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Chapter 4
Theory of Operation
NI PXI-4220 User Manual
4-12
ni.com
Notes
Half-bridge type II requires one strain gauge to undergo tensile strain while the
other strain gauge undergoes compressive strain of the same magnitude. This configuration
is often used to measure bending strain where the strain gauges are mounted on opposite
sides of a beam.
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 2-7,
Half-Bridge Type 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
.
Full-Bridge Type I
This section provides information about the full-bridge strain-gauge
configuration type I. The full-bridge type I measures bending strain only.
Figure 4-10 shows how to position strain-gauge resistors in a bending
configuration. Figure 4-11 shows the full-bridge type I circuit wiring
diagram.
Figure 4-10.
Full-Bridge Type I Rejecting Axial and Measuring Bending Strain
R
4
Axial
Bending
(+ )
(– )
(+ )
(– )
R
3
R
2
R
1
R
2
R
1
R
4
R
3
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