SV SLA-PA201, Manual, Page: 54 / 88

54
3. 1 Using the Velocity Graph or Displacement Graph to get a fit RPM
It is difficult that you find the fundamental frequency in the harmonic frequency of spectrum of
acceleration. The spectrum of rotational vibration with the fluctuation from 1610PRM to 1660RPM is
shown from [Fig. VI-3-3] to [Fig. VI-3-5]. As shown in [Fig. Vi-3-3], you can’t find the first order(or
fundamental frequency) easily and harmonic frequencies are so big that the calculated RPM is not
correct. If you find in the spectrum of a velocity or displacement, you can find it easily. But in the
integrating process, the noise of a low frequency is amplified. Consequently, though you see a first
order, you can be mixed between noise and first frequency. To solve this problem, the highpass filter
with 4Hz cutoff frequency is used. In other words, the frequency less than 4Hz has 0 magnitudes.
To get the graph of acceleration/velocity/displacement, you can set it in [Integral] of [Fig. VI-3-2].
A magnitude has RMS, units is fixed with
2
/ m s
,
/ cm s
and
mm
.
[Fig. VI-3-3] Acceleration [Fig. VI-3-4] Velocity [Fig. VI-3-5] Displacement
3.2 Integral in the spectrum of FFT
The relationships of acceleration, velocity and displacement are listed below equations:
2
Acceleration
velocity
f
π
=
<Eq. VI-3-2>
2
(2 )
Acceleration
Displacement
f
π
=
<Eq. VI-3-3>
where
f
is frequency. These equations represent the relationship of magnitude except phase. We
can rewrite these equations with frequency in the spectrum as below.
( )
( )
2
Acc
Vel
i
FFT i
FFT i
f
π
=
<Eq. VI-3-4>
2
( )
( ) ,
(2 )
Acc
Disp
i
FFT i
FFT i
f
π
=
0
2
N
i
≤ ≤
<Eq. VI-3-5>
where
( ) FFT i
is a magnitude at each frequency.
i
f
is frequency.
i
is the index of frequency
and
N
is a buffer size.
First Order
Amplified Noise