Chapter 4
Theory of Operation
4-10
ni.com
The following symbols apply to the circuit diagram and equations:
•
R
1
and R
2
are half-bridge completion resistors.
•
R
3
is the active strain-gauge element measuring compression from
Poisson effect (–
νε
).
•
R
4
is the active strain-gauge element measuring 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
R
g
is the nominal gauge resistance.
R
L
is the lead resistance.
ν
is the Poisson’s ratio.
GF
is the Gauge Factor.
To simulate the effect on strain of applying a shunt resistor across R
3
,
use the following equation:
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.
Strain gauges need not have a particular STC number.
As shown in Figure 4-9, 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.
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
.
The nominal values of R
3
and R
4
equal
R
g
.
strain
ε
( )
4
V
r
–
GF
1
v
+
(
)
2
V
r
v
1
–
(
)
–
[
]
--------------------------------------------------------------
1
R
L
R
g
------
+
×
=
ε
s
4
U
–
GF
1
v
+
(
)
2
U v
1
–
(
)
–
[
]
-------------------------------------------------------------
=