High Frequency Acoustic Emission (HF-AE)

happyman

The High-Frequency Acoustic-Emission (HF-AE) transducer is a very wide band piezo accelerometer which responds at frequencies up to several MHz.

Every time a defect in a bearing is over-rolled it produces  short impact type of pulse which is repeated at a fixed frequency determined by the geometry of the bearing and  its speed of rotation. The signal produced by the defect is therefore a train of pulses, which has as its Fourier transform another train of pulses. The frequency content of each pulse is very wide, but filtering in the 250 kHz to 350 kHz band reduces the amplitude of the pulses and also removes much of the “noise”. Although the pulses are reduced in energy, they still occur at the same intervals the repetition frequency of the defect signal is not changed by filtering.

The basis of the newly developed technique is very similar to enveloping – a monitoring technique well reported in the literature. The major differences with the new technique are the frequency range being treated (around 300 kHz), well away from the structural resonances, and the relatively largedistance between the frequency range of the enveloped low frequency signal and the frequency band (carrier) from whichthe information is derived.

happyman

The processed signal coming from the HF-AE monitor is a low frequency (0-20 kHz) rectified signal and can be processed by standard analysis equipment in either an analog or digital way. RMS value, peak value and Kurtosis can all be calculated.