vbSeries Demodulation Explained
The demodulation process looks for repetitive patterns created byimpact events that lie embedded within a 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 set up correctly. Ensure that:
- The accelerometer is mounted close to the signal 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 otherwise trend plots of overall RMS values will be meaningless.
The bandwidth you select determines which range of frequencies the demodulation process will use. We recommend you use the following procedure to determine an appropriate bandwidth.
- First take a 'quick snapshot' spectrum measurement. From the Main Menu press
Measure then press 
Spectrum Waveform. - Press
to create a new parameter set then set the Store Units to Acceleration and Fmax to 20 000 Hz (1200 kCPM). - Press
to begin measuring. Examine the chart for a region of broadband high frequency noise or 'haystack'. Such a haystack might look like the one below. - When you set up your demodulation measurement select a bandwidth which includes the haystack but excludes any large individual peaks such as those shown to the left of the haystack in the example diagram.
It is very important that all the large-amplitude/low-frequency peaks are rejected. If there are no significant haystacks displayed, we recommend a standard bandwidth of (2 to 10) kHz (120 to 600 kCPM. When you have determined an appropriate bandwidth proceed with the demodulation measurement.
Analyzing Demodulation Data
Measuring, recording and viewing data is exactly the same as for a Frequency/Acceleration spectrum. The demodulation spectra can also be transferred to Ascent and analyzed using the same techniques as for normal spectra e.g. trend plots, alarms, fault frequencies, bearings and gears. The following are points to keep in mind when analyzing demodulation data:
- Bearing tones appearing in the demodulated spectrum but not in the normal spectrum are a good early warning indicator. This often points to insufficient lubrication.
- As the bearing deteriorates the baseline noise level increases and bearing tones develop running speed sidebands.
- In the final stages of bearing wear the bearing tones may become less prominent as cracks and pits become more rounded and distributed over the race.
Last Reviewed: 11/Jun/2009 Source: vbSeries Reference Guide Revision 24 March 2009 - pages 66 - 68