Chapter 4
Theory of Operation
©
National Instruments Corporation
4-9
NI PXI-4220 User Manual
•
Compensates for the aggregate effect on the principle strain
measurement due to the Poisson’s ratio of the specimen material.
•
Sensitivity at 1000
µε
is
∼
0.65 mV
out
/
V
EX
input.
Figure 4-7.
Half-Bridge Type I Circuit Diagram
The following symbols apply to the circuit diagram and equations:
•
R
1
and R
2
are half-bridge completion resistors.
•
R
3
is the active strain-gauge element measuring compression from
Poisson effect (–
νε
).
•
R
4
is the active strain-gauge element measuring tensile strain (+
ε
).
•
V
EX
is the excitation voltage.
•
R
L
is the lead resistance.
•
V
CH
is the voltage measured.
To convert voltage readings to strain units, use the following equation:
where
•
R
g
is the nominal gauge resistance.
•
R
L
is the lead resistance.
•
ν
is the Poisson’s ratio.
•
GF
is the gauge factor.
To simulate the effect on strain of applying a shunt resistor across R
3
,
use the following equation:
R
1
R
2
R
L
+
–
–
+
V
EX
V
CH
R
3
(– )
R
4
(+ )
R
L
R
L
strain
ε
( )
4
V
r
–
GF
1
ν
+
(
)
2
V
r
ν
1
–
(
)
–
[
]
---------------------------------------------------------------
1
R
L
R
g
------
+
×
=
ε
s
4
U
–
GF
1
ν
+
(
)
2
U
ν
1
–
(
)
–
[
]
-------------------------------------------------------------
=
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