As performance capabilities for embedded computing products expand, the requirements for commercial-grade and MIL ruggedized systems blur into similar specifications. There are increasing demands to take feature-rich, successful industrial/commercial enclosure systems into MIL or other harsh environment applications. Similarly, engineers are designing up-front computing platforms that they can expand into harsher environments.

Figure 1. Commercial-grade OpenVPX chassis are commonly used for benign-environment defense applications and for prototyping. With powerful reverse-impeller blowers directly above the card guide, there is efficient cooling for even the hottest OpenVPX cards.

Preparing for Rugged Design During Commercial Chassis Development

It would be easy if the same system platform could be used in both commercial and MIL applications. One would just design and build one system and be done. Of course, the requirements for hardened designs are significantly costlier, thus not practical in most commercial applications. The rugged chassis platforms need to survive extreme temperatures, shock and vibration, EMI susceptibility, and more. Therefore, many engineers are starting with commercial-grade chassis platforms in the backplane architecture that meets both requirements.

OpenVPX is widely used in ruggedized designs and offers high-performance across the board. One such example is a design that started with an air-cooled enclosure for 3U OpenVPX boards. To keep power uniformity, the designer utilized VITA 62 power supplies in the commercial version. These power supplies can meet the harsh environments for avionics applications. The VITA 62 cards meet MIL-461 for EMI, MIL-810 for shock/vibration/environmental and MIL-704 for aircraft power. These PSUs were utilized in this enclosure by employing card guides that are designed to accept conduction-cooled boards. These card guides allow OpenVPX boards that are conduction-cooled to be used in the air-cooled system. This is advantageous during prototyping, where you may need to mix-and-match module types. Further, by employing a DC PSU in the commercial version, the designer was able to keep the same type of PSU for the rugged version.

The 9-slot backplanes featured some VITA 67 slots for RF. By only having the cutouts for the VITA 67 slots, the designer was able to keep flexibility during prototyping and for various end-customer requirements. The housings/contacts could be filled in as required by the application.

When RTM (Rear Transition Module) slots are required, it is beneficial to have a chassis with the fans above the card cage. With a reverse-impeller blower approach, the air is pulled from below the cards up to the fans directly above the modules with the heat exhaust blown 90 degrees out the rear of the enclosure. Most cabinet enclosures that hold these subracks employ this airflow configuration where cool air is directed to the front of the cabinet with the heat going to the back. The air is then recirculated in this fashion.

In the subrack chassis platform, the fans could potentially be placed in the rear of the box, but RTMs would impede the airflow. In that instance, the enclosure would need to increase in size and weight so that the fans can reside above the RTM section. But, the extra height and weight is typically not desired.

Figure 1 shows an OpenVPX chassis with hot-swappable RiCool blowers above the card cage. This enclosure has multiple RTMs plugged into the rear of the backplane. Another advantage of the placement of the fans is cooling efficiency. With the high-wattage boards of OpenVPX, this approach with the fans directly above the boards ensures adequate cooling. The dual fans are typically 110 CFM each, but for more extreme cooling needs, the fans can go up to 191 CFM each displacing up to 3.6 inches of water of static pressure.

Figure 2. For ruggedized versions of the enclosures, thicker extrusion rails with double screws can be used. They are also commonly used in commercial-grade chassis to prevent bowing from the extreme insertion forces of 6U OpenVPX boards.

Taking the chassis in the example above to a MIL-grade system requires a few changes. The sheet metal would be designed with thicker material and the extrusions would be reinforced. Figure 2 shows an example of a rugged OpenVPX extrusion that is used both in 6U OpenVPX designs and in MIL rugged designs. The very thick extrusion is specially designed to handle the high insertion forces of 6U OpenVPX boards. The hardened aluminum prevents bowing and cracking of the board interface point. With a double-screw design, the extrusion is further reinforced. The boards can be recessed in the card cage or have an outer panel with EMI filtering. The backplane and PSUs are typically conformal coated to protect against moisture, dust, salt-fog, etc.

Figure 3. Other elements of ruggedizing the enclosure include thicker walls, a recessed card cage for EMI protection, filtering, and MIL-grade versions of fans, connectors/ cabling, and PSUs.

Finally, the fans would need to meet MIL spec requirements. In this case, they can be placed in the rear with extra space allotted for the airflow for the RTM slots. Figure 3 shows the back of the same type of rugged enclosure, but with cabling from a VME64x P2 area as opposed to pluggable RTMs.

Another option for the designer is to shift to an ATR type of enclosure. This requires a completely different approach utilizing conduction-cooled modules. The type of ATR used depends on the number of slots and cooling requirements. 3U OpenVPX boards can fit in a ½ ATR with a width of 4.88’'. They are typically top-loaded, but for small systems, a front or rear-loaded approach can be beneficial.

Figures 4a & 4b. For medium to high slot counts, a top-loaded approach for OpenVPX boards is common, as shown in Figure 4b (bottom). For smaller slot counts, a rear-loaded configuration, as shown in Figure 4a (top), allows a compact size in a rugged and secure I/O format.

Figure 4a shows a rear-loaded ATR for a 3-slot 3U OpenVPX backplane. This allows the IO to go straight to the panel, in a secure and reliable format. Most importantly, it saves space and weight in the process. Figure 4b shows a top-loaded configuration for larger backplanes. The drawback of this size is you don't have the space for supplemental air-cooling. So, the OpenVPX boards cannot dissipate too much heat. The small ATR in Figure 4a was designed to cool at least 125W. If extra width is allowable and air-cooling is available, a heat exchanged version can provide the enhanced thermal management. The enclosure in Figure 4b is fully sealed, with a 2nd enclosure wall on the outside that allows supplemental airflow to pass over the fins of the enclosure. This design requires extra width such as a 5/8 ATR size. Versions have been simulated that cool up to 800W using this approach. (Although, in this case, the width increased beyond the 5.28 inches of the 5/8 width).