Why Stackable Systems Work Well in Rugged Environments

Designing for rugged applications presents its share of challenges, but there are form factors and manufacturing techniques that accommodate most requirements for either general or application-specific design. Mobility and environmental extremes are critical considerations for rugged board design in military, transportation, medical, industrial and surveillance applications, to name a few. And with the current trend in embedded system design of higher performance and lower power usage, stackable system standards continue to be updated to maintain relevance.

Stackable System Options

The two basic stackable modular design approaches, Single Board Computer (SBC) and Computer-on-Module (COM), are both popular options for rugged applications. Both design approaches have advantages depending on application requirements, and there are fundamental differences that are important to be aware of when choosing which design path to take.

It can be argued that small form factor design trends are paradoxical. As form factor size decreases, functionality requirements increase. And at the same time that processing power requirements heighten, lower power consumption and thermal output is expected. Now add to that the requirement for ruggedness to accommodate for the shock, vibration, humidity, and temperature extremes and variance inherent in mobile and outdoor applications.

The PC/104 embedded computing format has no backplane, instead allowing modules to stack together like building blocks⎯more rugged than typical bus connections in PCs (such as PCI or PCI Express slot cards). PC/104 delivers high performance combined with low power, stackable configurations and adherence to MIL-STD, and it meets key industrial and transportation standards for Electromagnetic Interface/Compatibility (EMI/EMC), e.g. EN50121, EN50155, EN610000-x, etc. The ability to build stacks of PC/104 modules create opportunities for developing a diversity of complex, often mobile, applications that range across industrial, transportation, and defense environments where PC/104's robust and reliable capabilities are required. In addition, PC/104's transition into vision and visual security monitoring systems is benefitted by PCI Express, as it has the capacity to directly meet the bandwidth needed to support multiple data streams. (Figure 1).

Figure 1: PC/104 Express can accept a wide variety of modules via either the ISA bus or the PCI bus.These modules can be rigidly attached for ruggedized systems. The variety of modules means that custom circuit design is rarely needed.

Though the number of stacks included in PC/104 systems has been decreasing, the small form factor continues its warm relationship with industries requiring rugged applications with high resistance to shock and vibration. In defense and transportation, legacy devices and ISA-BUS interface requirements are still plentiful. With high-speed serial I/O interfaces, such as PCI Express, supported in current PC/104-based standards, PC/104 boards are keeping pace with the movement toward consolidating workload on expansion modules, requiring fewer layers to fulfill application requirements.

The ability to withstand temperature extremes often associated with remote environments still allows PC/104 to excel in off-grid computing (e.g., defense apps). Stackable, mix-and-match modularity and the intrinsically rugged design of PC/104 is ideal for many of today's technology upgrade programs looking for Commercial Off-the-Shelf (COTS) options—especially those that value SWaP(-C). In addition to ruggedness, users of PC/104 have come to expect long lifecycle support. When considering shrinking DoD budgets, the robustness, longevity and compatibility of the PC/104 ecosystem ensure strong system support and minimized costs.

While PC/104 allows flexibility by combining cards to meet application requirements, the PC/104 format becomes less attractive when very high computing speed and network throughput is required—situations where VPX or CompactPCI formats are better suited. In cases where an application design requires very specific I/O or physical size/shape restrictions, then a Computer-on-Module (COM) approach would provide better results.

COMs are complete embedded computers built on a single circuit board for use in small or specialized applications requiring low power consumption or small physical size. Though they are compact (ETX/XTX at 114 x 95 mm and COM Express at 125 x 95 mm to 84x55 mm) and highly integrated, COMs can accommodate complex CPUs.

With the COM approach, all generic PC functions are readily available in an off-the-shelf foundation module, allowing system developers to focus on their core competencies and the unique functions of their systems. A custom designed carrier board complements the COM with additional functionality that is required for specific applications. The carrier board provides all the interface connectors for peripherals, such as storage, Ethernet, keyboard/mouse and display. This modularity allows the designer to upgrade the COM on the carrier board without changing any other board design features, and also allows more customization of peripherals as dictated by a specific application. (Figure 2).

Figure 2: A rugged COM Express solution securely mates a standard COM module with processor and memory to a custom carrier card, which implements the application-specific I/O functions

The COM Express form factor offers flexibility in the development and advancement of ultra-rugged embedded applications for a wide range of industries, including transportation. By using the modular processing block, the designer creates a price and value advantage; he/she isn't locked into a single vendor for board creation and can customize based on pricing and performance requirements. Because it is easily swapped from a carrier board and comes in one of the smallest form factors, COM Express is ideal for long-life embedded applications with a critical development cycle, as well as more progressive applications that require frequent processor upgrades without affecting other application design elements.

