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Chapter 4
Theory of Operation
NI PXI-4220 User Manual
4-10
ni.com
Notes
In half-bridge type I, R
4
is mounted along the principal axis of the stress field
and R
3
is mounted transverse to the axis of the stress field. Use this configuration in
applications where no stress exists along the axis of the transverse strain gauge.
The nominal values of R
3
and R
4
equal
R
g
.
Gauges need not have an STC number corresponding to the material type of the test
specimen.
As shown in Figure 2-6,
Half-Bridge Type I 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 II
This section provides information about the half-bridge strain-gauge
configuration type II. The half-bridge type II measures bending strain only.
Figure 4-8 shows how to position strain-gauge resistors in a bending
configuration. Figure 4-9 shows the half-bridge type II circuit wiring
diagram.
Figure 4-8.
Half-Bridge Type II Rejecting Axial and Measuring Bending Strain
A half-bridge type II configuration has the following characteristics:
•
Two active strain-gauge elements. One is mounted in the direction of
bending strain on one side of the strain specimen (top), and the other
is mounted in the direction of bending strain on the opposite side
(bottom).
•
Completion resistors provide half bridge completion.
R
3
R
4
R
3
(– )
R
4
(+ )
Axial
Bending
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