Design Criteria for Rugged Enclosures
by: Ram Rajan
Elma Electronic
The advent of COTS opened the market for ruggedized electronic enclosures to manufacturers of commercial enclosures. But the fact that products can be "Off-The-Shelf" does not preclude them from producing an enclosure that meets the MIL Standards with respect to performance. The merging of a military environment with commercial electronics necessitates a revolutionary approach and a departure from the traditional packaging solution. Therefore, while designing a COTS enclosure for the military, it is imperative that the design framework is geared towards meeting the objective of the COTS initiative.
Electronic packaging performs the following five essential functions:
1. It provides a reliable structure that physically supports and protects the circuit boards.
2. It cools the electronics (maintaining the proper temperature range and air quality for operation).
3. It supplies and distributes power.
4. It accommodates communication with other electronic devices via I/O cabling.
5. It furnishes proper shielding to make the electronic device electromagnetically compatible with other electronic devices.
Thus, there are a few key aspects to a packaging solution that has to meet the requirements for operating in a military environment such as:
· Shock and vibration resistant
· Electromagnetic Compatibility (EMC)
· Thermal management
· I/O cabling
· Resistance to harsh atmospheric environments
· Reliability and maintainability
And, as is the case with most other COTS components or subsystems, the problem is not to sacrifice system performance in order to design a modular packaging system that is flexible enough to meet varied COTS demands.
Shock and Vibration
A thorough understanding of the environment the equipment will need to survive in is essential before the design begins. Whether it is mounted on a ground-based vehicle, airborne vehicle, shipboard mounted or in a submarine, a ruggedized construction (with an optimal isolation scheme to mitigate the effects of shock and vibration endemic to these environments) should be a primary design criteria.
Vibration can be either random vibration or sinusoidal vibration. A practical example of the former is what vehicle mounted equipment experiences rolling across rough terrain. A rotating engine, like a helicopter rotor, would cause a continuous vibration, which is the latter type. In order to protect the equipment and its contents from its insidious effects, passive isolation systems such as elastomeric dampers, rope coil isolators and air springs are very effective.
Whatever the final solution may be, the goal should be to isolate the equipment from excitement at its natural frequency. (Natural frequency of a component or system is the frequency at which the displacement is the greatest). Isolation systems are designed to change the system's natural frequency so it is as far from the exciting frequency as possible, without creating instability. The natural frequency of a simple spring mass system can be calculated from the mass (m) and the spring rate (k) of the system.
A good isolator system has two components, a spring to support the load and a damping element to dissipate the input energy. The amount of damping has an impact on the transmissibility which is the ratio of output response to input excitation. Figure 1 shows a typical transmissibility curve. If the ratio is 0.2, that means the isolator can dampen or dissipate 80% of the input energy. If the input excitation frequency matches the natural frequency, resonance occurs causing amplification (transmissibility ratio >1) of response vibration and if unchecked, can lead to system destruction.
Some guidelines to adopt while choosing an isolator are:
· Use air springs for low frequency problems on the order of just a few Hz.
· Wire rope isolators (this is ideal) when the operating environment involves combined shock and vibration. This is due to the fact that they offer large deflections in relation to their size. Also they are not affected by temperature extremes and resist attack by solvents, chemicals ozone etc.
· Elastomeric isolators offer economical solutions if only vibration isolation is needed.
The performance characteristics of typical isolators are outlined in Figure 2. The enclosure's exterior frame or shell should be of sufficiently rigid construction so as to withstand the shock pulse without any buckling or distension. The mating parts of the frame should be bolted or welded together in a manner so as to lend the structural integrity to withstand vibration.
The key components of an electronic enclosure that need isolation are the card cage with circuit cards, backplanes, disk drives and power supplies. Since more often than not, both shock and vibration are prevalent in the operating environment, a design incorporating coil rope isolators ensures optimal performance. A number of established vendors such as IDC, and Enidine specialize in providing standard off-the-shelf as well as custom-engineered coil isolators. These are coils of stainless-steel braided wire constructed to carry a load. When deflected, friction between the wire strands produces the damping. Varying the size, shape, construction and the number of coils all affect the spring rate and damping, as does mounting orientation and whether it is in tension or compression.
