Unmanned vehicles for air, land, and water represent an increasingly-important capability for virtually all military services worldwide. Because each new generation of device technology promises capabilities and higher levels of overall performance for unmanned vehicles of all types, military customers expect these benefits in the latest UAV offerings. To meet these expectations, systems engineers must deal with increasingly difficult problems of thermal management, power budgeting, EMI emissions and susceptibility, weight, size, cabling, connectors, antenna management, and command/control functions. Several new technologies and innovative packaging concepts now provide better solutions to these issues.

Thermal Management Solutions

(Photo: U.S. Army)

Since unmanned vehicles have no requirements for human occupants, SWaP penalties for life-support systems and crew or pilot quarters are eliminated. The downside for system designers is that UAVs are therefore expected to operate over temperatures ranging from -50°C to +75°C. Often unappreciated is the fact that these limits can be more difficult to meet when the vehicle is on the ground and powered down for sustained periods of time between missions. Primary design concerns here are non-operational effects of mechanical stress and packaging integrity. These are usually well documented as storage temperature specifications, which can help predict survivability on mid-winter airstrips in Alaska and mid-summer runways in the mid-East.

There are numerous operational temperature issues. If the unit is not provided with a standby heating system, starting up from -50°C often requires a warm-up delay and the use of heaters or partially powering up some equipment to heat the more temperature-sensitive equipment. Notorious for problems at low temperatures are crystal oscillators, batteries, capacitors, and some semiconductor devices like A/D converters.

At high start-up temperatures, similar care must be taken to cool down the equipment before applying full power. Otherwise, components like processors and FPGAs can sustain permanent damage, completely disabling critical subsystems within the vehicle. Cooling strategies must transfer heat to a heat exchanger coupled to the outside surface of the vehicle or to outside air. Methods of moving the heat include heat pipes, carbon nanotubes, circulating air or liquid, refrigerators, thermionic coolers, and some promising new nano-technology cooling engines.

Figure 1. VITA 66 optical connector blocks for a 3U VPX module (top) and backplane (bottom) are installed in place of electrical RT connectors (black). Each holds an MT ferrule with 24 fiber cables. (Courtesy: TE Connectivity Ltd.)

Once the UAV is operational, these same structures can continue to regulate internal temperatures, often consuming relatively little energy because of self-heating of the payload systems and the normally cold skin temperature of the vehicle once it reaches operational altitude.

Often a simple, independent vehicle thermal management processor capable of operating across the entire temperature range controls the heating and cooling systems and initiates operational power to the UAV systems when ready.

Open Packaging Standards

Even though each UAV targets a specific class of applications and missions, systems designers can reap significant benefits by exploiting the latest open standards for the many internal subsystems. An outstanding example is the VITA OpenVPX standard, now also adopted by ANSI. It defines numerous mechanical and electrical profiles for circuit boards, backplanes, chassis, connectors, as well as cooling and power distribution methods, all capable of withstanding severe military environmental conditions.

Particularly appropriate for UAVs are the numerous cooling methods for OpenVPX defined in the VITA 48 standard, which includes conduction, liquid flow-through, air flow-through, air flow-by, and variants. Designers can select the most appropriate cooling technique for a given UAV by surveying vendors for availability of VPX solutions sharing a common VITA 48 cooling method. This can greatly simplify the overall thermal design of the vehicle.

Another important benefit of open standards is improved life cycle support, especially for military programs looking for multi-year acquisition and installation phases, followed by ten or more years of operational life, that can be fully supported with maintenance and repairs. Obsolescence of critical components like memories, processors, or FPGAs is all too common and quite difficult to predict. In some cases, redesign of modules or subsystems is the only solution, and compliance with a well-defined standard helps ensure success.

Upgrades become far easier when an older module can be replaced with a new one that exploits the latest technology and delivers new performance levels, but yet still complies with the OpenVPX infrastructure to minimize system integration efforts.

Making Good Connections

A peek inside a military UAV reveals a staggering array of wires, cables, harnesses, and connectors, accounting for a significant share of vehicle weight, and having a major impact on both operational costs and mission endurance.

Because UAVs are loaded with sensors, antennas, processors, cameras, telemetry systems, radios, radars, navigation systems, jammers, power supplies and cooling systems, the necessary interconnects are often highly specialized to match the link.

Some new power systems distribute higher voltages using smaller diameter wires to minimize weight. Many new classes of POL (point-of-load) switching regulators drop the nominal 24 or 48 VDC distribution bus to lower local voltages required for each subsystem, while maintaining high efficiency across a wide range of supply voltages and load currents. More complex devices can maintain regulation across high pulse current loads to support radar and countermeasure equipment.

Parallel digital lines for high-speed data connections are being replaced by gigabit serial links at virtually every level of embedded systems. The benefits are fewer wires, smaller space, and higher rates. Within board-level products, gigabit serial links join processors, FPGAs, data converters, network interfaces, and storage interfaces. Within a chassis, these same serial links stretch across the backplane for connecting boards and for bringing I/O signals to bulkhead connectors. UAV subsystems are now exploiting these serial links to replace fat, parallel data cables to save space and weight. Two of the most popular gigabit protocols in UAVs are PCIe and Ethernet at speeds of 1, 10, and 40 GB/sec.

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