RTC Magazine

The Advanced Mezzanine Card (AMC) is a small form-factor, hot-swappable module supporting high-speed serial fabric interconnect. Originally developed for PICMG AdvancedTCA platforms, AMCs were intended for communications applications, as a means to provide finer-grain I/O or processing scalability.

The recently ratified Micro Telecommunications Computing Architecture (MicroTCA) specification leverages this proven AMC form-factor and management infrastructure. MicroTCA defines a family of small, low-cost, flexible, high-bandwidth and highly scalable platforms comprised entirely of AMC modules.

With the ink barely dry on the MicroTCA specification, ideas are already in progress to broaden the adoption of the technology into applications beyond 19-inch rack packaging and telecom central office environments. Analogous to the penetration of VMEbus technology into multiple markets, ruggedized adaptations of the AMC and MicroTCA specifications for the defense and aerospace markets will spread adoption of these technologies into other harsh environments.

Example applications of rugged MicroTCA platforms include industrial automation equipment, outside plant (OSP) for telecommunications base stations and access devices, and high shock and vibration uses such as commercial vehicles, in addition to defense and aerospace applications.

Rugged, Open COTS

Certain trends are evident in today’s defense and aerospace markets, which will initially drive the need for ruggedized versions of these next-generation components and platforms. Firstly, there is the migration to open standards-based commercial off-the-shelf (COTS) technologies, and secondly, there is an emerging transition to a network-centric paradigm.

The impetus for the migration to open standards-based COTS technologies is simple—economics and time-to-market. Adoption of COTS can reduce program development costs and schedules, and improve interoperability. A network-centric architecture based on open standards will exert significant pressure for replacement of closed, proprietary elements, many of which exist at the edges of military networks where conditions are the harshest.

The adoption of COTS requires the integration of open standards-based COTS subsystems on many different mobile

platforms into a high-performance network. It will also require ruggedizing these platforms for the defense/aerospace environment.

For solution providers in these markets, developing proprietary subsystems and seamlessly integrating them into multiple mobile platforms is a daunting task, requiring significant investment of time and money. However, open standards-based COTS technologies are often not designed to operate in many military environments.

AMC and MicroTCA Enter the Mix

Many defense solution providers and prime contractors are interested in deploying platforms based on open PICMG specifications, specifically AMC and MicroTCA. This interest is fueled by the bandwidth, management and flexibility potential provided by these platforms.

As an open standards-based off-the-shelf technology, MicroTCA has a number of limitations for use in defense/aerospace environments. These include:

• Shock and vibration tolerance

• EMI/EMC emissions and immunity

• Operating temperature ranges

• Power input and conditioning

To allow AMCs and MicroTCA to become viable COTS technologies for all functional elements of the defense and aerospace architectures, ruggedization efforts must begin now. To that end, Hybricon and Motorola have teamed to develop a ruggedized version of Motorola’s first commercial MicroTCA platform, for use as a development and design-in vehicle for rugged applications.

In addition, Motorola and Hybricon have formed a Rugged MicroTCA Special Interest Group (SIG), comprised of several industry participants. The Rugged MicroTCA SIG is in the process of developing proposed draft specifications for rugged MicroTCA. These rugged-centric specifications build on, and as much as possible remain compliant with, the telecom-centric base specifications, AMC.0 and MicroTCA.0.

The ultimate goals of this effort are that COTS AMCs and MicroTCA-specific modules (MCH, PM) can be installed unmodified into ruggedized, air-cooled enclosures for more benign rugged environments. Alternatively, they may be mechanically converted for conduction cooling, with no change to the printed wiring board (PWB), components or edge connectors, and integrated in a conduction-cooled rack/enclosure for harsher environments.

How Rugged Is Rugged?

To establish a baseline for precisely how much vibration, shock, temperature, etc. a mobile platform should be expected to survive, adoption of the environmental requirements from the ANSI/VITA 47 standard is being proposed. This standard applies specifically to “plug-in units,” but may be applied somewhat more generally to the combination of a plug-in unit and the connector, subrack and enclosure into which it is installed. ANSI/VITA 47 draws from MIL-STD-810-F, considered the authority for military environmental testing, and specifies many of the test procedures therein as a means to verify compliance. It is, however, intended to address the potential environmental requirements of virtually any commercial, defense or aerospace application.

ANSI/VITA 47 specifies multiple environmental “classes,” each consisting of a set of subclasses related to specific environmental conditions, as illustrated in Table 1.

A wide variety of defense, aerospace, commercial and telecom rugged applications can be addressed with modules and platforms that meet the requirements of the EAC1 or EAC4 (air-cooled) classifications. Here, air-cooled platforms are expected to provide an operating temperature range of 0°C to +55°C (subclass AC1), immunity to random vibration of ~2g (subclass V1) or ~8g (subclass V2), and immunity to shock up to 20g for 11 msec. More stringent environmental classes such as EAC2 and EAC5 extend the minimum operating temperature to -40°C; this can be accomplished with chassis level heater provisions or with extended temperature range cards (Figure 1).

Air-Cooled, Rugged COTS MicroTCA

The MicroTCA.0 specification defines a number of possible chassis form-factors, but does not preclude alternatives, as long as the platform can accommodate and air-cool (or heat if necessary) a standard AMC module—one that is compliant to the PICMG AMC.0 specification—and standard MicroTCA-specific modules, such as the MicroTCA Carrier Hub (MCH) and Power Module (PM).

