Programming instructions

Chapter 21 Common SCXI Applications
© National Instruments Corporation 21-13 LabVIEW Data Acquisition Basics Manual
10 times the number of points acquired for each channel. After your device
averages the voltage data from the AI Read VI, it converts the voltage
values to temperature. After completing the acquisition, remember to
always clear the acquisition by using the AI Clear VI.
Measuring Pressure with Strain Gauges
Strain gauges give varying voltages in response to stress or vibrations
in materials. Strain gauges are thin conductors attached to the material
to be stressed. Resistance changes in parts of the strain gauge to indicate
deformation of the material. Strain gauges require excitation (generally
voltage excitation) and linearization of their voltage measurements.
Depending on the strain gauge configuration, another requirement for
using strain gauges with SCXI is a configuration of resistors. As shown in
Figure 21-7, the resistance from the strain gauges combined with the
SCXI hardware form a diamond-shaped configuration of resistors, know
as a
Wheatstone bridge
. When you apply a voltage to the bridge, the
differential voltage (V
m
) varies as the resistor values in the bridge change.
The strain gauge usually supplies the resistors that change value with strain.
Figure 21-7.
Half-Bridge Strain Gauge
Strain gauges come in full-bridge, half-bridge, and quarter-bridge
configurations. For a full-bridge strain gauge, the four resistors of the
Wheatstone bridge are physically located on the strain gauge itself. For a
half-bridge strain gauge, the strain gauge supplies two resistors for the
Wheatstone bridge while the SCXI module supplies the other two resistors,
as shown above. For a quarter-bridge strain gauge, the strain gauge only
supplies one of the four resistors for a Wheatstone bridge. For more
information on how to connect your strain gauge to SCXI, refer to the
Getting Started with SCXI
manual.
-
+
R
g
R
1
R
2
Physical strain gauge
is value at rest
=
Supplied by signal
conditioning hardware
DC Voltage
Excitation
V
m
R
1
R
2
R
g
R
g