©
National Instruments
21
NI WSN-3214 User Guide and Specifications
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 compressive strain (–
ε
).
•
R
4
—Active element measuring tensile strain (+
ε
).
•
GF
—Gage Factor, specified by the gage manufacturer.
•
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:
Full-Bridge Type I
This section provides information for the full-bridge strain-gage configuration type I. The full-bridge
type I only measures bending strain. Figure 19 shows how to position strain-gage resistors in a bending
configuration. Figure 20 shows the full-bridge type I circuit wiring diagram.
Figure 19.
Full-Bridge Type I Rejecting Axial and Measuring Bending Strain
A full-bridge type I configuration has the following characteristics:
•
Four active strain-gage elements. Two strain-gage elements are mounted in the direction of bending
strain on one side of the strain specimen (top) while the other two are mounted in the direction of
bending strain on the opposite side (bottom).
•
Highly sensitive to bending strain.
•
Rejects axial strain.
•
Sensitivity ~ 2
μ
V/V per
με
, for GF = 2.0.
Vr
VCH
VEX
------------
Shunt Calibration
×
Offset Null
–
=
strain
(
ε )
2
Vr
–
GF
------------
=
R
2
Reject
s
Axi
a
l
Me
asu
re
s
Bending
(+ )
(– )
(+ )
(– )
R
1
R
3
R
4
R
3
R
4
R
2
R
1
