X = 25000B / K(R + 0.5B) %
Since the equivalent input is here expressed as a percentage of full-scale output, you
must multiply it by the rated full-scale capacity of the transducer, in order to determine
the
actual input simulated by the shunt.
Shunt calibration is an easier though generally less accurate technique then two-point
(deadweight) calibration. It is useful, however, when overall “deadweighting” is impos-
sible or inconvenient, and is good for an accuracy of about 0.2% (depending, of
course, on the accuracy of the specified equivalent input, and on the resistor/bridge
tolerance and temperature).
The AA72 is equipped with a 100-kΩ, 0.1%
calibration resistor for each active channel.
These resistors are located on turret terminals on the Connector Assembly Board
(see Fig. 2). You may, if you wish, replace each channel’s installed 100K shunt resistor
with a resistor of another value (strain-gage transducer manufacturers often supply
such resistors with their instruments).
In System 10, a strain gage channel’s shunt resistor may be switched in and out by
means of the
SHUNT CALIBRATE—POSITIVE (SHP) or SHUNT CALIBRATE—NEGA-
TIVE (SHN)
command. A RESUME (RSM) command should then be applied to
remove the shunt and resume normal channel measurement. Since these are “run-
time” commands, the mainframe’s
EEPROM Write Protect Switch need not be on
for them to be effective. See Manual Section 1.G.6 for general instructions regarding
the “SHUNT CALIBRATION” technique in System 10.
NOTE: Per-channel shunt calibration for the AA72 can be “remotely” controlled, if
desired, as an alternative to using the software “SHUNT CALIBRATE” commands pro-
vided by the system. This remote calibration control is accomplished by means of
logic-level inputs to the AA72 card. The relevant connections are given in Fig. 6.
Fig. 6(a) shows how the “CALIBRATE POSITIVE” and “CALIBRATE NEGATIVE” com-
mands can be independely applied to any active AA72 channel, without the need of
an external logic reference supply.
Closing the switch in Fig. 6(a) to contact point “A” will produce a
Logic 0 level at the
“NOT +CAL COMMAND” terminal. Since this is a negative-true logic line, the
Logic 0
input will activate the “+CALIBRATE” condition of the channel. That is, it will switch in
the channel’s shunt resistor for a
positive up-scale reading. Opening the switch to dis-
connect the “NOT +CALIBRATE” line from POWER COMMON will then return the chan-
nel to the “NO +CALIBRATE” condition.
Similarly, closing the switch to contact point “B” will produce a
Logic 0 level at the
“NOT –CAL COMMAND” terminal, thereby switching in the channel’s shunt resistor for
a
negative up-scale reading. Opening the switch to disconnect the “NOT –CALI-
BRATE” line from POWER COMMON will then return the channel to the “NO –CALI-
BRATE” condition.
You may also use active TTL logic, as illustrated in Fig. 6(b), to produce the “+CALI-
BRATE” or “–CALIBRATE” condition for either AA72 channel.
SETUP AND/OR OPERATING CONSIDERATIONS AA72-2/4.11
DUAL / QUAD DC STRAIN GAGE CARD AA72-2 / AA72-4
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