Case study: Dual-CPU PC/104 stack meets Marines vehicle OBC upgrade challenges

Story

April 29, 2011

Jonathan Miller

Diamond Systems Corporation

A recent military contract called for a compact, rugged, wide-temperature embedded system to be used in upgrading the U.S. Marine Corps already-vehicle-deployed Onboard Computers (OBCs). The challenge of upgrading the OBCs was met by integrating two CPU subsystems within a single PC/104 stack, while utilizing a new approach to conduction cooling.

U.S. Marine Corps vehicles serve as the workhorses of U.S. global peacekeeping activities. The Onboard Computer (OBC) monitors the health of the vehicle’s engine and drive train, to alert crew and maintenance staff to critical problems that might strand the vehicle during operation. If allowed to occur, such failures would present a mortal risk to the vehicle’s occupants and, of course, to the vehicle itself.

Each vehicle’s OBC gathers data through various onboard sensors, data acquisition subsystems, embedded computers, and remote user interfaces, and caches the data in local mass storage. Later, the OBC must rapidly transfer its collected data wirelessly to an off-board server, which aggregates and analyzes the diagnostic data collected throughout the fleet.

Accordingly, a large defense prime contractor recently required a compact, rugged, wide-temperature embedded system for upgrading a diagnostics Onboard Computer deployed on U.S. Marine Corps vehicles. As the following case study describes, the challenge was to upgrade the OBC’s user interface, performance, communications, and internal storage, while meeting the stringent size, thermal, ruggedness, and other constraints of an existing enclosure (Figure 1). The solution involved integrating two independent CPU subsystems within one PC/104 stack, plus an innovative approach to conduction cooling.

Figure 1: The upgraded OBC embedded computer needed to fit within an existing sealed enclosure

(Click graphic to zoom by 1.5x)

Evolving system requirements

As initially envisioned, the required embedded system electronics upgrade would consist primarily of a PC/104 form factor SBC based on an 800 MHz National LX800 CPU – a custom PC/104 I/O module for interfacing via CAN to the vehicle’s electronics – and a custom Uninterruptable Power Supply (UPS). These modules needed to fit within an existing 7" x 7" x 4" sealed enclosure. Additionally, the system was required to operate over the extended temperature range of -40 ºC to +71 ºC (measured at the enclosure’s surface), and had to withstand MIL-STD-202G shock and vibration conditions.

During the defense contractor’s design phase, it was determined that the anticipated architecture of initial upgrade design would not satisfy the diagnostic system’s performance requirements. Consequently, a second embedded processor, dedicated to the tasks of wirelessly uploading data to the depot and managing the system’s operator interface, was added to the architecture. The new dual-processor architecture also necessitated two additional Ethernet ports, through which the pair of CPUs would communicate with each other.

Based on the design team’s estimate of CPU bandwidth needed for managing both the high-speed wireless data uploads and the OBC’s operator interface, an Intel 1.6 GHz Atom Z5xx was designated to be the system’s second processor.

This turn of events significantly complicated the diagnostic system’s heat dissipation requirements because of the added power consumption of the new components (particularly the Atom CPU and its associated chipset), and because the number of boards in the PC/104 stack packed within the compact, sealed enclosure now needed to be doubled from two to four.

In short, the need for a second embedded processor appeared to be both a space and budget buster. The key challenges now would be:

Fitting the expanded set of electronics into the small enclosure
Meeting the application’s operating temperature and shock/vibration requirements

After exploring various alternatives, the design team concluded that an all-COTS system upgrade could not provide a satisfactory solution for two main reasons. For one thing, too many boards would be needed in order to meet to the application’s I/O and dual-CPU requirements, making it impossible to fit everything inside an existing enclosure.

Additionally, the conventional method of cooling PC/104-sized SBCs without forced air is to use a large, finned heat sink above the module, thermally attached to the CPU and chipset. However, this method would be unable to provide adequate cooling for reliable operation at the high end of the required external temperature range (+71 °C). A better thermal solution would be needed to maintain the CPU’s junction temperature (Tj) below its specified maximum limit of +90 °C to prevent thermal runaway.

Dual-SBC architecture

The solution to the contractor’s dilemma turned out to be an innovative system architecture that solved both the size and heat-dissipation problems, while meeting the project’s performance and cost targets. The approach involved physically combining the two subsystems into a single PC/104-style stack, while isolating them from each other functionally (see Figure 2).

