Chapter 2
Using the Module
©
National Instruments Corporation
2-19
The following symbols apply to the circuit diagram and equations:
•
R
1
through R
4
—Active elements of the bridge, located inside the sensor.
•
R
s
—Shunt calibration resistor located inside the NI PXIe-4330/4331.
•
R
L
—Lead resistance.
•
V
CH
—Measured voltage (sensor output).
•
V
EX
—Excitation voltage provided by the NI PXIe-4330/4331.
•
V
r
—Offset compensated ratiometric bridge output defined by the following equation:
Note
The ratio of the bridge output voltage and the excitation voltage is calculated
internally on the NI PXIe-4330/4331.
Converting module readings to physical force, pressure, or torque readings can be performed
using linear, table, or polynomial scaling.
In NI-DAQmx, linear scaling for bridge-based force, pressure, and torque sensors is done
based on two points which are specified as pairs of corresponding physical and electrical
values: (
electrical
1
,
physical
1
) and (
electrical
2
,
physical
2
). These should be based on the
calibration certificate of the sensor, if one is available; otherwise, they can be based on the
specifications or data sheet of the sensor. Any two points can be used, assuming that they are
far enough apart to accurately determine the slope of the linear scaling equation. For example:
•
physical
1
—The zero point of the sensor: zero force, pressure, or torque.
•
electrical
1
—The electrical output corresponding to the zero point of the sensor, in mV/V
or V/V.
•
physical
2
—The maximum physical reading of the sensor, or capacity: maximum load,
pressure, or torque.
•
electrical
2
—The electrical output corresponding to the maximum physical reading of the
sensor, in mV/V or V/V.
Note
Some sensor calibration certificates specify the electrical output in mV or V, not
mV/V or V/V. If this is the case, divide the specified electrical output by the excitation
voltage at which the calibration was performed.
V
r
V
CH
(under load)
V
CH
(no load)
–
(
)
V
EX
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=
