Understanding Demodulation (Envelope Analysis)

Definition: What is Demodulation?

In the context of vibration analysis, demodulation is a powerful signal processing technique used to detect repetitive, low-frequency impacts that are “hidden” within a machine’s high-frequency vibration signal. It is the core process behind the more commonly known term, Envelope Analysis. The two terms are often used interchangeably.

The technique works by isolating a high-frequency band of vibration, which acts as a “carrier” signal, and then extracting the “envelope” of that signal. This envelope reveals the underlying low-frequency patterns of repeating impacts, such as those generated by microscopic faults in bearings or gears.

The Process of Demodulation

Demodulation involves a three-step process:

1. Band-Pass Filtering: First, the raw vibration signal is passed through a high-frequency band-pass filter. This removes the strong, low-frequency vibrations (like unbalance and misalignment) and focuses only on a high-frequency region where the stress waves from bearing or gear impacts excite structural resonances.
2. Rectification: The filtered, high-frequency signal is then rectified. This process flips the negative portion of the waveform to positive, essentially creating a signal that represents the absolute amplitude.
3. Low-Pass Filtering (Enveloping): Finally, this rectified signal is passed through a low-pass filter. This smooths out the high-frequency resonant “carrier” signal, leaving behind only the low-frequency “envelope” that traces the peaks of the rectified signal. This resulting envelope signal directly represents the repetition rate of the underlying impacts.

An FFT is then performed on this final envelope signal. The resulting spectrum, called the envelope spectrum or demodulated spectrum, shows clear peaks at the exact fault frequencies of the bearing or gear components.

Why is Demodulation So Effective?

Demodulation is one of the most important techniques for early fault detection because of how it handles impact signals.

• Early Warning: When a tiny spall on a bearing race is struck by a rolling element, it produces a small, low-energy impact. This impact causes a very brief, high-frequency burst of vibration as the machine structure “rings” at its natural frequencies.
• Separating the Signal from the Noise: In a normal FFT spectrum, the tiny amount of energy from these early-stage impacts is completely buried by the massive energy from low-frequency vibrations like unbalance.
• Focusing on the Repetition Rate: Demodulation ignores the powerful low-frequency signals. It focuses on the high-frequency “ringing” and, most importantly, the *repetition rate* of that ringing. It is this repetition rate that directly corresponds to the bearing or gear fault frequencies (e.g., BPFO, BPFI, BSF, GMF).

Applications

The primary applications for demodulation are:

• Rolling-Element Bearing Analysis: It is the definitive method for detecting and diagnosing faults in ball and roller bearings, often providing a warning months before the fault becomes critical.
• Gearbox Analysis: It is highly effective at detecting issues like cracked or broken gear teeth, which generate a clear impact signal at 1X the gear’s rotational speed in the demodulated spectrum.
• Other Impacting Events: It can also be used to detect other repetitive impacting phenomena, such as steam traps opening and closing or reciprocating engine valve timing issues.

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