D-8
D Functional Description of the Controller
D.5.3 Operating Principle
The functional structure of the displacement decoder is shown schematically
in
.
The subassembly phase multiplier is only included in the high resolution
models OVD-20 and OVD-40. In the displacement decoder OVD-10, the
modulated 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
modulated signal
ϕ
mod
. As a result, the pulses COUNT and DIRECTION are
generated. 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 forward-reverse counter, the status of which thus corresponds with
the instantaneous position of the object. The counting range is of 12 bit which
corresponds to 4,096 increments. As each increment is equivalent to
316.4 nm, total displacements of up to approximately 1.28 mm (
±
0.64 mm
amplitude) can be measured in the so-called direct counting mode with a
resolution of 316.4 nm. 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
, 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 12 bit digital-to-analog converter
into an analog voltage with a step resolution of approximately 4 mV. The steps
are smoothed with a low pass filter at high frequencies. A subsequent
amplifier stage scales the signal according to the measurement range set.
In the high resolution displacement decoders OVD-20 and OVD-40, the
additional 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 2 nm is attained. As the
counting range is always 12 bit, the total measurement range is decreased by
the respective multiplier. In the lowest range thus displacements up to
approximately 8
µ
m (
±
4
µ
m amplitude) can be measured. Bear in mind
Figure D.3: Block diagram of the displacement decoder
Q u a d r a t u r e
D e m o d u l a t o r
P h a s e
M u l t i p l i e r
C o u n t e r
D
A
D / A C o n v e r t e r
L o w P a s s
D i s p l a c e m e n t
S i g n a l
C L E A R
C o n t r o l B u s
1 2 b i t
D I R E C T I O N
C O U N T
n x
j
m o d
j
m o d
j
r e f
O s c i l l a t o r
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...
