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Chapter 4
Theory of Operation
NI PXI-4220 User Manual
4-18
ni.com
•
R
3
is an active strain-gauge element measuring compressive Poisson
effect (–
νε
).
•
R
4
is an active strain-gauge element measuring the tensile strain (+
ε
).
•
V
EX
is the excitation voltage.
•
R
L
is the lead resistance.
•
V
CH
is the measured voltage.
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 III is sometimes used for axial strain measurement. R
2
and R
4
are
positioned along the beam axis and on opposite sides of the beam, and R
1
and R
3
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 2-10,
Full-Bridge Type III 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.
NI PXI-4220 Theory of Operation
This section provides information about the circuit features of the module.
Refer to the block diagram in Figure 4-16, while reading this section.
strain
ε
( )
2
–
V
r
GF
ν
1
+
(
)
V
r
ν
1
–
(
)
–
[
]
-----------------------------------------------------------
=
ε
s
4
U
–
GF
ν
1
+
(
)
U
ν
1
–
(
)
–
[
]
----------------------------------------------------------
=
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