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.
