Designing An Enclosure For Shock And Vibration

All electronic equipment needs to be able to withstand the physical demands of the environment in which it is to be used. And since applications range from office and industrial use to full military deployment, the design requirements can vary greatly. Typical shock values for industrial use might be in the 5G to 10G range whereas systems located on naval ships need to withstand shocks in access of 100G’s. Based on the application, material types and construction techniques can be employed to ensure the unit can withstand the environment.

Types of Shock and Vibration

A thorough understanding of the environment in which the equipment will need to survive is essential before the design for shock and vibration begins. Whether the equipment is mounted on a ground-based vehicle, airborne vehicle, shipboard mounted, or just needs to withstand seismic events, a ruggedized construction should be a primary design criteria.

Vibration can be either random or sinusoidal. An example of the former is the vibration that vehicle-mounted equipment experiences rolling across rough terrain. Sinusoidal vibration, on the other hand, is a continuous vibration like a rotating engine in a helicopter. A well-designed rugged chassis needs to protect against both types. To ensure this happens, a rugged chassis can undergo severe shock and vibration testing like MIL-STD 810F, MIL-STD-167, and MIL-STD 901D. Figure 1 shows an Elma Electronic Rugged COTS enclosure about to go under shock and vibration testing.

The main goal in handling shock and vibration is 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. In order to protect the equipment and its contents from potential damage a designer can use passive isolation systems such as elastomeric dampers, rope coil isolators and air springs.

Key Components

Isolators are critical elements used to reduce the effects of shock and vibration in embedded system platforms. 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 transmissibility, which is the ratio of output response to input excitation. 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.

Tips For Using Isolators

Air springs should be used for low-frequency applications on the order of just a few Hertz. For heavy shock and vibration, wire rope coil isolators are very effective. This is due to the fact that they offer large deflections in relation to their size. Also, rope coil isolators are not affected by temperature extremes and resist attack by solvents, chemicals ozone, and so forth. Elastomeric isolators offer economical solutions if only vibration isolation is needed.

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 rope coil isolators ensures optimal performance. These are coils of stainless-steel braided wire constructed to carry a load. When deflected, friction between the wire strands produces the damping effect. Varying the size, shape, construction and the number of coils all affect the spring rate and damping, as well as the mounting orientation and whether the coils are in tension or compression. Figure 2 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.

The Enclosure Frame

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.

In order to have a box that is modular and incorporates any required MIL standards, a key design feature would be to use ruggedized side plates in pre-configured sizes. Providing spot welded aluminum extrusions lends structural integrity to the frame and provides high precision contact surface for reinforcing panels. The result is a modular rugged COTS chassis design. This allows a wide range of sizes and configurations to be incorporated cost effectively and with short lead times. Further, if the base platform has been MIL-tested, you can feel confident that a customized version is built upon a proven core design.

MIL Standards

For Mil/Aero applications, the 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-810F – Environmental shock and vibration

· MIL-STD-167 - Shipboard vibration

Cabinet Enclosures

While Eurocard enclosures or similar embedded system platforms have various design solutions for shock and vibration, a cabinet enclosure has its own set of considerations. Since cabinet enclosures can stand 42U or higher, the focus shifts to limiting frame displacement. Also, the corner bezels and base must be ruggedized to provide a secure foundation.

A cabinet for seismic zones or rugged applications needs to have heavy-duty construction throughout. To prevent the frame from bending, the cabinet designer can run a back stiffener from one side of the frame to the other. Often, these stiffeners will be employed near the top of the cabinet, in the middle, and near the bottom. Next, you can add cross-bracing to protect from movement in different axis. The photo of the Elma Optima Seismic Cabinet in Figure 3 illustrates the use of these stiffeners and cross bracing. Another consideration is to have double walled construction where the frame has two layers of sheet metal for extra strength.

Welding and crimping all of the corner sockets and corner members also provides resistance to bending and vibrational stresses. The welding should also meet ANSI 329 and Bellcore TR-63 certification specifications. Double-cavity aluminum extrusions can also be used to strengthen interconnection points of the piece parts. Of course, it is important to also use vibration-resistant hardware and thicker gage steel. The welded base should be very strong with a thick steel up to 7 gauge. The rails should be thick as well—11-gauge steel can provide strong support and 10-gauge can be used for extra heavy-duty reinforcement.

Cabinets for rugged or Zone 4 seismic applications need to go through stringent testing. This includes shock loading, vibration test, shaker table, and more. Figure 4 shows an example of an Elma Optima Seismic cabinet under test. The shaker table test recorded results were obtained by simulating Zone 4 loading on a model with a 600 lb. weight, distributed so the center of gravity was 4" above center, to match the actual test conditions. This simulation verifies the actual shaker table-test findings, and reveals that the stress level was less than 60% of the allowable maximum amount. Modeling can also show levels of stresses under test. A vibrations test has the cabinet subjected to Seismic Zone 4 vibration loading in the “X” (side to side) and the “Y” (front to rear) directions. Colors in the modeling, ranging from blue to red, indicate increasing stress.

Conclusion

There are many elements to designing an enclosure for shock and vibration. Once the specific application environment has been analyzed, the enclosure designer can incorporate a wide range of options. These include isolators, stiffeners, braces, heavy-duty materials, and more. Thorough testing for shock and vibration is an important element. Finally, it is important to remember that while you are resolving one problem (a solution for shock and vibration), you do not impede the performance in another area (airflow, EMC, etc.). It’s best to work with a knowledgeable vendor with experience in a wide range of enclosure solutions.

Justin Moll is Director of Marketing at Elma Electronic Inc. You can reach him by phone at 510-490-7388 x516.