Stackable design for harsh environments

Rugged solutions are most often housed outdoors or in moving vehicles, where exposure to a variety of climates dictates the need to operate in extended temperatures and to power up in any extreme. The easiest initial step is to select a rugged board or system that is designed for harsh environments from the ground up. To support the extremes of shock, vibration, humidity, and temperature, care is given to component selection, circuit design, Printed Circuit Board (PCB) layout and materials, thermal solutions, enclosure design, and manufacturing process. Robust test methods, including Highly Accelerated Life Testing (HALT), ensure optimal product design phases in order to meet a product's stringent requirements, such as -40°C to +85°C operating temperature range, MIL-STD, shock and vibration, and long-term reliability.

Conformal coating can also reduce degradation from exposure to outside elements. A variety of conformal coating materials (such as acrylic, polyurethane, epoxy, and silicone) and application methods (such as brushing, spraying, and dipping) are currently used to protect against moisture, dust, chemicals, and temperature extremes that can potentially damage electronics. The correct coating or application method varies depending on established standard operating conditions for an application. With transportation applications, different coatings may be selected based on a primary need for moisture resistance versus abrasion resistance versus temperature stability.

Maintaining performance while mobile

Rugged computing solutions also demand more memory space than ever before for both data storage and application performance. Options for storage include rotating hard disk drives (HDDs) for economy or solid-state drives (SSDs), which are truly rugged, but also come at a higher price point (cents per GB for HDDs versus dollars per GB for SSDs). HDDs contain spinning disks and movable read/write heads, whereas SSDs use microchips that retain data in non-volatile memory chips and contain no moving parts, making them less susceptible to physical shock, altitude, and vibration issues. SSDs have faster access time and lower latency than do HDDs, but SDDs cannot provide the capacity of an HDD; because of the higher cost per GB, SSDs are typically no larger than 120GB, while HDDs average 500GB-1TB. Higher performing HDDs also require heavier materials than either a standard HDD or the flash memory and circuit board materials of SSDs. Both PC/104 and COM can accommodate added storage through built-in extensibility and customization options.

With rugged, in-vehicle applications, vibration control is critical for performing functions like capturing video or securing targets. Some rugged SBCs offer a thicker PCB fabrication to add rigidity so the board can withstand higher levels of vibration strain. The thicker PCB offers stability to the overall surface area, protecting electronic components from damage due to vibration. The thicker PCB also offers the ability to use more copper between layers for thermal considerations. Heat is a common unwanted by-product of processing power. In addition to cooling fans and large heat sinks, which may not always be possible for compact, mobile transportation designs, PCBs with adequate amounts of integrated copper facilitate heat conduction away from temperature-sensitive electronic components to prevent performance degradation.

Case study: Rugged, Intelligent Bus Network

A leading designer of innovative technology solutions for all modes of public transportation implemented an on-board smart system enabling transit agencies to communicate with customers and dispatch, maintain its fleet and collect and analyze operating data. The numerous control inputs included vehicle run switch, front and rear door, wheelchair ramp, stop request, odometer and emergency alarm. The solution also required GPS with driving recorder and support for both wireless and cellular transmission. Finally, the company required a Class A device for testing against SAE International standards.

Due to both space constraints within transit vehicles and the highly specialized application requirements, the COM Express form factor was selected for this particular embedded solution. The solution consists of a rugged COM Express module plus custom baseboard with an Intel® Atom™ processor. Mini PCI Express slots support 802.11 a/b/g/n and cellular modems for connectivity and specified operating and storage temperature, shock and operating and non-operating vibration requirements were all designed into and extensively tested to create the custom solution.

Considerations for developing rugged, stackable systems include physical size and interface requirements, I/O needs, computing power, staff engineering capabilities, and, of course, budget. PC/104 and COM formats offer different design advantages, but are both highly effective solutions for building advanced rugged, often mobile, applications. In the case of the intelligent bus network above, COM Express best addressed overall requirements and specifications. But PC/104 continues to be a key form factor in rugged solutions across industries that require flexibility with mix-and match expansion cards, as well as support for both legacy and advanced interfaces.

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