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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 calcu-
lated very precisely by external signal integration after cutting off the interfer-
ence frequencies below 1kHz with a suitable high pass filter.
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UDQJH
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 band-
widths 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
29'
. As long as the frequency remains below 250kHz, 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 250kHz only the top three mea-
surement 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
29'
the technical limits for acceleration have to be
taken into consideration in some velocity measurement ranges. According to
the relation
Equation 4.2
in these ranges the permissible frequency decreases with increasing ampli-
tude. If the condition 4.2 is infringed upon, the signal is seriously distorted. In
this case, a higher velocity measurement range has to be selected.
Vibration to be measured: e.g.
,
⇒
(refer to equation 4.1)
Background vibration: typically
,
⇒
f
Signal
100kHz
=
x
Signal
1µm
=
v
ˆ
Signal
628, 10
1
–
m/s⋅
=
f
Background
100Hz<
x
Background
10µm<
v
ˆ
Background
628, 10
3
–
m/s⋅<
a
2π fv
⋅⋅=
a
…acceleration amplitude
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