Envelope Detection

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 typically has an amplitude-modulating effect on the vibration signal of the bearing. You thus can use the envelope detection technique to perform fault detection on rolling-element bearings.

You can use an accelerometer and a tachometer to measure the vibration signal and the rotational speed, respectively, of a rolling-element bearing. 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 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.

A fault on a rolling-element bearing generates a low-level impulse every time the local fault contacts another part of the bearing. 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 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 lower frequency of the band range is typically greater than ten times the rotational speed in order to eliminate common harmonics of the rotational speed. The upper frequency of the band range is usually sixty times the outer race frequency or two hundred times the rotational speed in order to attenuate high frequency noise and vibration components. 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 usually use the results of envelope detection in trend analysis. The trend of the magnitude of faulty components can indicate the condition of the bearing. You must use the same transducers mounted on the same location to ensure consistent measurement of the vibration signal and the rotational speed. Trend analysis is ideal for early fault detection because the magnitude of the faulty component might decrease when a fault becomes severe.