Chapter 4
Theory of Operation
©
National Instruments Corporation
4-17
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), 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) and the other
on the opposite side (bottom).
•
Compensates for temperature.
•
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
2
(+ )
(+ )
(– )
(– )
Axial
Bending
R
4
R
3
R
1
R
4
R
3
R
2
R
1
R
1
(– )
+
+
–
–
V
EX
(+ )
(– )
(+ )
R
2
R
4
R
3
V
CH