NI WSN-3214 User Guide and Specifications
26
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Note
The ratio of the bridge output voltage to the excitation voltage is calculated internally on the
NI WSN-3214. Shunt Calibration is only supported for quarter-bridge. If you are switching from
using a quarter-bridge to a half-bridge or a full-bridge configuration, you need to specify 1 for Shunt
Calibration, and then perform Offset Null in the software.
The NI WSN-3214 converts the readings to strain using the following equation:
Analog Input Circuitry
Excitation Voltage
The NI WSN-3214 uses a fixed 2 V excitation to supply the bridges. This is generated internally from
the external power supply or batteries. Bridge excitation current is limited to 6.4 mA per channel. The
excitation current for a single bridge is
where
R
is the total resistance of the bridge.
For a quarter-bridge or half-bridge,
R
is equal to two times the resistance of each element. For a
full-bridge,
R
is equal to the resistance of each element. If you are using a 350
Ω
full-bridge, the
excitation current is 5.7 mA. If you are using a 1 k
Ω
half-bridge and quarter-bridge, the excitation
current is 1.1 mA. You cannot use 120
Ω
bridges, because they require more than 6.4 mA. Refer to the
section for more information about how the battery life changes with the type
of bridge used.
Connection Options to Correct for Resistance Errors
Wiring resistance can create gain errors in bridge circuits. The NI WSN-3214 provides two mechanisms
to correct for these errors: remote sensing and shunt calibration. The gage and completion resistance
tolerances create offset errors. The NI WSN-3214 provides offset nulling to correct for this error.
Remote Sensing
Remote sensing continuously and automatically corrects for errors in excitation leads, and is only
supported for half-bridge and full-bridge sensors.
Note
The NI-WSN 3214 does not support remote sense with quarter-bridge sensors.
Long wire and small gauge wires have greater resistance, which can result in gain error. The resistance
in the wires that connect the excitation voltage to the bridge causes a voltage drop, which is a source of
gain error. The NI WSN-3214 includes remote sensing to compensate for this gain error. Connect remote
sense wires to the points where the excitation voltage wires connect to the bridge circuit. Figure 25
shows how to connect remote sense wires to the NI WSN-3214.
strain
ε
( )
2
–
Vr
GF
ν
1
+
(
)
Vr
ν
1
–
(
)
–
[
]
------------------------------------------------------------
=
2
V
R
-------
