Chapter 5
Developing Your Application
©
National Instruments Corporation
5-7
NI PXI-4220 User Manual
the configuration of the global channel(s). Refer to the
Creating a Strain
Global Channel or Task
section of Chapter 3,
Configuring and Testing
,
for information about offset null compensation when in MAX. If you are
not measuring strain, or would like to adjust the offset to an arbitrary
voltage, you can manually adjust the coarse and fine potentiometer settings
using properties.
For more information about offset null compensation, refer to the
Offset
Null Compensation
section of Chapter 4,
Theory of Operation
.
Perform Shunt Calibration
Shunt calibration is a process used to obtain a gain adjust factor, which is
used to correct for system gain error and discrepancies between nominal
gauge factor and actual gauge factor of the strain gauge. If you are
measuring strain, you can use a strain task or global channel to perform
shunt calibration. The shunt calibration is performed during the
configuration of the global channel(s). Refer to the
Creating a Strain
Global Channel or Task
section of Chapter 3,
Configuring and Testing
,
for information about shunt calibration when in MAX.
To manually perform shunt calibration, refer to the
Shunt Calibration
section of Chapter 4,
Theory of Operation
.
Acquiring, Analyzing, and Presenting
After configuring the task and channels, you can start your acquisition, read
measurements, analyze the data returned, and display it according to the
needs of your application. Typical methods of analysis include digital
filtering, averaging data, performing harmonic analysis, applying a custom
scale, or adjusting measurements mathematically.
Some custom scaling applications require the actual excitation voltage
applied to the bridge instead of the nominal excitation voltage output by the
NI PXI-4220. You can scan the remote sense pins RS
X
+ (pin 3) and
RS
X
– (pin 8) with the DAQmx physical channels
Dev
X/_pPosX
and
Dev
X/_pNegX
to find the actual excitation voltage. Take the difference of
the two physical channels to determine the actual excitation applied to the
bridge, and use this value in your scaling equation.
Note
If RS
X
+ and RS
X
– are not wired to the bridge where P
X
+ and P
X
– connect, then
_pPos
X
and
_pNeg
X
only measure the internal excitation. Measuring this voltage does not
correct for the voltage drop in the excitation leads.
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