Military systems represent extreme environments for COTS electronic equipment. Many systems involve multiple enclosures, often from different suppliers. Equipment layout, the selection of racks, whether isolation is used, and how the electronics are “housed” can vary widely. There are no standards regarding maximum allowable shock and vibration levels. Design factors are often based on estimates of equipment fragility and expected loads, both of which can be uncertain.
Over 100 g can be experienced in the Navy’s Mil S 901D test and more than 60 g in Mil Std 810G. The levels at which shock and vibration can damage or degrade equipment is a basic concern in packaging. The proper selection of an isolation system combined with a “rugged” enclosure is the basis for effective shock and vibration control.
Protecting COTS Electronics
Two choices for protecting COTS electronics are:
- strengthen the electronics through “hardening” of components, or
- “tailor” the environment by means of the enclosure, thereby reducing the environmental loads to which the equipment is exposed.
The term “rugged enclosure” is not well defined, but we’ll use it here to differentiate from a commercial rack intended for industrial handling. By rugged rack, we mean one that can experience 20 — 25 g or more with up to 1000 lbs of equipment in a high shock environment. Confusing the two can result in a poor selection of enclosure and isolation.
Features of isolated enclosures are proven reduction of severe shock and vibration to levels well within COTS limits, which are typically 20 g in shock and 1.5 g in vibration. Enclosures can be used “stand alone” or “ganged” together. Rugged racks generally have heavy-duty structural frames, stiffening members and corner brackets to reinforce the structure and properly transfer load to the isolators. Shock rated slides and equipment supports are used.
Rugged designs also allow for separating the equipment into “hard” and “isolated” compartments. Accessories include fan assemblies and slide-out shelves. Using CAD/FEA software, enclosures can be configured and populated in the design stage to evaluate equipment layout including stress analysis of critical parts and thermal distribution at COTS electronics.