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Chapter 4
Theory of Operation
NI PXI-4220 User Manual
4-16
ni.com
To convert voltage readings to strain units, use the following equation:
where
•
GF
is the gauge factor.
•
ν
is the Poisson’s ratio.
To simulate the effect on strain of applying a shunt resistor across R
3
,
use the following equation:
Notes
Full-bridge type II is sometimes used for strain measurement of bending beams.
R
3
and R
4
are positioned along the beam axis and on opposite sides of the beam, and
R
1
and R
2
are positioned transverse to the beam axis and on opposite sides of the beam.
The nominal values of R
1
, R
2
, R
3
, and R
4
equal
R
g
.
Gauges need not have an STC number corresponding to the material type of the test
specimen.
As shown in Figure 2-9,
Full-Bridge Type II Circuit Diagram
, for greatest calibration
accuracy, use separate wires between the bridge and the shunt calibration pins SCA (pin 4)
and SCCOM (pin 5). Do not directly short SCA (pin 4) or SCCOM (pin 5) inside your
connector unless the strain-gauge leads are short and have minimal lead resistance.
Full-Bridge Type III
This section provides information about the full-bridge strain-gauge
configuration type III. The full-bridge type III only measures axial strain.
Figure 4-14 shows how to position strain-gauge resistors in an axial
configuration. Figure 4-15 shows the full-bridge type III circuit wiring
diagram.
strain
ε
( )
2
–
V
r
GF
1
ν
+
(
)
--------------------------
=
ε
s
2
U
–
GF
1
ν
+
(
)
--------------------------
=
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