SPM leonova emerald - Page 138

E
E:16 Vibration measurement
FFT type
FourdifferentFFTtypesFFTtypes(1)canbeselected:
‘Linear spectrum’ consists of RMS values on a linear
scale. Each value is the equivalent of the RMS value of
a sinusoidal signal of the correspondent frequency. If
the spectrum unit is velocity, v, the amplitude scale is
accordingly: v
rms
.
‘Amplitude spectrum’ is closely related to the Linear
spectrum scale. The Amplitude spectrum shows the
peak value instead of the RMS value. If the spectrum
unit is velocity, v, the amplitude scale is accordingly:
v
peak
.
‘Powerspectrum’(Autopower).Eachvalueistheequiv-
alent of the RMS square value of a sinusoidal signal of
the correspondent frequency. If the spectrum unit is
velocity, v, the amplitude scale is accordingly: v
2
rms
.
‘PowerSpectralDensity’(PSD).Thisisapowerspec-
trum where each amplitude value is divided by the FFT line width times
the used window’s equivalent noise band width. If the spectrum unit is
velocity, v, the amplitude scale is accordingly: v
2
rms
/Hz.
As general advice, use:
- Linear or Power spectrum on periodic signals.
- PSDspectrumonnon-periodic(noise)signals.
Number of spectrum lines
Thenumberofspectrumlines(2)affectstheresolutionandthemeas-
uring time. Doubling the number of lines also doubles the measuring
time. In cases where different fault symptoms, such as bearing frequen-
cies and multiples of 1X, are close together, a high resolution spectrum
is preferred.
Spectrum to save
Savingatimerecordorafullspectrumrequiresmorememory.Unless
the spectrum lines in damage patterns have fairly high amplitudes, the
pattern is not visible.
Thus, saving only peaks will preserve the essential data while reducing
the amount of redundant data. A peak is a spectrum line that has a line
with a lower amplitude on either side.
Ifyouselect‘Peaks’under‘Save’(3),Leonovawilldisplaythefull
spectrum before saving, with the peaks as black lines while the lines to
be deleted are grey. The maximum number of peaks that can be saved
is half the number of spectrum lines. The number of peaks to be saved
(4)isinputonthenumberpad.
1
2
3
1
3
2
4