©
National Instruments
19
NI WSN-3214 User Guide and Specifications
Figure 16.
Half-Bridge Type I Circuit Diagram
The following symbols apply to the circuit diagram and equations:
•
R
1
and R
2
—Half-bridge completion resistors located inside the NI WSN-3214.
•
R
3
—Active element measuring compression from Poisson effect (–
νε
).
•
R
4
—Active element measuring tensile strain (+
ε
).
•
GF
—Gage Factor, specified by the gage manufacturer.
•
ν
—Poisson’s ratio, defined as the negative ratio of transverse strain to axial strain (longitudinal)
strain. Poisson’s ratio is a material property of the specimen you are measuring.
•
V
CH
—Measured voltage of the bridge.
•
V
EX
—Excitation voltage provided by the NI WSN-3214.
•
V
r
—Offset compensated ratiometric bridge output defined by the following equation:
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 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:
V
CH
V
EX
R
1
R
2
R
3
(
–
ν
ε
)
R
4
(+
ε
)
+
–
NI W
S
N-
3
214
R
COMP
/EX–
AI+
EX+
R
S
+
R
S
–
+
–
Vr
VCH
VEX
------------
Shunt Calibration
×
Offset Null
–
=
strain
ε
( )
4
Vr
–
GF
1
ν
+
(
)
2
Vr
ν
1
–
(
)
–
[
]
---------------------------------------------------------------
=
