Increases in equipment density and the need for an easier maintenance and versatility are causing changes in the designs of chassis and power systems for military electronic systems.
By Michael Long
Military leaders face significant challenges from electronic equipment that is becoming more and more complex, budgets that are stretched, and maintenance that becomes more crucial every day.
In test equipment, for example, one of the stated goals of the Air Force's Automatic Test Systems (ATS) Roadmap (USAF, WR-ALC/LEA) is to reduce costs, improve efficiency, and increase the effectiveness and commonality of the service's inventory of automatic test systems. At the same time power densities are increasing, as is the demand for improved reliability.
One approach can be seen in the way some of the newest equipment handles power distribution. Instead of incorporating a large power supply in the back of the rack, today's equipment uses multiple power supplies that plug in from the front of the rack, usually in an N + 1 or N + 2 configuration; this is much easier to replace than power supplies buried in the back of a rack.
This architecture distributes prime power on a heavy-duty DC bus. Power supplies are becoming intelligent; they report on their conditions, can be commanded remotely, and can cause system-cooling fans to speed up before they become overheated and begin to degrade.
Hand in hand with the need for better monitoring and control of power supplies is the need for speedy replacement or repair. This means that systems designers can no longer bury power supplies in the backs of equipment racks where technicians must disconnect many wires slowly before they can replace them. Military users prefer removable hot-swappable power supplies.
LCAC navigation system upgrade
Not long ago, officials of the U.S. Navy embarked on an upgrade to the electronic systems of their Landing Craft Air Cushion (LCAC) as part of a service-life extension program (SLEP). LCAC is a large hovercraft that can transport 60 tons of troops and equipment at 40 knots over land and water.
Part of this upgrade involved replacing electronic systems with as much commercial off-the-shelf (COTS) equipment as possible. When it came to one particular enclosure in the vessel's navigation system upgrade, the Navy wanted to use the CompactPCI form factor but the amount of power involved, the system complexity, and the small amount of space available presented a considerable challenge.
Designers at Tracewell Systems of Westerville, Ohio, came up with an answer that involved a significant rearrangement of components. Prime power to the unit aboard the LCAC is 28-volts DC, distributed via a rail in the back of the cabinet.
A great departure from past military practice was to eliminate the large rear-mounted power supply and go to front-mounted pluggable power supplies. With front access there is no need to disconnect a welter of cables to get at and remove a power supply; technicians simply need to pull it out of the panel. Output from the front-mounted power supplies feeds via paddle cards that plug into the back of the backplane. Attached to the paddle cards are additional cable harnesses that go to circular connectors on the back panel.
Tracewell engineers had to ruggedize the entire unit and meet military electromagnetic interfere specifications so the unit would operate reliably aboard a combat vessel. Cooling is via forced air with washable filters that technicians can change easily from the front panel by removing a pair of thumbscrews.
Third Echelon Test System
The U.S. Marine Corps uses a wide variety of electronic equipment, including radios of various types, shoulder-fired weapons, night vision, and more. Force readiness depends upon keeping that equipment in good condition but the Corps had run into a problem: due to the lack of front-line diagnostic equipment, Marines had to transport malfunctioning equipment to the rear, even though much of that equipment was filled with line-replaceable units (LRUs) that could be replaced in the field — if only there were a way to determine which ones were at fault.
The Third Echelon Test System (TETS) from ManTech International Corp. in Fairfax, Va., with chassis design from Tracewell Systems, is the Corps' answer to that need. Mounted in the back of a high-mobility multi-wheeled vehicle (HMMWV, or pronounced humvee), it can follow the troops into forward areas and can screen LRUs to find failed circuit-card assemblies for replacement.
TETS consists of two VXI chassis of instrumentation, a ruggedized laptop computer running Windows NT and associated power supplies built into a pair of transit cases that can be set up for operation in the back of a humvee.
The VXI systems have a common-interface test adapter and all new Marine Corps equipment interfaces with it. For example, the electronic equipment in the expeditionary fighting vehicle (EFV, previously called the Advanced Amphibious Assault Vehicle, or AAAV) has LRUs that can be tested using TETS. The EFV is a true amphibious vehicle that can move through water at speeds of 20 to 25 knots and on land at speeds around 45 miles per hour.
TETS can be used in three configurations:
- basic, which can perform a Go/No Go screen on analog, digital, and hybrid modules and circuit card assemblies;
- radio frequency (RF), which adds the ability to screen RF modules and circuit cards; and
- EO configuration, which consists of the Basic configuration plus the ability to screen electro-optical (EO) modules and circuit cards.
The system can also do some limited testing of discrete components.
The system runs on 28-volts DC from the humvee's electrical system via a standard NATO slave receptacle; it can also run on 108- to 220-volts AC, 40 to 400 Hz. As befits its mission, it is ruggedly built and in tests has operated normally after bouncing around as unrestrained cargo in the back of a humvee.
The EFV itself follows the same philosophy that went into the design of TETS. It uses the open-system architecture in which technicians can easily remove and replace components, subsystems, and LRUs — with successor equipment as they become available or with equipment from competing manufacturers that are potentially less costly. The use of COTS and non-developmental items helped to reduce development and ownership costs.
Transportable automatic test system
Another example of the use of COTS equipment and the need to make provisions for future upgrading is a transportable automatic test system (ATS) built for the military by ManTech and Tracewell. The designers set out to build an ATS that would keep up with future equipment changes without being fitted with ad hoc add-ons to accommodate individual equipment. Such methods had been common in the past but were no longer accepted. Instead, the design team set out to make a modular system that could accept a wide variety of test equipment.
The General-Purpose Interface Bus (GPIB), VXI, and PXI all have advantages but none can satisfy all needs. GPIB-based systems, while very capable, tend to be physically large. VXI-based systems are smaller but may not be sufficiently portable. PXI systems can be quite portable but may not provide all the features required.
The solution was a system that can accommodate IEEE-488 (GPIB), VME/ VXI, and CompactPCI/PXI equipment all at the same time and all in the same chassis. The system is built around six universal inner chassis, each of which can be used as a VXI/PXI card cage, a System Power Chassis, a DC UUT Power Chassis, or a 19-inch subframe for IEEE-488 equipment. VME and CompactPCI subchassis are incorporated within a single lightweight structure using the latest in backplane technology.
Power flows through the system via a 48-volt DC rail, while power to the individual chassis comes from redundant 1600-watt VXI/PXI power supplies that plug in from the front. Signal wiring is distributed via an intrasystem connector block and all intrasystem wiring is routed through fixed position, blind-mateable connectors.
The trend to greater complexity and power density, along with the need for high reliability and easier maintenance, are driving equipment designers to search for innovative solutions that will meet these objectives and cut costs at the same time. Indications are that the methods they are developing will spread to more and more classes of equipment.
Michael Long is a product manager for Tracewell Systems Inc. in Westerville, Ohio. He can be reached by e-mail at [email protected].
