Chapter 4
Theory of Operation
©
National Instruments Corporation
4-3
NI PXI-4220 User Manual
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
impact 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 signal’s voltage.
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:
Software Scaling and Equations
Once you have acquired the voltage signal
V
CH
, you can scale this voltage
to the appropriate strain units in software. This is done automatically for
you in MAX using a strain task or strain channel. You also can scale the
voltages manually in your application using the voltage to strain conversion
equations provided in this document for each configuration type.
Finally, there are voltage to strain conversion functions included in
LabVIEW and NI-DAQmx. In LabVIEW, the conversion function,
Convert
Strain
Gauge
Reading.vi
, is in the
Functions»All
Functions»NI Measurements»Data Acquisition»Signal Conditioning
subpalette.
U
R
g
–
4
R
S
2
R
g
+
-------------------------
=
V
r
V
CH
strained
(
)
V
CH
(unstrained)
–
V
EX
-------------------------------------------------------------------------------
=
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