DNx-AI-208 Analog Input Layer
24
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Vers:
4.6
Date: November 2013
File:
AI-208 App B.fm
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United Electronic Industries, Inc.
Appendix B
Shunt Calibration Support in Framework
B.1
Introduction
Strain-gauges and load cell measurements are typically based on the Wheat-
stone bridge, which allows the measurement of the very small resistance
changes that characterize strain gauges.
The values measured from a Wheatstone bridge are very sensitive to the resis-
tance of its branches and there can be signal attenuation caused by lead resis-
tances.
Shunt calibration is used to compensate for the loss of sensitivity (The strain
gauge is “desensitized”).
Shunt calibration is the action of simulating a load on one of the branches of a
Wheatstone bridge with a resistor of a known value and comparing the mea-
sured value to the calculated ideal value.
The ratio between the ideal value and the measured value is called “Gain
Adjustment Factor”. It should be very close to 1. Multiplying the measurement
value by the gain adjustment factor compensates for the loss of sensitivity intro-
duced by the lead resistances in a four-wire gauge.
B.2
Theory
Load cell and strain gauge measurement are normally done through a Wheat-
stone bridge.
For load cells, the Wheatstone bridge is built into the cell.
For Strain Gauges
,
the bridge is part of the wiring.
Figure B-1. Strain Gauge Bridge
Vex is the excitation voltage applied to the bridge by the instrument.
Vout is the output voltage measured by the instrument.
The formula to calculate Vout, knowing Vex, is:
Vex
Vout
R4(Rg)
R1
R2
R3
Vout
Vex
R4
R3
R4
+
---------------------
R1
R1
R2
+
---------------------
–
=
Eq. 1:
