What to expect for ruggedized MicroTCA enclosure platforms
While there have been some custom rugged designs for MicroTCA, the Rugged MicroTCA specifications are still in draft form, as noted in this month’s Specification Update column. Justin and Eike explain what changes to expect, factoring in the level of ruggedization and cooling method.
MicroTCA is gaining acceptance in more applications and markets. Although originally geared for telecom core and edge applica-tions, MicroTCA's form factor, manageability, wealth of vendors, performance/cost potential, open specification, and other factors make it a strong fit for many designs. It is expected to do well in telecom, medical, industrial, and military/aerospace markets (once the PICMG MicroTCA MIL/Aero specifications are completed). Some of the larger design wins today have already come in military programs, although in relatively benign environments, typically addressed by "cocooning" MicroTCA.0 solutions into an ATR box.
It is believed by many in the industry that MicroTCA could be well-suited for mil-aero applications. There are only a few high-bandwidth architectures with manageability addressed that have MicroTCA's potential in military designs, particularly with its smaller form factor possibilities. Size and weight are common concerns in the mil-aero apps, and with MicroTCA's well-known AdvancedMCs widely available, the architecture could be a hit. Questions to answer are: "How can we make MicroTCA rugged enough for military environments?" and "What is the definition of rugged?"
Rugged MicroTCA specifications are currently being developed by PICMG subcommittees. (See the Specification Update column in this issue.) Although some companies like Elma have announced ruggedized MicroTCA versions, one should not claim to have an actual specification-compliant rugged MicroTCA enclosure at this time. The PICMG MicroTCA specifications that define ruggedized versions are in draft stage.
An ATR box for ruggedized MicroTCA applications
When the specification is complete, a rugged MicroTCA.X Extended Environment air-cooled chassis might look similar to the unit in Figure 1. Figure 1 shows an Elma Mektron air-cooled ruggedized MicroTCA chassis concept that passed MIL-STD-810E shock/vibration requirements and MIL-STD-461 for electromagnetic interference.
The chassis example is an ARINC 404A full-size ATR long enclosure, often used in commercial and military aviation. It seems that it will be very difficult for MicroTCA to pass military shock/vibration test standards without some modifications. Nobody wants to change the AdvancedMCs, which have been in the marketplace for several years now, thus the chassis and/or front panels will have to change. The unit in this example was tested with modified front panels on the AdvancedMC modules (to become an option to be specified in MicroTCA.X Extended Environment) secured to the subrack for extra shock and vibration protection. Testing confirmed that this "standard" chassis configuration met the MIL-STD-810E shock/vibration requirements and MIL-STD-461 for electromagnetic interference. The vibration and shock tests were performed according to the IEC 61587 1 and VITA 47 standards in six axes (three spatial axes, with the system rotated by 180 degrees for each axis).
Although in this case modified front panels were used, in the forthcoming MicroTCA.X Extended Environment specification, the required subrack-to-module interface will be incorporated as an option. Noteworthy is that for this option to work, the subrack overall depth test dimensions will need to be reduced by 0.8 mm. This subrack depth reduction may result in an update to a later MicroTCA.0 issue (Figure 2). The proposed subrack/module interface for Rugged MicroTCA has the subrack overall depth dimensions reduced by 0.8 mm. Therefore, the chassis and/or face plates will likely be slightly different than nonrugged MicroTCA.
The new subrack/module interface allows the use of existing module face plates (AMC.0 defined) and/or the optionally defined face plates with retention (defined in MicroTCA.X Extended Environment).
Recapping MicroTCA's evolution
MicroTCA grew out of the original AdvancedTCA specification (PICMG 3.0), which was aimed at the Telco Carrier Grade environment. AdvancedTCA needed the feature of mezzanine boards, which led to the development of the AdvancedMC standard (AMC.0). Telco and industrial, as well as mil and aerospace applications, demanded AdvancedMC modules outside the AdvancedTCA/AdvancedMC carrier environment. This led to the expansion of AdvancedMC modules role as direct-plug backplane technology - which became MicroTCA - with AdvancedTCA management functionality incorporated via MicroTCA carrier hubs and power modules. It made sense, given MicroTCA's roots, that up to this point the contributing committee members were telco-centric, ensuring that the telco environmental needs were addressed.
Attention has been drawn to the needs of more extreme environments, including a considerably more severe random vibration environment (versus less severe seismic Zone 4 or sinusoidal vibration testing), ESD protection, bench handling, and rear I/O, just to name a few. These needs require specific mil testing and documentation not yet undertaken.
This issue presents a classic case of whose shoes you wear. Telco applications are known to be high volume and cost sensitive, while mil-aero applications are typically lower quantity and solution driven.
Conduction-cooled Rugged MicroTCA
For the conduction-cooled Rugged MicroTCA.Z chassis, the subcommittee's work has just begun. The chassis may look very similar to a typical conduction-cooled chassis. Figure 2 shows a chassis design concept in use now.
We may see a wide range of cooling designs for Rugged MicroTCA. A conduction-cooled MicroTCA chassis with an external fan can be seen in Figure 3. The conduction-cooled boards are in a sealed compartment with a separate compartment with a fan to expel excess heat. The airflow runs front-to-rear in this configuration. Again, what emerges from the MicroTCA.2 specification might differ from these examples.
Other chassis types
Rugged MicroTCA may come in various forms. For example, rack-mount versions are on the horizon. See Figure 4, which depicts a ruggedized MicroTCA chassis concept in a 19" rack-mount format.
Like the ATRs, the 19" rack-mount Rugged MicroTCA chassis can leverage the same type of shock-isolated card cage and device mounting that exists in other rugged designs.
Liquid-cooled versions are certainly a possibility for systems with more intense cooling demands. One concern would be the increased costs. Liquid cooling can be very complex and costly. However, there are ways to get improved thermal performance in applications where forced airflow is not available. One such possibility is with Liquid Heat Exchange (LHE). Instead of having the liquid go through the individual modules or via a spray method, given that method's associated costs and complications, the liquid can simply go through the outer chassis walls.
For cooling the standard MicroTCA conduction-cooled boards, the chassis sidewalls can be specially designed to carry various liquids (de-ionized water, kerosene, sea-water, PAO, alcohols, and similar) to transfer the heat. (See Figure 5 for a close-up of liquid-cooled chassis sidewalls.) We might see applications that require more advanced cooling techniques for MicroTCA. This rugged concept features liquid heat exchange via the chassis sidewalls to dispel up to 150 W per slot
The conduction-cooled boards would transfer the heat to the outer shell of the wedge locks, where the liquid-cooled sidewalls would carry away the heat and recirculate the liquid. An estimated 100 W to 150 W of heat can be dissipated per module in the chassis. With a modular design, either one or both of the independently cooled sidewalls can be implemented. This may be particularly important for Rugged MicroTCA, where the cost/performance ratio is likely to be a critical factor in its implementation.