This has allowed Hybricon to develop a ruggedized MicroTCA Air Transport Rack (ATR) chassis designed to meet ANSI/VITA 47 EAC1 or EAC4 requirements (Figures 2 and 3). This chassis leverages the backplane interconnect and module payload specifications of a commercial MicroTCA platform being developed by Motorola. The rugged platform also goes a step further to accommodate double-width modules. This ruggedized ATR platform remains compliant with the specification and addresses many of the limitations of commercial MicroTCA for defense applications:

• Innovative locking bars firmly retain the MicroTCA cards into the card cage, providing significant additional resistance to shock and vibration.

• The ruggedized ATR chassis has a shock-isolated MicroTCA card cage that attenuates the level of shock and vibration at the MicroTCA cards. This, coupled with module locking, allows the chassis to meet ANSI/VITA 47 class V1 and V2 shock and vibration requirements.

• A secondary EMI barrier, in addition to aggressive power line filtering, allow the ruggedized chassis to meet stringent MIL-STD-461 EMI/EMC requirements.

• Military circular-style I/O connectors allow the chassis to meet military ruggedization requirements for external connectors.

• Military power supply front-end converters allow the ruggedized chassis to utilize COTS MicroTCA power modules, yet meet specific military power supply requirements such as MIL-STD-704 aircraft power or MIL-STD-1275 vehicle power.

The actual operating temperature range of this platform is limited by the specifications of the AMC and MicroTCA cards that are used for a particular application. It is, however, designed to provide sufficient airflow for virtually any module that is rated up to 55°C, with sufficient mass flow rate for operation at 10,000 ft.

Extended Temperature and Vibration

As the current MicroTCA.0 specification only requires immunity to sinusoidal vibration, testing is in process now by connector manufacturers to determine the maximum ANSI/VITA 47 vibration and shock classifications that single and double AMC modules and connector assemblies can withstand, within MicroTCA.0-compliant subracks.

Also, neither the AMC.0 nor MicroTCA 1.0 specifications require an AMC to be anything other than air-cooled, nor to operate above roughly 60°C ambient (55°C + 5°C margins).

Existing COTS AMC or MicroTCA modules are unlikely to survive EAC3 or EAC6, the hottest air-cooled classes of operation (+70°C) without modification, but extended temperature range versions could be developed. Testing will verify whether air-cooled double modules can meet the V2 vibration class required by EAC4/5/6, but this is likely to require chassis-level shock and vibration isolation.

More demanding applications could be addressed with conduction-cooled modules and platforms that meet the requirements of the ANSI/VITA 47 ECCn classifications, which must provide class V3 vibration immunity of ~12g, and shock up to 40g for 11 msec. ECC2 thru ECC4 specify maximum operating temperatures from +55°C thru +85°C.

Besides reducing platform complexity and increasing reliability by doing without mechanical fans and allowing higher operating temperatures, conduction cooling has been a popular choice in this application space for several other reasons. For one thing, conduction-cooled modules include a rugged frame with wedge locks that provide a robust mechanical connection to the chassis. Due to these features, conduction-cooled modules can survive in environments with higher levels of shock and vibration. Secondly, rugged applications often encounter sand, dust, salt, fog and condensing humidity that can be problematic for air-cooled assemblies. Conduction cooling allows the modules to be isolated from the external environment since air is not flowing over them.

Thus, it is very desirable to develop specifications for conduction-cooled versions of AMCs and MicroTCA-specific modules (MCH and PM), along with conduction-cooled racks and enclosures (most likely AMC.n and/or MicroTCA.n). These specifications should reference ANSI/VITA 47 for the requirement levels and test procedures.

The objective is to allow conversion to conduction-cooled modules without changing the underlying PWB, thus remaining compliant with the current specifications for PCB form-factor and edge connectors. Existing AMC and MicroTCA module designs must be leveraged to prevent fragmentation of the ecosystem.

Figure 4 illustrates the addition of conduction cladding to COTS AMC modules. It is expected that most AMCs in production today can be adapted without PWB modification to conduction cooling. However, future modules could be designed to make better use of this cooling method by selecting extended temperature components, and moving thermally sensitive components toward the module’s edge and closer to the thermal transfer surfaces. This has the added benefit of locating these typically higher-mass components near the card guides where the mechanical support provided by the module and chassis is highest.

Due to the width of the required heat transfer surface (cooling frame), conduction-cooled modules would not fit in standard MicroTCA subracks. However, a conduction-cooled chassis could support the same 3HP, 4HP and 6HP pitch options, as these are primarily driven by the height of the module’s component envelope. Further, retaining standard pitch allows both air-cooled and conduction-cooled platforms to employ the same backplane.

The focus of research to date has been on cooling and physical mounting. More work is needed to develop front panel and latch designs that leverage the LED and hot-swap switch placement of existing modules, while paying attention to the human factor issues unique to harsh environments (such as gloved hands).

Similar approaches are anticipated for conduction-cooled power modules as well as single and multi-mezzanine MCH modules.

Motorola Inc., Embedded Communications Computing
Tempe, AZ.
(800) 759-1107.
www.motorola.com/computing.

Hybricon
Ayer, MA.
(978) 772.5422.
www.hybricon.com