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Chapter 4
Theory of Operation
NI PXI-4220 User Manual
4-14
ni.com
To convert voltage readings to strain units, use the following equation:
where
•
GF
is the gauge factor.
To simulate the effect on strain of applying a shunt resistor across R
3
,
use the following equation:
Notes
The nominal values of R
1
, R
2
, 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 2-8,
Full-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.
Full-Bridge Type II
This section provides information about the full-bridge type II strain-gauge
configuration. The full-bridge type II only measures bending strain.
Figure 4-12 shows how to position strain-gauge resistors in a bending
configuration. Figure 4-13 shows the full-bridge type II circuit wiring
diagram.
Figure 4-12.
Full-Bridge Type II Rejecting Axial and Measuring Bending Strain
strain
ε
( )
V
–
r
GF
--------
=
ε
s
U
–
GF
--------
=
R
3
(+ )
(– )
(– )
(+ )
Axial
Bending
R
2
R
4
R
1
R
4
R
1
R
3
R
2
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