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.
Types of Isolated Enclosures
There are two main types of isolated enclosures: external and internal. Both are commonly constructed of aluminum or steel. External isolation (Figure 1, left) describes a rack with an isolation system usually consisting of four to eight mounts at the base and two to four isolators at the rear for stabilizers. The number of isolators and their placement is based on the weight and CG (center of gravity) of the populated rack. Isolators are normally attached to the rack near its corners where the structure is fairly rigid. The internally isolated enclosure (Figure 1, right) is a more complex design involving an inner platform that houses the equipment and an outer enclosure that is hard-mounted to the foundation. Mounts support and “float” the inner platform from the outer rack.
Figure 2 shows a hard-mounted workstation with an isolated section. The design involves side panels that are the primary structural support frame with two lower subsections for equipment. The displays are directly in front of the operator. In the isolated section, the equipment is “floated” with elastomer Arch mounts.
Example of Shock Reduction
In the Navy’s 901D heavyweight barge test, an electronic rack is installed in a canopied barge and an underwater explosion is initiated near the barge. Shock measurements (Figure 3) show the input as nearly 45 g and the response (Figure 4) at slightly over 12 g. This is typical of the shock performance of isolated racks.
Externally Isolated Enclosure
This type of rack is widely used in the military. Multi-axis mounts that are capable of 4 to 4.5 inches vertical deflection are preferred for most Navy enclosures. Isolation enables the rack to remain upright and relatively stable while the foundation or platform moves vertically and laterally beneath the enclosure. Damping rapidly decreases the response of the system before additional shocks occur.
The predominant frequency of the isolation system is set to “decouple” from the driving frequency of the applied shock in order to reduce the force at the equipment. Stiffness and damping of the mounts are selected for a specified resonant frequency range, minimum amplification at resonance and decreased transmissibility as the disturbing frequency increases beyond resonance. Equipment is easily accessed by means of extended slides. The door is full-length opening. There are provisions for forced air cooling. Side, rear and electrical connector panels are removable.
Internally Isolated Enclosure
Features of the internally isolated enclosure are fixed outer rack (no sway space) and absence of secondary structure between the ship and the rack. The design basically consists of an open inner platform that houses the equipment and an outer rack that is hard mounted to the deck. The platform is suspended by means of the isolators and is relatively free to move within the outer enclosure. In addition to shock and vibration reduction, the isolated open frame allows for directional control of cooling air at different elevations of the equipment.
Secured only at its base, the enclosure can be positioned on an open floor or even located against a wall or at a corner. The absence of stabilizers simplifies ship’s architecture and enables racks to be placed side by side. The isolators are compliant, moderately well damped elements with repeatable stiffness characteristics. Shock is simultaneously reduced in all directions.
Isolation mounts are typically the elastomer Arch or cable type. For high shock Navy applications, isolators are typically 7 inches tall with 4.5 inch of free space. Proprietary elastomer compounds exhibit broad temperature range: 30°F to 140°F with a 25 percent variation in stiffness. Damping is relatively constant at 0.1 to 0.15% depending on the material, strain rate and temperature. Elastomer Arch mounts are highly stable with excellent return to position after shock. Cable mounts are made of preformed stainless steel rope having interlaced multiple strands and contoured in a biased loop arrangement. Damping is 0.1 to 0.15%. The stiffness rates of both types of mounts vary with load and direction.
Properly selected low frequency isolators can reduce the shock by more than 65%. Deflection depends on amplitude g and the ratio of the input pulse duration versus the response period. Shock reduction is less effective as the ratio decreases. A relatively stiff isolation system can result in amplification of the shock.
The effective resonant frequency of the typical isolation system is 7-10 hz. The vibration resonant frequency and the shock response frequency can be different if the stiffness rates of the isolation are non-linear and change with relative displacement across the mount. Some mounts also exhibit shifts in damping values. Elastomer mounts can exhibit 1.25 to 2.5 dynamic to static stiffness ratio.
The frequency characteristics of the rack can also affect the input to the equipment. The objective is to widely separate the predominant frequencies of equipment and isolation system, however, the construction of the rack and the layout of equipment can result in a relatively flexible rack with a predominant mode only 2 to 2.5 times greater than the isolation system. The interaction can amplify the dynamic response above the isolators. There are examples in vibration where isolation resonance at 11 to 13 hz coupled with the rack resonance at 17 to 20 hz resulted in a combined input to a disk drive above its 1.5g limit. The same isolation/rack system had an effective shock response frequency of 6 to 7 hz and was very successful in shock.
Isolation and Rack Characteristics
Enclosure manufacturers work closely with the customer to ensure that the rack meets military requirements. For example, Navy shock test will produce extremely high bending stresses at corner connections and provisions have to be made for upper restraints in almost all installations. This may require additional strengthening. There are no industry standards that define “military” and “commercial” enclosures. “Rugged,” therefore, may have different meanings, depending on the application. Not all manufacturers verify that their designs meet military specifications and standards. Even racks qualified to seismic standards may not be adequate for military use.
Requirements for validating shock and vibration performance often involve simulation and analysis by the enclosure manufacturer. Shock is usually defined in the form of shock response spectra (SRS) and acceleration (g) input. Analytical methods such as modeling and simulation of a multi-degree of freedom system can provide SRS information for the equipment at different locations of the ship or vehicle. Shock at individual equipment can then be determined by comparing test measurements to numerical simulation. Calculation methods involving modeling, transient time analysis, and modal superposition are commonly used.
Mil S 901D and Mil Std 810 G
Mil S 901D covers equipment installed on military ships. The 901D heavyweight test is performed on a floating shock platform (FSP). Also known as the barge test, FSP deck frequencies are important factors on shock input levels. Peak g-forces range from 110 to 130 g for the 25 hz deck, over 60 g for the 14 hz deck and 35 to 45 g for the 8 hz deck. Mil Std 810 tests range from combined-axis sine and random vibration to 60 g crash shock for flight and ground equipment.