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2. Measurements and analysis of diagnostic signals.

Question 2.2.

Why when you measure the envelope spectrum of high frequency vibration you preliminary extract only part of the high frequency signal by a band pass filter, but do not use all the high frequency components that could be extracted by high frequency pass filter?
The question was asked by "SKF" specialists.

The question is answered by Natalia A. Barkova:

The frequency band pass filter that extracts the high frequency vibration before the spectrum analysis of its envelope is used to decrease the distortion of the form of the vibration modulation process. Such distortion appears in the following main cases:
 

  • When a strong harmonic component appears in the high frequency vibration, extracted to form the envelope and its subsequent spectral analysis. This leads to decrease of the total signal power oscillation and decrease of the real high frequency vibration modulation depth evaluation.

  • When two harmonic components appear in the high frequency vibration, extracted to form the spectrum of its envelope. It leads to additional oscillation of the power of the total signal with the frequency of beats (with the difference frequency of these two components).

  • When several harmonic components appear in the high frequency vibration extracted to form the spectrum of its envelope. It leads to even more complicated additional oscillations of the total signal with the frequencies of beats (with the difference frequencies of these components).

  • When the resonances appear in the high frequency vibration, extracted to form the spectrum of its envelope. It leads to redistribution of the spectrum of enveloped signal and the low frequency components of the modulation process increase, because the more high frequency components (similar to side components in the modulated harmonic signal) do not coincide by the frequency with the resonance frequency band.

  • When there are large (more than 10 dB) differences of the spectral density of high frequency vibration in the frequency band, extracted to form the spectrum of its envelope. It also leads to redistribution of the spectrum of enveloped signal as it was indicated in the previous paragraph.


Because of these reasons the frequency band pass filter is used. Its bandwidth must be sufficient to measure without any distortions the modulation process in the necessary for diagnostics frequency band, viz. any harmonic components, resonances and differences of spectral density in the selected frequency band must not exceed 6-10 dB.

 

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