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Chapter 4
Theory of Operation
©
National Instruments Corporation
4-17
NI PXI-4220 User Manual
Figure 4-14.
Full-Bridge Type III Measuring Axial and Rejecting Bending Strain
A full-bridge type III configuration has the following characteristics:
•
Four active strain-gauge elements. Two are mounted in the direction of
axial strain with one on one side of the strain specimen (top), and the
other on the opposite side (bottom). The other two act together as a
Poisson gauge and are mounted transverse (perpendicular) to the
principal axis of strain with one on one side of the strain specimen
(top), the other on the opposite side (bottom).
•
Compensates for specimen temperature variation.
•
Rejects bending strain.
•
Compensates for the aggregate effect on the principle strain
measurement due to the Poisson’s ratio of the specimen material.
•
Compensates for lead resistance.
•
Sensitivity at 1000
µε
is
∼
1.3 mV
out
/
V
EX
input
Figure 4-15.
Full-Bridge Type III Circuit Diagram
The following symbols apply to the circuit diagram and equations:
•
R
1
is an active strain-gauge element measuring compressive Poisson
effect (–
νε
).
•
R
2
is an active strain-gauge element measuring tensile strain (+
ε
).
(+ )
R
4
(+ )
(– )
(– )
Axial
Bending
R
3
R
2
R
1
R
4
R
3
R
2
R
1
R
2
R
1
(– )
+
–
–
+
V
EX
V
CH
R
4
(+ )
R
3
(– )
(+ )
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