Chapter 4
Theory of Operation
4-18
ni.com
•
R
3
is an active strain-gauge element measuring compressive Poisson
effect (–
νε
).
•
R
4
is an active strain-gauge element measuring the tensile strain (+
ε
).
•
V
EX
is the excitation voltage.
•
R
L
is the lead resistance.
•
V
CH
is the measured voltage.
To convert voltage readings to strain units use the following equation:
where
GF
is the Gauge Factor.
ν
is the Poisson’s ratio.
To simulate the effect on strain of applying a shunt resistor across R
3
,
use the following equation:
Notes
Full-bridge type III is sometimes used for axial strain measurement. R
2
and R
4
are positioned along the beam axis and on opposite sides of the beam, and R
1
and R
3
are
positioned transverse to the beam axis and on opposite sides of the beam.
Strain gauges need not have a particular STC number.
As shown in Figure 4-15, for greatest calibration accuracy, use separate wires between the
bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals
inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.
The nominal values of R
1
, R
2
, R
3
, and R
4
equal
R
g
.
SCXI-1520 Theory of Operation
This section includes a brief overview and a detailed discussion of the
circuit features of the module. Refer to Figure 4-16 while reading this
section.
strain
ε
( )
2
–
V
r
GF v
1
+
(
)
V
r
v
1
–
(
)
–
[
]
----------------------------------------------------------
=
ε
s
4
U
–
GF v
1
+
(
)
U v
1
–
(
)
–
[
]
---------------------------------------------------------
=