Polytec OFV-3001, User Manual

Page: 32 / 101

4-4
4 Making Measurements
4.2 Selecting Suitable Settings
4.2.1 Velocity or Displacement Measurement ?
The vibrometer can provide both velocity and displacement signals
independently of each other. If the vibrometer is equipped with both velocity
and displacement 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
affecting the choice of the quantity to be measured which depend on the
measurement procedure. These are explained in the following.
Dynamic
range Due to the 12 bit 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 66 dB. The background noise lies generally below the
resolution and therefore does not affect the measurement.
In contrast, the resolution in velocity measurement is only limited by the
background 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 100 dB below full scale range. This corresponds to a
dynamic range which is about 100 times higher than that of the displacement
measurement.
Resolution If the absolute, noise limited resolution of the velocity decoder (approximately
0.2 ) is rearranged according to equation 4.1 to obtain the
corresponding amplitude of a sinusoidal vibration with a frequency of 100 kHz,
this results to approximately mm or 0.3 pm ! This means that with high
frequencies in particular, significantly higher resolutions can be attained with
the velocity measurement.
Signal-to-
noise ratio The vibration to be measured is usually superimposed by interference
vibrations from the surroundings or from the object itself. These background
vibrations (e.g. building vibrations) often have low frequencies but high
displacement amplitudes. To prevent overranging, the displacement
measurement range must be selected taking the amplitude of the background
vibrations into consideration. If the required signal is then resolved at all, at
least a bad signal-to-noise ratio is obtained at the output.
vˆ 2
π
f xˆ
⋅ ⋅
=
vˆ
…
velocity amplitude
xˆ
…
displacement amplitude
f
…
frequency
µ
m
s
--------
Hz
⁄
3 10
13
– ⋅