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Chapter 4
Theory of Operation
©
National Instruments Corporation
4-7
NI PXI-4220 User Manual
•
Completion resistors provide half-bridge completion.
•
Compensates for specimen temperature variation.
•
Sensitivity at 1000
µε
is
∼
0.5 mV
out
/
V
EX
input.
Figure 4-5.
Quarter-Bridge Type II 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 quarter-bridge temperature-sensing element (dummy gauge).
•
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.
•
GF
is the gauge factor.
To simulate the effect on strain of applying a shunt resistor across R
3
,
use the following equation:
The dummy gauge element must always be unstrained and mounted to the
same type of material as the active gauge, but not strained. The dummy
gauge temperature must closely track the temperature of the active gauge.
R
1
R
2
R
L
R
3
+
–
V
EX
–
+
V
CH
R
4
(+ )
R
L
R
L
strain
ε
( )
4
–
V
r
GF
1 2
V
r
+
(
)
-------------------------------
1
R
L
Rg
-------
+
×
=
ε
s
4
U
–
GF
1 4
U
+
(
)
------------------------------
=
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