This brief write-up is intended to explain and to take some of the mystery out of the Predictive Maintenance (PdM) analyst's tool called "Demodulation".
First, let's take a look at the hypothetical time waveform as shown below. Note that in addition to the normal low frequency of the running speed of the machine that there are repetitive noise bursts occurring that might indicate bearing issues. Now we are presented with the challenge to separate out that information.
This is where we introduce the term "Envelope Demodulation", sometimes also referred to as "High Frequency Enveloping". The Demodulation process looks for the repetitive patterns created by impact events that lie embedded within the time waveform. The PdM instrument will then display a demodulated frequency spectrum that will greatly emphasise these fault frequencies. But how does it work?
The first step in the process is to apply a high-pass filter that removes the large low-frequency component(s). The illustration below gives an idea of what the FFT analyzer then "hears" - analogous to the tapping on the water-filled glass:
Next, we "rectify" the signal so that all negative-going pulses are made positive, and we trace an "envelope" around the positive-going bursts in the waveform to identify the impact events as repetitions of the same fault. The resulting waveform appears as shown below.
And now this enveloped signal is passed through the FFT process to obtain a frequency spectrum. In this example, the signal now clearly presents the BPFO peaks (and harmonics) as we first hoped for.
After the work has been done by the instrumentation, as we have illustrated in the step-by-step process, then the questions become: "What do I do with this information and how am I to interpret it?"
Think of "Demodulation" data to be as an "early warning" indicator. Look for peaks at known bearing fault frequencies in both the normal vibration spectra and the demodulated spectra. Below are some general guidelines for action:
1. No peaks in either spectrum: Condition is good, use as a baseline for future comparisons. 2. Peaks appear in Demod only: Early warning indication that defects exist (or the bearing needs lubrication). 3. Peaks appear in Normal and Demod spectra: Plan replacement at next maintenance period. 4. Peaks appear in Normal spectra only, combined with a rise in the Demod noise floor: Replace the bearing now!