Figure 2: The solution hinged on integrating dual SBC subsystems within a single PC/104-style stack, and removing heat from the Atom Z5xx processor/chipset via a large heat spreader.

(Click graphic to zoom by 1.8x)

In addition to the two PC/104-sized SBCs and the contractor’s custom I/O module, the new architecture added a newly developed multifunction I/O module, which was to become a COTS offering from Diamond Systems. The new module integrated a collection of additional requirements of the contractor’s application, including an 802.11a/b/g wireless radio, two 10/100 Mbps Ethernet ports, a SATA-interfaced 32 GB Solid-State Disk (SSD), and an SDVO-to-VGA video output converter.

Referring to Figure 2, SBC 1 gathers real-time data from various sensors and data acquisition components throughout the vehicle via the CAN interface residing on the contractor’s custom module. SBC 2, meanwhile, manages the vehicle’s operator interfaces, processes the vehicle data sent to it by SBC 1, caches it on the SATA SSD, and later rapidly uploads it via high-speed WiFi to the off-board server. While it might not be apparent from the photo, the PC/104 bus connector on the contractor’s custom module (one down from the top) is implemented with a “non-stack-through” bus connector. Consequently, its PC/104 bus only connects upward to the stack-through pins of SBC 1’s PC/104 bus, but not downward to the PC/104 bus of the modules below it. This creates the required functional isolation between Subsystem 1 and Subsystem 2.

So, the resulting four-module PC/104 stack satisfied all of the system’s functional requirements. But without fans or exotic heat-extraction methods, how could a dual-processor stack situated within such a small, sealed enclosure be expected to operate reliably amid external environments of up to +71 ºC?

Conduction cooling meets PC/104

The heat dissipation challenge was addressed with the aid of the unusual conduction-cooled thermal design methodology of Diamond’s Atom-based “Aurora” PC/104 form factor SBC. The SBC’s main heat-generating chips – the Intel Atom Z530 along with its US15W chipset – are positioned on the bottom of the board, in contrast to the topside CPU and heat sink positioning generally used on PC/104 SBCs.

In this SBC’s case, the underside processor and chipset are conduction cooled by means of a large, flat heat spreader. A layer of Z axis-compliant Thermal Interface Material (TIM) between the heat spreader and the underside IC packages ensures a low thermal resistance. In addition to efficiently cooling the SBC’s hottest components, the board’s bottom-mounted heat spreader also provides a standardized pattern of four mounting holes, with which the entire PC/104 stack can be ruggedly bolted to a system chassis.

Although not employed previously on PC/104-expandable SBCs, conduction cooling is a common practice in COM-based designs. Figure 3 compares the effectiveness of Aurora’s conduction cooling to that of the typical convection method used in the design of most PC/104-expandable SBCs. Lab tests confirmed that the SBC’s conduction-cooling design successfully lowered CPU junction temperature (Tj) by more than 20 °C, compared with a conventional heat sink-based convection-cooling approach.

Figure 3: A comparison of typical topside, heat sink-based PC/104 CPU convection cooling and Aurora’s bottom-side conduction cooling

(Click graphic to zoom by 1.9x)

Mission accomplished

The contractor’s objectives in upgrading the OBC’s performance, communications, and storage capabilities – without an enclosure redesign – were successfully accomplished despite stringent size, thermal, ruggedness, and other constraints. The key elements of the solution involved:

Fitting a dual-processor system architecture into a single PC/104-style module stack
Developing a new PC/104-sized wired/wireless communications and SSD module product
Applying conduction-cooling technology to the problem of removing heat from a PC/104-sized SBC’s 1.6 GHz Intel Atom CPU and chipset

The end result is enhanced capability to monitor the engine and transmission health of a fleet of USMC ground vehicles, thus protecting the vehicles and their occupants. This success also demonstrates the continued viability of PC/104-style modules as a pragmatic solution to COTS development challenges.

Jonathan Miller is founder, president, and chief technology officer of Diamond Systems Corporation. He held various technical positions before launching Diamond Systems in 1989, and is Chairman Emeritus of the PC/104 Embedded Consortium. He holds a B.S. in Computer Science from the Massachusetts Institute of Technology. He can be contacted at [email protected].

Diamond Systems Corporation 800-36-PC104 www.diamondsystems.com