Page 26 - Shock and Vibration Overview

Equipment Setup

1) Ground Loops
Unwanted differences in the electrical potential between the sensor and the
instrumentation will result in erroneous DC offsets and voltage drops. The entire
measurement chain should be grounded at only one point (which is the very purpose
of grounding!). To prevent these ground loops there should be sufficient insulation
between the accelerometer body and ground. Unfortunately this insulation may
mechanically dampen the coupling of the accelerometer which will also influence
vibration data. Another way to mitigate this issue is to shorten the cable length as
much as possible.

2) Electromagnetic Noise
In particular EMI laden environments (like those found on surface ships)
electromagnetic noise can greatly sully the data quality. Again, the best practice is to
avoid long cable lengths where possible; but sometimes special cabling may be

needed that is has reinforced shielding to offer EMI protection. This of course comes
with an added price tag; but of only a few hundred dollars in most instances.

3) Mechanical Noise
Cable motion will literally induce mechanical strain on the accelerometer which the

accelerometer will measure and erroneously report as vibration in your structure.
This strain and acceleration will influence the data quality of the sensor as it tries to
measure the vibrating structure directly without interference. To prevent this motion
which causes erroneous data the cable should be secured as close as possible to the
accelerometer. O-rings, cable clamps, and adhesive cable clamps are typically used.

4) Risk of Losing Connection
The last challenge wiring presents is the possibility that you may lose connection
during a test. So strain relieving your cabling is very important to prevent mechanical
noise but also to help ensure connection is not lost. In large setups there can be a
concern about wire mismatching where the engineer may think he/she is analyzing
data from one accelerometer when in fact he/she may be looking at data from a
completely different area of the structure. It’s very important to double check that all

accelerometers are properly wired, labeled, and strain relieved to prevent losing or
confusing data sources.

MEGGITT offers a nice resource on sensor wiring and cabling that is worth a read.

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