Polytec OFV-3001, User Manual

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4-5
4 Making Measurements
The situation for the velocity measurement is however quite different. For the
same displacement amplitude but a higher frequency, the velocity amplitude
of the required signal is a factor of 2
π⋅
f higher than the background vibration
(refer to equation 4.1 ). Thus at velocity measurement the signal-to-noise ratio
is higher per definition. A realistic ultrasound application should make this
clearer:
Thus the signal-to-noise ratio here for velocity measurements is two orders of
magnitude higher, even with a 10 times higher amplitude of the background
vibrations. If the displacement signal is expressly required, it can be
calculated very precisely by external signal integration after cutting off the
interference frequencies below 1 kHz with a suitable high pass filter.
4.2.2 Settings for Velocity Measurement
Measurement
range When selecting a suitable velocity measurement range the maximum
expected values for velocity, acceleration and frequency have to be taken into
consideration. Orientation purely on the velocity is often not enough, as the
various velocity decoders and measurement ranges have different
bandwidths and maximum accelerations. The respective values are given in
the specifications (refer to
section 7.1.4 ).
It is easiest to select the velocity measurement range for the universal
decoder OVD-02. As long as the frequency remains below 250 kHz, any of the
four measurement ranges can be selected. To maximize the signal-to-noise
ratio however, the smallest possible range should be used in which the output
signal is not clipped. For frequencies above 250 kHz only the top three
measurement ranges are suitable. Acceleration limits usually do not have to
be taken into consideration. Even with maximum amplitude, it can still process
the highest specified frequency in every measurement range.
With the decoder OVD-01 the technical limits for acceleration have to be
taken into consideration in some velocity measurement ranges. According to
the relation
Equation 4.2
Vibration to be measured: e.g.
,
⇒
(refer to equation 4.1 )
Background vibration: typically
,
⇒
f
Signal
100kHz
= xˆ
Signal
1
µ
m
=
vˆ
Signal
6 28
,
10
1
–
m/s
⋅
=
f
Background
100Hz
<
xˆ
Background
10
µ
m
<
vˆ
Background
6 28
,
10
3 –
m/s
⋅ <
aˆ 2
π
f vˆ
⋅ ⋅
=
aˆ
…
acceleration amplitude
vˆ
…
velocity amplitude
f
…
frequency