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Chapter 4
Theory of Operation
NI PXI-4220 User Manual
4-8
ni.com
The nominal value of R
3
is equal to
R
g
.
Gauges need not have an STC number corresponding to the material type
of the test specimen.
As shown in Figure 2-5,
Quarter-Bridge II Circuit Diagram
, for greatest
calibration accuracy, use separate wires between the bridge and the shunt
calibration pins SCA (pin 4) and SCCOM (pin 5). Do not directly short
SCA (pin 4) or SCCOM (pin 5) inside your connector unless the
strain-gauge leads are short and have minimal lead resistance.
You can neglect lead resistance (
R
L
) of the wiring if shunt calibration is
performed or if lead length is very short (
∼
<10 ft), depending on the wire
gauge. For example, 10 ft of 24-AWG copper wire has a lead resistance
of 0.25
Ω
.
Half-Bridge Type I
This section provides information about the half-bridge strain-gauge
configuration type I. The half-bridge type I measures either axial or
bending strain. Figure 4-6 shows how to position strain-gauge resistors in
an axial and bending configurations. Figure 4-7 shows the half-bridge
type I circuit wiring diagram.
Figure 4-6.
Half-Bridge Type I Measuring Axial and Bending Strain
A half-bridge type I has the following characteristics:
•
Two active strain-gauge elements. One is mounted in the direction
of axial strain, and the other acts as a Poisson gauge and is mounted
transverse (perpendicular) to the principal axis of strain.
•
Completion resistors provide half-bridge completion.
•
Sensitive to both axial and bending strain.
•
Compensates for specimen temperature variation.
R
3
(– )
R
4
(+ )
R
4
(+ )
R
3
(– )
Axial
Bending
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