Polytec OFV-511 User Manual

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The functional structure of the displacement decoder is shown schematically
in figure D.3.
The subassembly phase multiplier is only included in the high resolution mod-
els OVD-20 and OVD-40. In the displacement decoder OVD-10, the modu-
lated signal ϕ
mod
is processed directly in the functional block quadrature
demodulator. Here the displacement information is digitally reconstructed
from the phase difference between the reference signal ϕ
ref
and the modu-
lated signal ϕ
mod
. As a result, the pulses COUNT and DIRECTION are gener-
ated. Each COUNT pulse corresponds to one period of the interferometer
signal i.e. a displacement of the object by λ /2. The binary value of the signal
DIRECTION provides the sign. Both pulses are counted by the binary for-
ward-reverse counter, the status of which thus corresponds with the instanta-
neous position of the object. The counting range is of 12bit which
corresponds to 4,096 increments. As each increment is equivalent to
316.4nm, total displacements of up to approximately 1.28mm ( ± 0.64mm
amplitude) can be measured in the so-called direct counting mode with a res-
olution of 316.4nm. To make use of the full counting range, the counter is
reset as appropriate with an external CLEAR pulse at each zero crossing of
the object (refer to section 4.2.3, CLEAR function).
To be able to present the displacement signal in real-time with an oscilloscope
e.g., the counter content is converted by a 12bit digital-to-analog converter
into an analog voltage with a step resolution of approximately 4mV. The steps
are smoothed with a low pass filter at high frequencies. A subsequent ampli-
fier stage scales the signal according to the measurement range set.
In the high resolution displacement decoders OVD-20 and OVD-40, the addi-
tional functional block phase multiplier realizes an even higher resolution. In
this subassembly, the interferometer signal ϕ
mod
is pre-processed in the sense
of an interpolation before the quadrature demodulator converts it into a pulse
train with a higher resolution. This generates integer multiples of the phase
deviation ∆ϕ and thus increases the resolution of the displacement decoder
by the same factor. This means that in the lowest measurement range
(0.5 µ m/V) an increment of approximately 2nm is attained. As the counting
range is always 12bit, the total measurement range is decreased by the
respective multiplier. In the lowest range thus displacements up to approxi-
mately 8 µ m ( ± 4 µ m amplitude) can be measured. Bear in mind however, that
due to the phase multiplication both the original bandwidth of the modulated
Figure D.3: Block diagram of the displacement decoder
Q uadrature
D em odulator
Phase
M u ltip lie r
C ounter
D
A
D /A C onverter
Low Pass
D isplacem ent
Signal
CLEAR
C ontrol Bus
12 bit
DIRECTION
COUNT
n x
ϕ
mod
ϕ
mod
ϕ
re f
O scillator

Polytec OFV-511 Specifications

General

Brand	Polytec
Model	OFV-511
Category	Measuring Instruments
Language	English