Polytec OFV-512 User Manual

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The most important consideration for selecting the displacement measure-
ment range is of course the maximum expected displacement. To prevent the
displacement decoder overranging however, a higher displacement measure-
ment 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 ± 8V (refer
also to section 4.2.3, CLEAR function).
Another aspect is the maximum expected velocity. For technical reasons, the
absolute velocity limit of the vibrometer (10m/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 section D.5.3). 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 approxi-
mately 2.5kHz. Above this value the maximum measurable amplitude
decreases with increasing frequency according to
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 measure-
ment range. The specified maximum frequency can only be measured in the
upper displacement measurement ranges. In the lower ranges there are tech-
nical limits which can not be exceeded, even when in compliance with condi-
tion 4.4. Furthermore, attention must be paid to the fact that the specified
accuracy is only complied with up to a characteristic maximum frequency
(refer to section 7.1.5).
The resulting range diagram for all measurement ranges is shown in
figure 4.6.
x
ˆ
v
ˆ
2π fx
ˆ
⋅⋅ const.
==
v
ˆ
…velocity amplitude
x
ˆ
…displacement amplitude
f…frequency

Polytec OFV-512 Specifications

General

Brand	Polytec
Model	OFV-512
Category	Measuring Instruments
Language	English