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Chapter 4
Theory of Operation
4-2
ni.com
Wheatstone bridge. The Wheatstone bridge configuration is used to help
measure the small variations in resistance that the sensing elements
produce in response to a physical changes in the specimen.
Acronyms, Formulas, and Variable Definitions
In the figures and equations in this document, the acronyms, formulas, and
variables are defined as:
ε
is the measured strain (+
ε
is tensile strain and –
ε
is compressive strain).
ε
S
is the simulated strain.
GF
is the Gauge Factor, which should be specified by the gauge
manufacturer.
R
g
is the nominal gauge resistance, which should be specified by the gauge
manufacturer.
R
L
is the lead resistance. If lead lengths are long,
R
L
can significantly affect
measurement accuracy.
R
s
is the shunt calibration resistor value.
U
is the ratio of expected signal voltage to excitation voltage with the shunt
calibration circuit engaged. Parameter
U
appears in the equations for
simulated strain and is defined by the following equation:
ν
is the Poisson’s ratio, defined as the negative ratio of transverse strain to
axial strain (longitudinal) strain. Poisson’s ratio is a material property of the
specimen you are measuring.
V
CH
is the measured voltage of the signal.
V
EX
is the excitation voltage.
V
r
is the voltage ratio that is used in the voltage to strain conversion
equations and is defined by the following equation:
U
R
g
–
4
R
s
2
R
g
+
------------------------
=
V
r
V
CH
strained
(
)
V
CH
(unstrained)
–
V
EX
-------------------------------------------------------------------------------
=