Figure 3 shows a 12R2 enclosure incorporating coil isolators that support a platform. The platform houses the card cage, disk drives and power supply. The sway space (due to displacement) required in all three axes should be calculated based on the total mass to be isolated, shock spring rates for the isolators in all three axes and the magnitude of the input shock pulse.
As mentioned earlier, any COTS enclosure should be designed and preferably tested to meet the MIL Standards relating to system performance under shock and vibration. The applicable tests are:
· MIL-S-901D (Navy) - High impact shock testing for shipboard machinery and equipment
· MIL-STD-810E - Environmental shock and vibration
· MIL-STD-167 - Shipboard vibration
In order to have a box that is modular and yet incorporates all of the above, a new design would be to use ruggedized side plates in pre-configured sizes with spot welded Aluminum extrusions that lend structural integrity to the frame and provides high precision contact surface for reinforcing panels. An example is the 12R2 - 9U unit, VME chassis shown below. <Photo 1>
Electromagnetic Compatibility (EM)
Electromagnetic Interference (EMI) is of paramount importance, especially in a military environment with mission critical applications where spurious emissions can seriously impair a system's integrity. Therefore, shielding for electromagnetic compatibility requires a step-by-step approach that addresses immunity (susceptibility) and emissions, both conducted as well as radiated. When an enclosure is packaged properly such that it is resistant to EMI from neighboring devices, it has achieved immunity. When it does not exceed certain levels of radiated energy to neighboring devices (mandated by applicable standards), it has controlled its emissions.
A design program for equipment that must meet both an emission and an immunity requirement consists of:
· Suppression: Reducing the interference at its source.
· Isolation: Isolating the offending circuits by filtering, grounding and shielding.
· Desensitization: Increasing the immunity of any susceptible circuits.
Some of the most effective and commonly employed design techniques for enhancing the shielding effectiveness (dB)<<need an explanation of this>> of an enclosure are:
· Integrating shielding gaskets into the enclosure design to provide a continuous conductive/ground contact surface for all removable panels thereby restricting the aperture size of any seams. These gaskets should be selected based on their attenuation characteristics, RF impedance, material compatibility (for corrosion control), compression range, compressibility and environmental sealing. Figure 4 illustrates the design of the 12R2 enclosure sides featuring extruded sections with channels to accept gasket material. The gasket is a braided wire mesh with elastomer core in a compression configuration under a flat panel. Closely spaced screws preserve the shielding effectiveness of the enclosure.
· Incorporating line filters that provide a high degree of attenuation (insertion loss) to conducted common mode (line to ground) and differential mode (line to line) emissions and meets MIL-STD-461D.
· Integrating honeycomb's EMC filters on all air intake and air exhaust openings. These are constructed of small wave-guides in parallel and produces a material that is approximately 97% open area. Providing the best airflow and attenuation (up to 110dB) characteristics. Figure 5 illustrates the rear panel assembly of the 12R2 enclosure with fans and honeycomb filters.
· Using high-grade power supplies that meet or exceed the conducted emissions, typically FCC part 15 class B, EN55022 class B or CISPR 22 class B.
· Employ proper grounding techniques and adequate number of ground points.
· Careful selection of components like fans, power on/off switches, indicator LEDs etc. based on their shielding effectiveness or compliance to MIL-STD-461D.
· Using shielded connectors and cable assemblies for I/O cabling requirements.
· Routing of primary AC wiring away from the DC wiring inside the enclosure and keeping the ground wires to minimum length.
Notwithstanding the type of shielding panel or material used, its effectiveness depends upon how well it is sealed into the enclosure, i.e. the quality of the RF bond across the seam. An access or ventilation panel that is not adequately grounded to the enclosure will often behave as an antenna. Grounding the panel at only one point will generally prevent it from acting as an antenna, but it will not eliminate leakage from the rest of the seam. Thus, the attenuation can be limited to 20dB instead of 80 to 100dB.
MIL-STD-461D establishes in detail the design requirements for emissions and susceptibility, both conducted and radiated, under a variety of operating environments.
MIL-STD-462D establishes the test methods to be used for measurement of EMI characteristics of the equipment as required by MIL-STD-461D.
By virtue of incorporating all of the key EMC design elements into a packaging approach that allows modularity and replaceability, consistent shielding effectiveness is ensured.
