Isolators: Considerations for Shock and Vibration Requirements
Isolation systems or isolators can be used to:
Reduce the effects of externally generated shock and vibration energy from negatively affecting the performance of electronic equipment
Reduce the amount of structure-borne noise transmitted from electronic equipment into the surrounding structure or ship’s hull
Many isolators fall into the latter category, and while they may reduce transfer of vibration energy between the enclosure and the surrounding structure, they are not suitable for meeting the strict performance requirements called out in MIL-S-901D (Shock) and MIL-STD-167 (Vibration).
There are a number of isolator options available with isolators that vary in performance, material, size, durability and cost. A brief discussion of each of these options will allow customers to select the best-value isolation system.
Performance — An isolation system intended to reduce the effects of externally generated shock and vibration energy from negatively affecting the performance of electronic equipment must comply with the requirements of MIL-S-901D (Shock) and MIL-STD-167 (Vibration). Selection of an effective isolation system depends on the equipment payload weight, the vibration deck frequency of interest and the degree of attenuation or dampening desired.
Payload Weight — The isolation system must be able to support the weight of the internal payload and the weight of the enclosure, including any mounting hardware and I/O cabling, in both a static condition and a dynamic shock/vibration loading condition. As an enclosure experiences shock/vibration loading forces, the isolation system will be subjected to both compression forces and tension forces. The isolation system must not experience excessive or permanent deformation under either compression or tension loading. Some isolation systems, especially those constructed from thinner designs using composite materials, have been found to collapse or fold in upon themselves under compression loading conditions.
Deck Frequency — Any isolation system must have a natural resonant frequency greater than the deck frequency of interest to prevent the enclosure and isolation system from going into resonance. Isolation systems are also effective at isolating vibratory energies over fairly limited frequency ranges. They are most effective at a particular target or deck frequency, but this effectiveness (and hence the degree of protection) diminishes as the externally imposed excitation frequency is increased or decreased relative to the frequency of optimal isolation.
Since no single isolator can perform adequately at all frequencies of interest, the choice of isolators must be adjusted or tuned for the specific intended application. This tuned isolation system will provide some protection for excitation frequencies somewhat above or below the target frequency, but this protection drops off quickly the more the excitation frequency differs from the target/deck frequency of interest.
Never allow an isolator vendor to convince you their system works at all frequencies. This is a physical impossibility. Understand what your target or deck frequency requirements are and choose an isolation system that is tuned accordingly.
Attenuation / Dampening — The degree of attenuation or dampening of the excitation energy required will depend on the vulnerability of the electronic equipment to shock & vibration. Many types of electronic equipment, especially those designed for military or rugged applications, can withstand 70-80 G’s of acceleration without difficulty.
However, much of the Commercial-Off-The-Shelf (COTS) electronic equipment in wide use in Defense applications requires more protection. The vast majority of COTS electronics can function effectively with G-loads in the 30-40 Gs range. Typical attenuation requirements call for isolation systems to ensure that COTS electronics see no more than 20-30 G’s of acceleration during testing.
There has been some discussion and experimentation regarding development of isolation systems that could provide protection down to 15 G’s or even 10 G’s of acceleration. This research has shown that striving for 10 G’s of acceleration exceeds the theoretical limits of performance for today’s isolation system technologies and is extremely cost prohibitive. The important take away is that COTS electronics don’t need any such exotic or cost prohibitive isolation system as existing systems that deliver no more than 20-30 G’s of acceleration are more than adequate.