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Synch
Pin 21 is a synchronising pin to be used in systems using more than
one card. Link pin 21 on all cards together with short wires to
synchronise the oscillators.
If this is not done it is possible to create beat notes between oscillators
causing fluctuations in the d.c. outputs.
Potentiometers
If it is required to mount the span or zero potentiometers remotely, the
pins are available on the edge connector to do so. Wire lengths should
be kept short to avoid pick up electrical noise, and lengths greater than
0.5m may cause some degradation of performance.
The potentiometer leads should be wired to the pins specified in
external connections, ensuring the clockwise and anticlockwise leads
are not reversed.
Example
As an example, take a dual channel card with A±B/2 facility, wired to a
pair of 1.0mm transducers. The A + B output should operate a current
output driving a remote meter.
Figure 8
(A±B)/2 wiring example
CAH card
Current
[
driver
A + B output
[
Demodulator A
Oscillator
Oscillator
return
[
Demodulator B
Setting up
The section describes how to set up the card to work with the specific
transducer. Facilities exist on the card for adjusting oscillator frequency,
output filter frequency, demodulator input impedance, span and zero.
This is carried out by moving links to short out elected pairs of vertical
pins. These links simply lift off and push on to the pins. In case of span
and zero there are also potentiometers for fine adjustment.
Figure 9
Card layout
Channel B
selector
LK4
LK2
+15
32
Power
-15
supply
28
0V
-
30
Current
+
meter
27a
25a
7a
White
16
Green
Channel A
14
Red
Blue
22
20
Red
Blue
19a
Channel B
White
Green
17a
Channel A
selector
Frequency
select
Fine span
adjust
Fine zero
A
adjust
Fine span
adjust
B
LK3
LK1
Fine zero
adjust
Oscillator frequency
Two oscillator frequencies are selectable using the four way pin pairs.
If the links are placed over the two centre pairs the oscillator will run at
5kHz, and, if over the other pairs, at 10kHz.
Figure 10
5kHz:
Frequency should be selected to be near the zero phase shift
frequency of the transducer for minimum temperature drift. Most
transducers are calibrated at 5kHz and so this frequency can be relied
upon for good results. However if a faster speed of response is
required, then the 10kHz oscillator frequency can be used with most
transducers (not a.c. long strokes).
Output filter frequency
Each demodulator has alow pass filter on the output to remove the a.c.
signal used to energise the transducer. The cut off frequency of this
filter can be set to either 500Hz or 1kHz using pin pairs LK1 - LK4.
Under normal use the 500Hz setting would be used with the 5kHz
oscillator frequency and 1kHz with the 10kHz oscillator. The benefit of
using the higher frequency is that the output will follow the movement
of the transducer armature more quickly, but use of the lower frequency
results in less ripple on the d.c. output. The best compromise is with the
settings above, but if, for example, a fast response with a long stroke is
1
required, it will be necessary to use a 5kHz oscillator and a 1kHz filter.
The disadvantage is an increase in ripple. LK1 and LK3 are used for
channel A. Put the link on LK1 for 1kHz filter frequency or LK3 for
500Hz.
1
LK2 and LK4 are used for channel B. Put the link on LK2 for 1kHz filter
frequency, or on LK4 for 500Hz.
Demodulator input impedance
Different transducers are calibrated into different loads. For instance
most LVDTs are calibrated with 100kΩ loads, but half bridges use 1kΩ.
For this reason the input impedance of the demodulator can be set to
100kΩ, 10kΩ, or 1kΩ, using the three pin pairs at the top of the
eighteen way selector. The channel A and B selectors are laid out as
follows:
Figure 11
A link should be place over one of the 100kΩ pin pairs to select
the correct impedance. If unsure, use the 10kΩ setting.
Oscillator frequency
10kHz:
Channel A amd Channel B selectable links
100kΩ
Select demodulator
10kΩ
input impedance
1kΩ
5 - 15mV/V
10 - 30mV/V
Selected sensitivity
25 - 75mV/V
range
50 - 150mV/V
100 - 300mV/V
250 - 750mV/V
Spare
These are not connected,
Spare
and can be used to store
Spare
unwanted links.
Spare
x10 gain
Z3
Spare
Select coarse zero
Z2
Z1
V7779
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