Chapter 4
Theory of Operation
4-18
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The following symbols apply to the circuit diagram and equations:
•
R
1
is an active strain-gauge element measuring compressive Poisson
effect (–
νε
).
•
R
2
is an active strain-gauge element measuring tensile strain (+
ε
).
•
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.
The nominal values of R
1
, R
2
, R
3
, and R
4
equal
R
g
.
Strain gauges need not have a particular STC number.
As shown in Figure 2-9,
Full-Bridge Type III 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.
strain
ε
( )
2
–
V
r
GF
ν
1
+
(
)
V
r
ν
1
–
(
)
–
[
]
-----------------------------------------------------------
=
ε
s
4
U
–
GF
ν
1
+
(
)
U
ν
1
–
(
)
–
[
]
----------------------------------------------------------
=
