Designers should carefully consider the alternatives for shock and vibration protection of military electronics

Aug. 1, 1999
Today`s trend of using commercially available state-of-the-art electronics for military control systems is pervasive from airborne avionics to shipboard weaponry. It enables designers to integrate the latest technology that not only enhances their systems, but also adds smaller, lighter — and more fragile — electronic components.

By Thomas Miller

Today`s trend of using commercially available state-of-the-art electronics for military control systems is pervasive from airborne avionics to shipboard weaponry. It enables designers to integrate the latest technology that not only enhances their systems, but also adds smaller, lighter — and more fragile — electronic components.

Better known as COTS — short for commercial off the shelf — this initiative provides more choices to the design engineer. But with it are greater shock and vibration challenges because COTS products most often are noticeably more breakable than are mil-standard parts.

To get around this problem, designers often use standard shock and vibration products such as wire rope isolators, elastomeric mounts, and hydraulic dampers. But designers of consoles and enclosures should carefully analyze their applications, understand the strengths and weaknesses of each, and realize that in the final analysis a hybrid solution may be the best choice.

Designers must deal with a wide range of conditions when isolating equipment against shock and vibration. For example, designers of naval applications today are realizing that ship decks are much less rigid than past designs.

Traditionally, systems designers subjected equipment on decks to MIL-STD-901D, Class I, heavyweight Navy FSP test — 25 Hz (20 ms 1/2 sine pulse) 60 G`s peak. Today, most experts consider this to be a `hard deck,` but designers now realize that decks can be much softer (14 Hz, 22 G`s). Vessels such as mine hunters may have even lower deck frequencies (7 Hz range). Designers must take these new `soft deck` characteristics into account.

It is easy to see that using traditional, straightforward isolation components such as elastomeric mounts or wire rope isolators is not always the best answer. Instead, the designer needs to understand the true isolation benefits of several component families and select or combine the best elements for his situation (see table).

Military systems today range from the simple and rugged to the complex and delicate. The following examples offer today`s design engineer insight into some of the most practical approaches to isolating military and aerospace electronic equipment.

HUMVEE-mounted controls

Designers of vehicle electronics for platforms such as the Avenger and LOSAT typically package COTS electronic equipment in boxes. They mount these boxes directly to the vehicle, usually over the wheel-well area. In these and similar cases, wire rope isolators provide solid support and shock/vibration isolation for good reasons. They are rugged, accommodate multi-axis inputs, work in extreme temperature conditions, are non- corrosive, and offer long, maintenance-free service life. Wire rope isolators are capable of dealing with MIL-STD- 810 for on- and off-road shock conditions as well as MIL-STD-167 vibration inputs.

Missile control electronics

The logistics of the Army Tactical Missile System, or ATACMS, involve frequent transportation and handling. This poses a risk of bumping or dropping the missile canister and breaking some of its electronic components. Rugged, multi-axis elastomeric skid isolators can shock-isolate the missile`s electronics even when subjected to a severe rail impact test at an impact velocity of 5 feet per second.

These isolators use Enitemp IV, a unique elastomer that provides effective damping at very low, but realistic field combat operating temperatures. This is a real and tangible electronic system survivability benefit.

Navy Q70 control systems

This group of operator-interface consoles and controllers in cabinet enclosures will control the heart of numerous naval ship operating functions for many years to come.

Enclosures, for example, will use COTS equipment extensively and must accommodate multi-axis shock (MIL-STD-901D), multi- axis vibration (MIL-STD-167) and attenuation of high-frequency noise. The ship deck conditions range from 25 Hz 60 G`s `hard deck` to 14 Hz 22 G`s `soft deck`. The isolation system, of course, must use a minimal amount of space.

This is an example of how a designer of enclosures might need to combine several shock and vibration components. Assuming that a designer mounts an electronic inner cabinet within an outer enclosure, the designer could configure an innovative system-level solution by using a hybrid of hydraulic, elastomeric, and wire rope components. Below are some examples.

Vertical Axis — suspend the inner cabinet from the outer cabinet with preloaded shock-isolation devices. These self-centering hydraulic dampers can provide efficient viscous damping, stroke in recoil, and rebound directions. They also are preloaded with mechanical springs to support the inner cabinet (normally at the 2-to-4-G level). This element is at the heart of the isolation system and typically reduces input G`s by more than 80 percent. Additionally, spherical rod end bearings at either end of the damper often use an elastomeric interface to inhibit high-frequency noise transmission through to the outer cabinet.

Lateral Axes — maximum shock inputs are normally in the 30-G peak range and may be in the front, back, or middle of a ship. A wire rope isolator is an ideal component to absorb such shock input. Designers can easily mount this multi-axis unit on a slide arrangement or with rollers to accommodate the inner cabinet vertical motion relative to the outer cabinet. They can enhance this approach with an elastomeric isolation barrier deal effectively with vibration and noise transmission.

Engineers should consider several criteria when evaluating an application for shock and vibration isolation. Designers should consider this checklist before beginning a project design using COTS equipment:

- determine the system dynamic performance criteria to determine what they expect their shock and vibration components to do (military or other standards in all axes);

- determine the shock and vibration inputs to the system (e.g. pulse, duration, G level);

- identify the environmental conditions that will influence performance (e.g. salt, temperature, altitude, fungus, ozone, and chemicals);

- determine the maximum shock that electronic components being isolated can handle;

- determine if high-frequency vibration (noise) requires attenuation;

- determine how the electronics are mounted within the system; and

- take note of what space is available for isolation components in the application.

The COTS initiative adds flexibility and complexity to the job of the design engineer. By carefully identifying the parameters of the application, and understanding the benefits and limitations of each component, design engineers will have the information they need to consult with a shock and vibration specialist who can help them identify the best solution. Whether it is a standard component or a hybrid product, consult with a supplier who is willing to understand the specifics of your application and work with you to develop a flexible solution to suit all of your application requirements.

Tom Miller is the aerospace business development manager of Enidine Inc. in Buffalo, N.Y. He joined Enidine as a sales manager in 1983 and has more than 25 years of experience in aerospace and defense engineering, sales, and management. His primary responsibilities today include working with customers to develop custom shock and vibration products to help military electronic systems withstand harsh shock and vibration environment

Traditional shock and vibration isolation components

- Component

Hydraulic damper (shock)

- Benefits

Most efficient damping Optional spring provides preload Can be self centering

- Limitations

Unit length Single axis operation

- Component

Elastomeric isolator (shock, vibration, and high frequency-noise)

- Benefits

Multi-axis damping Low profile Various shapes

- Limitations

Operating temperature range Operational life

- Component

Wire rope isolator (shock and vibration)

- Benefits

Multi-axis performance Wide temperature operational range Long operational life

- Limitations

Unit size Weight

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