Section 7: Advanced Operation
101
Demodulation
A useful technique for measuring and analyzing data is
demodulation. The demodulation process looks for repetitive
patterns created by impact events that lie embedded within the time
waveform. The process works by extracting the low amplitude, high
frequency impact signals and then tracing an 'envelope' around
these signals to identify them as repetitions of the same fault. The
resulting spectrum, with the low frequency data removed, will now
clearly show the high frequency impact signals and harmonics.
Demodulation is also useful as an early warning device, as it detects
bearing tones before they are visible in a normal spectrum.
The high frequency signals that demodulation aims to extract are
very localized, therefore extra care must be taken to ensure the
accelerometer is setup correctly. Ensure that:
• The accelerometer is mounted close to the fault source.
• The accelerometer is well coupled, using either stud
mounting or a very strong magnet on bare metal. A handheld
probe or stinger is not recommended.
• The accelerometer mounting is consistent between visits. If
not, a trend plot of overall rms values will be meaningless.
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