Justin Moll,†Director of Marketing
There are times when the design engineer needs to take a commercial platform and ruggedize it for a Mil/Aero or other extreme application. The task seems easy at first, but the simple approaches typically add size and significant weight, which are strong disadvantages (especially in Mil/Aero). On a chassis-level, sometimes it requires re-designing from the ground up. A good example is the AdvancedTCA (ATCA) architecture, which was initially designed to be a Telco platform.
AdvancedTCA started as a Telco-centric platform, utilizing 8U by 280 mm boards in a 19 inch rackmount system offering up to 14-slots. The architecture typically used a load of packet processors and high-end processors across 10G backplanes. The specification now utilizes 40G backplanes for multiple Terabit aggregate performance across the shelf.†† Letís look at how this chassis can be ruggedized to meet a MIL Rugged application.
Ruggedizing the Chassis
Since it was designed for commercial applications, a standard ATCA chassis would not hold up to the EMI and shock/vibration requirements of a defense platform.† However, the tremendous processing performance of ATCA is ideal for the heavy-duty back end data processing in many Mil/Aero applications. To provide the ruggedness and EMI performance, the card cage in this example was machined with a lightweight aluminum construction. The result was an optimal 4:1 strength-to weight ratio supporting a 160 lbs payload inclusive of electronic modules, power supplies and internal I/O cables.
The design included high strength card guides to support electronic payloads in excess of the PICMG 3.0 subrack requirements of 70.5 lbs front module payload while being deployed in severe vibration, shock and acceleration environments encountered in military applications. †Testing the Rugged ATCA chassis showed it met MIL-STD-810G for shock (Method 516.6 Procedure I 40Gís, 11ms Half Sine Pulse) and MIL-STD-810G (Method 514.6 Procedure I) for vibration.† By providing a rugged card cage, it was not necessary to add items that may require a larger chassis, such as rope-coil isolators and specialized dampeners.
A commercial ATCA chassis would typically be 13U-14U tall using a sheet metal construction.† By changing to a 12U height and using aluminum, the weight can be kept low and size as limited as the cooling requirements would allow.
With a machined design and Faraday cage approach, designers were able to incorporate EMI tabs and seat the gasketing as needed to meet MIL-STD-461 for EMI requirements. Using MIL-grade filtering connectors on the I/O panel is another approach to handle EMI.
Cooling the Chassis
With the unique construction of the Rugged ATCA chassis, it was necessary to confirm the chassis cooling. The ATCA specification requires cooling of at least 200 W/slot. With the rugged card guides, an unique airflow-balance baffle provision was implemented with the front and Rear Transition Module card guides. Typical sub-rack payload configurations may be composed of a mixture of high-impedance and low-impedance front boards and RTMs which tend to have uneven airflow through the slots with air from high impedance slots often diverted to neighboring low impedance slots. In order to counteract this effect, the card guides were designed with an integral baffle provision to enable system integrators to balance the airflow for optimal performance within the Shelf.† Using Computation Fluid Dynamics (CFD) simulation, there was confirmation that the chassis would easily meet the required cooling levels. †(See Figure 1 for an image of the Rugged AdvancedTCA chassis)
Choosing the Right Path
There are various ways to skin the cat, but the methods in this example required several changes to the chassis construction to achieve the ruggedization levels required. †This included changing the material to a lightweight aluminum, machining the card cage for a stronger design that suppresses EMI, as well as ribbed constructed panels for strength.† In the end, the customer got a ruggedized chassis solution and was able to utilize legacy software and modified hardware for their solution.
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