4-10
4 Making Measurements
An additional time delay is caused by the velocity decoder. It depends on both
velocity decoder and velocity measurement range and is approximately a few
microseconds. The resulting overall phase shift
∆Φ
can be estimated using
the following simple equation:
Equation 4.3
4.2.3 Settings for Displacement Measurement (optional)
Measurement
range
The most important consideration for selecting the displacement
measurement range is of course the maximum expected displacement. To
prevent the displacement decoder overranging however, a higher
displacement measurement range often has to be selected if low frequency
interference vibrations are present. Basically the displacement measurement
range should be selected such that the output signal is as high as possible
but with its peak values definitely remaining below the maximum signal
amplitude of
±
, CLEAR function).
Another aspect is the maximum expected velocity. For technical reasons, the
absolute velocity limit of the vibrometer (10 m / s) can only be made use of in
the upper displacement measurement ranges (80
µ
m / V; 320
µ
m / V; 1,280
µ
m /
V; 5,120
µ
m / V). In the lower displacement measurement ranges, however,
several counts per fringe are generated by interpolation whereby bandwidth
and count frequency multiply correspondingly (refer to
). As a
result, the maximum permissible velocity decreases to about the same extent
as the displacement resolution increases. Due to this correlation, the peak
values of the lower displacement measurement ranges (0.5
µ
m / V; 2
µ
m / V;
8
µ
m / V; 20
µ
m / V) can only be made use of for frequencies of up to
approximately 2.5 kHz. Above this value the maximum measurable amplitude
decreases with increasing frequency according to equation
Equation 4.4
From the correlations described, it can be concluded that the frequency also
has to be taken into consideration when selecting the displacement
measurement range. The specified maximum frequency can only be
measured in the upper displacement measurement ranges. In the lower
ranges there are technical limits which can not be exceeded, even when in
compliance with condition
. Furthermore, attention must be paid to the fact
that the specified accuracy is only complied with up to a characteristic
maximum frequency (refer to
The resulting range diagram for all measurement ranges is shown in
.
∆Φ
100
°
f
f
c
---
p
d
f
⋅
+
⋅
=
f
…
frequency in kHz
f
c
…
cutoff frequency of the low pass filter in kHz
p
d
…
specific phase roll-off
xˆ
vˆ
2
π
f xˆ
⋅ ⋅
const.
=
=
vˆ
…
velocity amplitude
xˆ
…
displacement amplitude
f
…
frequency
Summary of Contents for OFV-3001
Page 28: ...3 16 3 First Steps...
Page 54: ...5 12 5 Operating the Vibrometer...
Page 94: ...D 10 D Functional Description of the Controller...