Thermal Management
Thermal management of packaging has become increasingly critical, not to mention challenging, given the rapid escalation in component densities and power hungry electronics coupled with the demand for smaller enclosure sizes. Moreover, to withstand harsh environments, the use of dust and air filters, honeycomb EMC filters etc. further limits the air intake and air exhaust openings.
The use of forced air convection methods are still predominant in rugged enclosures. The selection of appropriate fans is an important step to ensure adequate cooling.
In order to calculate the amount of airflow (Q) in CFM needed to cool the enclosure, the total amount of heat to be dissipated (W) and the allowable temperature rise (T) in degree centigrade should be known. From the formula Q = (1.76 x W)/T, the amount of volumetric airflow required under free air conditions is calculated without accounting for pressure losses.
The selection of the fans should be based on the volume of air they are capable of moving under static pressure rather than in free air conditions. This is due to the fact that typical electronic enclosures experience a static pressure of 0.15" of water caused by resistance to airflow. Figure 6 shows the airflow characteristics of a 6" DC fan from Rotron that is used in a 12R2 enclosure. Another important factor is the ability of the fan to operate under a wide temperature range of -10 to +70 <<degree>>C at a minimum.
The location of the fans (Intake vs. Exhaust) must also be evaluated before finalizing the design. If the fans are exhausting air, then proper baffles and <<plenums>> should be designed to direct the airflow over the heat dissipating circuit cards and minimize leakage. Removable and washable dust and air filters should be used to protect the electronics inside. This will also serve to increase the life of the fan.
The use of fans with locked rotor output or tachometer output is recommended to enable monitoring the fan status and provide fan fail indication via a red LED located on the front panel. Similarly, overtemp monitoring can be facilitated by using temperature sensors at critical exhaust points. These guidelines will contribute significantly to an efficient thermal management scheme.
By selecting a high volume fan that has superior performance under high static pressure conditions, compared to the traditional commercial muffin fan solutions, the system achieves optimal performance under hostile operating conditions.
I/O Cabling
I/O cabling is a key component for almost all enclosures, enabling them to interact with a variety of equipment. The number of cables, the type of connectors, the bend radius required and the routing must all be analyzed and evaluated before the design begins.
Cable channels or clamps should be provided to manage the cable bundle and keep it from interfering with the air flow and access to internal components. A patch panel approach to mounting of the connectors on the rear panel will lend tremendous flexibility to the cabling configuration while at the same time, facilitating easy maintenance. Figures 3 and 5 illustrate this concept on a 12R2 rear panel. Optimal spacing of the connectors by adhering to human engineering aspects of enclosure design ensures easy access and repairability. As mentioned previously under EMC, the connectors patch panel and cabling should all be adequately shielded.
Operation under Harsh Environmen :
Salt fog, fungus and moisture in environments can have corrosive effects on the enclosure over a period of time. This is prevalent in ship board and coastal installations and necessitates the use of the following:
· Protective corrosion resistant film over the metallic parts like chemical conversion coatings per MIL-C-5541E.
· Stainless steel hardware for assembly.
· Conformal coatings over PCB material like backplanes etc.
· Galvanic compatibility of materials in contact with each other.
· Hermetically sealed power switches, reset switches and other actuators.
This is one area where traditional methods continue to provide good solutions.
Reliability and Maintainability
Reliability of the system is of paramount concern owing to the mission critical nature of <<the>> application. Mean Time Between Failures (MTBF) and Mean Time To Repair (MTTR) the system are measures of a system's reliability. The MTBF of the system is dependent on the failure rate of its components (MTBF (hrs) = 1/ Failure rate per million hrs).
Therefore, the choice of major components like power supply, fans, alarm sensors and circuit cards should be predicated on their failure rates among other factors. There should be empirical or analytical data to support these numbers as a minimum. The 12R2 model incorporates Vicor power supplies and Rotron fans which have low failure rates while operating under adverse conditions of temperature, shock, vibration and humidity. Using the reliability data of the components, the reliability of the system can be calculated per MIL-HDBK-217F in ground benign, ground fixed or naval sheltered environments at a specified temperature.
Another key consideration for packaging is maintainability. The major components like power supply, fans and media disk drives should be located so that they are easily accessible for repair or replacement. The air filter should be located such that it can be removed for cleaning without opening any panels or doors. The fact that the circuit cards be easily accessible through a door need hardly be emphasized.
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