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The phase of the interferometer signal is the carrier of the displacement infor-
mation. To be directionally sensitive, Polytec’s vibrometers work on the het-
erodyne principle i.e. the phase is modulated onto a carrier signal. The
information required thus rides on the phase difference between the driver
signal for the Bragg cell and the modulated signal at the photo detector. A dis-
placement of the object by
±λ
/2 produces a full demodulation period (a fringe
passage) at the photo detector.
The number of fringes counted is thus a measure of the displacement of the
object with an accuracy and resolution of
λ
/2 which is 316.4nm for the helium
neon laser. The mode of operation in which merely the number of fringes is
counted is called the direct counting mode. It corresponds with the measure-
ment range 80
µ
m/V.
As the interferometric phase changes continuously with the displacement of
the object, displacements of less than
λ
/2 can also be resolved with the aid of
electronic interpolation techniques. This is realized in the measurement
ranges below 80
µ
m/V in which resolutions down to 2nm can be attained. The
specified resolution corresponds to one increment (count pulse) which is
accumulated directionally sensitive by a 12bit counter in the displacement
decoders OVD-10 and OVD-20.
For certain applications, in particular for low frequency vibrations, the limita-
tion of the measurement range to 1.3mm in the direct counting mode due to
the 12bit counting range may not be sufficient. The displacement decoder has
therefore been equipped with three adjustable range extensions which cover
a displacement measurement range up to 82mm (
±
41mm amplitude). The
measurement range is extended by digitally dividing the number of counts by
integers. This procedure of course decreases the resolution, however the
accuracy is retained.
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29'
29'
In the following sections the digital phase modulation of the standard dis-
placement decoders OVD-10 and OVD-20 will be described.
29'
The displacement decoder OVD-30 is suitable for ultrasonic measurements at
frequencies down to 20Hz and operates with analog phase demodulation. It is
described in a separate manual.
2)9
The PC-based displacement decoder OFV-600 extends the counting range of
the internal displacement decoders OVD-10 and OVD-20 to 32bit and thus
offers a significant extension of the measurement range. The counter content
is handled directly as a numerical value thus increasing the measurement
accuracy. The PC-based displacement decoder is also described in a sepa-
rate manual.
29'
The model OVD-40 has been designed exclusively for operation with the
external PC-based displacement decoder OFV-600. It provides the same res-
olution as the OVD-20, but does not generate an analog displacement signal.
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