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National Instruments Corporation
4-1
NI PXI-4220 User Manual
4
Theory of Operation
This chapter discusses strain-gauge concepts, the theory of operational
measurement concepts, and timing and control concepts.
Strain Gauge Theory
This section discusses how to arrange, connect, and scale signals from
bridge-based sensors, especially strain gauges.
Wheatstone Bridges
All strain-gauge configurations are based on the concept of a Wheatstone
bridge. A Wheatstone bridge is a network of four resistive legs. One or
more of these legs are active sensing elements. Figure 4-1 shows a
Wheatstone bridge circuit diagram.
Figure 4-1.
Basic Wheatstone Bridge Circuit Diagram
The Wheatstone bridge is the electrical equivalent of two parallel voltage
divider circuits. R
1
and R
2
compose one voltage divider circuit, and R
4
and
R
3
compose the second voltage divider circuit. The output of a Wheatstone
bridge is measured between the middle nodes of the two voltage dividers.
A physical phenomenon, such as a change in strain or temperature applied
to a specimen, changes the resistance of the sensing elements in the
Wheatstone bridge. The Wheatstone bridge configuration is used to
measure the small variations in resistance that the sensing elements
produce corresponding to physical changes in the specimen.
+
V
CH
R
1
R
2
R
4
R
3
V
EX
+
V
EX
–
–
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