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Chapter 4
Theory of Operation
©
National Instruments Corporation
4-13
NI PXI-4220 User Manual
A full-bridge type I configuration has the following characteristics:
•
Four active strain-gauge elements. Two are mounted in the direction of
bending strain on one side of the strain specimen (top), and the other
two are mounted in the direction of bending strain on the opposite side
(bottom).
•
Highly sensitive to bending strain.
•
Rejects axial strain.
•
Compensates for specimen temperature variation.
•
Compensates for lead resistance.
•
Sensitivity at 1000
µε
is
∼
2.0 mV
out
/
V
EX
input.
Figure 4-11.
Full-Bridge Type I Circuit Diagram
The following symbols apply to the circuit diagram and equations:
•
R
1
is an active strain-gauge element measuring compressive
strain (–
ε
).
•
R
2
is an active strain-gauge element measuring tensile strain (+
ε
).
•
R
3
is an active strain-gauge element measuring compressive
strain (–
ε
).
•
R
4
is an active strain-gauge element measuring tensile strain (+
ε
).
•
V
EX
is the excitation voltage.
•
R
L
is the lead resistance.
•
V
CH
is the measured voltage.
+
–
–
+
V
EX
V
CH
R
4
(+ )
R
3
(– )
R
1
(– )
R
2
(+ )
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