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Chapter 4
Theory of Operation
NI PXI-4220 User Manual
4-4
ni.com
The names given the strain-gauge types in the following sections directly
correspond to bridge selections in MAX and the LabVIEW Convert Strain
Gauge Reading VI.
Quarter-Bridge Type I
This section provides information about the quarter-bridge strain-gauge
configuration type I. The quarter-bridge type I measures either axial or
bending strain. Figure 4-2 shows how to position a strain-gauge resistor in
an axial and bending configurations. Figure 4-3 shows the quarter-bridge
type I circuit wiring diagram.
Figure 4-2.
Quarter-Bridge Type I Measuring Axial and Bending Strain
A quarter-bridge type I has the following characteristics:
•
A single active strain-gauge element is mounted in the principle
direction of axial or bending strain.
•
A passive quarter-bridge completion resistor (dummy resistor) is
required in addition to half-bridge completion.
•
Temperature variation in specimen decreases the accuracy of the
measurements.
•
Sensitivity at 1000
µε
is
∼
0.5 mV
out
/
V
EX
input.
Figure 4-3.
Quarter-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 quarter-bridge completion resistor (dummy resistor).
R
4
(+ )
Axial
Bending
R
4
(+ )
+
–
–
+
V
EX
R
1
R
L
R
2
R
3
V
CH
R
4
(+ )
R
L
R
L
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