Fault Detection (Sound and Vibration Measurement Suite)
December 2007
NI Part Number:
372416A-01
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This book discusses how to use the envelope detection technique to perform fault detection on rotating machinery.
Envelope Detection
Envelope detection is a technique to extract the modulating signal, or envelope signal, from an amplitude-modulated signal. You can use the envelope detection technique to identify mechanical faults that have an amplitude-modulating effect on the vibration signal of a machine. The power spectrum of the envelope signal exhibits peaks that can reveal the source of faults.
A fault on a rolling-element bearing or gear tooth typically has an amplitude-modulating effect on the vibration signal of the bearing or gear. You thus can use the envelope detection technique to perform fault detection on rolling-element bearings or gears.
Measuring a Vibration Signal for Envelope Detection
You can use an accelerometer and a tachometer to measure the vibration signal and the rotational speed, respectively, of a rolling-element bearing or gear. Choose a sampling rate that is at least 2.56 times higher than the highest frequency component of the signal of interest. Also choose an accelerometer with a high cutoff frequency to cover the frequency band that the envelope detection technique uses. Then place the accelerometer close to the bearing or gearbox so the distance between the fault location and the accelerometer does not affect the signal transmission. When the accelerometer is far away from the fault location, the envelope detection technique might fail.
Extracting an Envelope Signal
A fault on a rolling-element bearing or gear tooth generates a low-level impulse every time the local fault contacts another part of the bearing or gear. This low-level impulse has an amplitude-modulating effect on the vibration signal. The modulating effect spreads over a wide frequency range because the impulse has a short period. In the low frequency band of the vibration signal, imbalance, misalignment, or mechanical looseness of the bearing or gear might generate vibration signals that overwhelm the low-level impulses. Thus the envelope detection technique focuses on a narrow band range in the high frequency band, which is useful for detecting the low-level impulses that are below the noise level in the normal spectrum. The envelope detection technique involves the following three steps: shifting the narrow band range in the high frequency band to the base band, filtering the frequency-shifted signal using a lowpass filter, and calculating the envelope signal of the lowpass-filtered signal.
You can use the OAT Envelope Detection VI to perform envelope detection on a rolling-element bearing or gear when the rotational speed is constant or when the rotational speed varies. You then can use the SVFA Power Spectrum VI or the OAT Order Power Spectrum VI to display the power spectrum of the resulting envelope signal.