach maintains positional accuracy even in highvibration environments.
The proposed coax and fiber VPX standards target level 2 line-replaceable modules (LRMs) typically found in space and air vehicles as well as many military platforms. This allows for quick replacement of defective components. The task is much more difficult if front-panel connections were involved. Having all I/O via the backplane makes replacement simple and reliable. It can also simplify the issues associated with cooling and environmental isolation, since all connectors are at one end of the board.