D-7
D Functional Description of the Controller
D.5 Displacement Decoder
D.5.1 Mode of Operation
The phase of the interferometer signal is the carrier of the displacement
information. To be directionally sensitive, Polytec’s vibrometers work on the
heterodyne 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
displacement 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 316.4 nm 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
measurement 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 2 nm can be attained. The
specified resolution corresponds to one increment (count pulse) which is
accumulated directionally sensitive by a 12 bit counter in the displacement
decoders OVD-10 and OVD-20.
For certain applications, in particular for low frequency vibrations, the
limitation of the measurement range to 1.3 mm in the direct counting mode
due to the 12 bit 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 82 mm (
±
41 mm
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.
D.5.2 The Various Displacement Decoders
OVD-10
OVD-20
In the following sections the digital phase modulation of the standard
displacement decoders OVD-10 and OVD-20 will be described.
OVD-30
The displacement decoder OVD-30 is suitable for ultrasonic measurements at
frequencies down to 20 MHz and operates with analog phase demodulation. It
is described in a separate manual.
VibSoft FC
The PC-based displacement decoder VibSoft FC extends the counting range
of the internal displacement decoders OVD-10 and OVD-20 to 32 bit 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
separate manual.
OVD-40
The model OVD-40 has been designed exclusively for operation with the
external PC-based displacement decoder VibSoft FC. It provides the same
resolution as the OVD-20, but does not generate an analog displacement
signal.
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...
