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Electronics Protection

Ruggedizing Enclosures: From Military to Seismic to Mobile Applications
Justin Moll, Marketing Manager
Optima EPS, An Elma Company

Ruggedizing an enclosure is often significantly more complex than one imagines. One typically thinks of thicker gauge metal and aluminum or metal shock-resistant mounts/isolators. However, many Mil-Aero and other applications require ruggedization on several levels. This includes an enclosure that will survive sand, salt-fog, humidity, extreme temperatures or offending EMC emissions.

Figure 1

Seaborne Applications
Different applications bring their own challenges. When it comes to design, sometimes it’s the little things that make the biggest difference. The sway of a ship can bring a different obstacle than a mobile command center. For shipboard applications, an enclosure may be external where it is directly accessible to sea conditions or deep inside the ship. For ship-deck and other external shipboard applications, one can protect from the elements with a sealed-gasket design. Using sealed gasketing shown in Figure 1, and having the cabinet fully enclosed can protect against harsh elements. A special alodine irridite pretreatment process along with a durable acrylic powder coated finish protects against corrosion based on thermo setting, cross linking resins. The frame of the enclosure needs to survive heavy sway. Compliant to MIL-STD-810F and 901D, the enclosure should employ thicker gauge aluminum extrusions.

Figure 2

The MIL-STD-810F testing of the shipboard MIL-SPEC cabinet unit shown (see Figure 2) included vibration and shock in the vertical, longitudinal, and transverse axis with up to 10G of shock for the profile measurement. In larger cabinet enclosures, the corners of the cabinet (points of weakness) can be welded or to reduce costs utilize reinforcing corner cavity members. (See Figure 3). Another weak point is the middle of the long enclosure vertical frame. Bracing bars, including cross-bracing, provide additional reinforcement. The base may be mounted to shock mounts to further reduce the effects of shock and vibration. Additional welding may be employed for more shock resistance.

For internal shipboard electronics, the enclosure may need to meet MIL-STD -461 for EMC against interference. This specification represents very stringent requirements requiring high levels of shielding performance (Tempest). Chassis inside the enclosure will typically have additional protection against EMC like honeycomb filters and stamped gasketing along the extrusion. Additional strip gaskets can be applied. The internal chassis would also meet MIL shock and vibration specifications. They can employ wire rope-coil isolators which deflect shocks in three axis. The elastomeric shock dampeners also provide protection to various deck frequencies. In shipboard applications, the sway of the ship can cause heavy electronics to slide violently if the enclosure is on slide rails within the rack, potentially causing harm to the crew or to the electronics. There using locking slide rails that are highly ruggedized are advised. The rails should be able to handle 800 N to 1,000N force with a weight of over 220 lbs. Locking versions allow the enclosure to slide out in a lockable position while open prevent the enclosure from sliding back and forth. Using an abrasion-resistant paint for corrosion protection on the slides also adds another measure of reliability.

Avionics Applications
Avionics applications have their own set of challenges. they need to meet the heavy G forces that can be found in performance aircraft. For rack and console applications in larger aircraft, they must survive the heavy shock and vibration. The same type of thicker gauge material, bracing and mounting techniques can be used. Often, the enclosure will need a balance of EMC and shock/vibration protection. Ruggedized extrusions can be used that incorporate the gasketing required. Balancing costs can be a challenge, so the customization with a modular platform helps keep customization costs and development time low. Plus, creative techniques can be employed to improve EMC performance without costs rising too high. This can be done with techniques such as tin plating, using a more conductive metal as a coating rather than use highly expensive materials. To protect against the wide range of EMI/RFI conditions, the enclosure can employ advanced filters including honeycomb filters, line filters and more. Using tin-coated, copper-clad steel wire mesh knitting over a double silicon elastomeric core provides a high level of filtering.

Figure 3

For smaller enclosures like ATR (Austin Turnbull Radio or Air Transport Racks), cooling presents some challenges. At higher altitudes there is less available air and combined with space limitations, forced air cooling may not be feasible. Therefore, conduction cooling is commonly used. With the daughter cards inserted into thick aluminum wedge locks, the heat is transferred out to the edges of the enclosure where grill-like “fins” maximize the surface space for the enclosure and allow more dissipation. For a simple solution to achieve additional heat removal, specialized liquid-flow-through ATRs have piping through the outer enclosure walls. This allows extra cooling, without the high costs and difficulty of having liquid transfer through the daughter cards and special liquid connectors.

Ground Applications
Ground applications have similar requirements as avionics and seaborne applications, particularly when it comes to shock and vibration. More and more advanced electronics are being housed inside more units. This is especially true for assault and transport vehicles. However, increasingly there is demand for electronics in mobile units for communications, command and control and medical/labs. Electronics needs to make its way into the theatre to communicate the tremendous amount of data from C4ISR (command, control, communications, computers, intelligence, surveillance and reconnaissance). Depending on the type of vehicle and its use, these can be very small racks mounted or welded to the internal structure or larger cabinets and desk systems. Regardless, space is always at a premium. Simply welding the units to the floor can cause stability and wear problems over time, so employing shock-mounting is advisable.

Particularly true for the dry and desert conditions of today’s battlefields, dust is the enemy for ground-based electronics. Providing dust-proof protection, while balancing EMC, light-weight and shock/vibration is critical. An extrusion-based modular design makes customization and adhering to the wide range of enclosure dimension requirements quicker and more cost-effective. Perhaps more importantly, this allows the designer to have customization, but based on a proven and tested base platform.

Extreme Conditions
In the frozen high altitudes of Afghanistan, to the desert heat of Iraq, to the salt water breeze off the coast of Libya, electronics need to survive diverse conditions. Electronic enclosures alone need to meet the MIL application temp ranges typically from at least – 20°C to 85°C (storage). The enclosure also needs to be protected for these ranges and heating or cooling elements are often applied. In colder environments, condensation can be an issue and the sandy desert conditions, filtering are more critical. Gasketing like EPDM, Neoprene or others prevent dust intrusion and are resistant to water (including saltwater), vapor and salt-fog. They are also weather-resistant and won’t harden or crack. EPDM has better temperature properties than materials like urethane and is resistant to compression set and abrasion.

The use of aluminum versus sheet metal offers some advantages for extreme conditions. Aluminum is lighter and provides better strength, thermal and corrosive protection. Sheet metal can be sprayed or dipped in primers and protective coatings. But, any nicks or scratches can lead to corrosion over time. The extrusions and other parts of the enclosure may also be anodized to increase the resistance to corrosion and wear.

For many harsh environments, using NEMA 12 components will effectively do the job. Using this gasketing in a well enclosed rack will meet a wide range of application’s needs. For some outdoor applications, NEMA Type 3R may be required. This ensures the enclosure won’t allow be affected by falling water (rain) or solid objects (dirt) and by the external formation of ice on the enclosure. NEMA 4 and 4x goes a bit farther by ensuring the enclosure is fully watertight.

Rugged Design
Whether the application is a military requirement, outdoor, mobile or other rugged requirement, you’ll need to balance various factors to meet your design goal. This includes marrying the features for shock/vibration, EMC, size/dimensional flexibility, weight and costs. There are always a wide range of choices and options. A modular design can help achieve the design goals for Mil/Aero or other rugged applications in as versatile a way as possible.

Optima EPS has been designing, manufacturing, and distributing quality electronic enclosures, enclosure systems, server racks, cabinets, security consoles, vertical racks, and telecom enclosures since its founding by Scientific-Atlanta, Inc. in 1962. As of April 1, 2004 Optima was acquired by Elma Electronic Inc. For more information visit www.optimaeps.com.