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The vibrometer can provide both velocity and displacement signals indepen-
dently of each other. If the vibrometer is equipped with both velocity and dis-
placement decoders, for many measurements a decision then has to be made
on which is the optimal quantity to be measured. This applies in particular to
harmonic vibrations, as in this case the velocity and the displacement signal
provide the same information according to
Equation 4.1
In contrast, transient movements in most cases are shown much more clearly
by the displacement signal.
Apart from these application-specific aspects, there are some aspects affect-
ing the choice of the quantity to be measured which depend on the measure-
ment procedure. These are explained in the following.
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UDQJH
Due to the 12bit digital resolution of the fringe counter system, the relative
resolution of each displacement measurement range is 2048 steps with a
symmetric output voltage swing. This corresponds to a dynamic range of
approximately 66dB. The background noise lies generally below the resolu-
tion and therefore does not affect the measurement.
In contrast, the resolution in velocity measurement is only limited by the back-
ground noise. With good optical signals (e.g. on reflective film) and a spectral
resolution bandwidth of several Hertz, the background noise is typically more
than 100dB below full scale range. This corresponds to a dynamic range
which is about 100 times higher than that of the displacement measurement.
5HVROXW L RQ
If the absolute, noise limited resolution of the velocity decoder (approximately
0.2 ) is rearranged according to equation 4.1 to obtain the corre-
sponding amplitude of a sinusoidal vibration with a frequency of 100kHz, this
results to approximately mm or 0.3pm! This means that with high fre-
quencies in particular, significantly higher resolutions can be attained with the
velocity measurement.
6LJQ DO WR
QRLV H UD WL R
The vibration to be measured is usually superimposed by interference vibra-
tions from the surroundings or from the object itself. These background vibra-
tions (e.g. building vibrations) often have low frequencies but high
displacement amplitudes. To prevent overranging, the displacement measure-
ment range must be selected taking the amplitude of the background vibra-
tions into consideration. If the required signal is then resolved at all, at least a
bad signal-to-noise ratio is obtained at the output.
v
2π fx
⋅⋅
=
x
…displacement amplitude
µm
s
--------
Hz
⁄
310
13
–
⋅
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