CRBasic Example 65. Four-Wire Full-Bridge Measurement and Processing
'This program example demonstrates the measurement and processing of a four-wire resistive
'full bridge. In this example, the default measurement stored in variable X is
'deconstructed to determine the resistance of the R1 resistor, which is the variable
'resistor in most sensors that have a four-wire full-bridge as the active element.
'Declare Variables
Public X
Public X1
Public R1
Public R2 = 1000 'Resistance of fixed resistor R2
Public R3 = 1000 'Resistance of fixed resistor R2
Public R4 = 1000 'Resistance of fixed resistor R4
'Main Program
BeginProg
Scan(500,mSec,1,0)
'Full Bridge Measurement:
BrFull(X,1,mV2500,1,Vx1,1,2500,True,True,0,_60Hz,1.0,0.0)
X1 = ((-1 * X) / 1000) + (R3 / (R3 + R4))
R1 = (R2 * (1 - X1)) / X1
NextScan
8.1.2.4.1 Ac Excitation
Some resistive sensors require ac excitation. Ac excitation is defined as excitation
with equal positive (+) and negative (–) duration and magnitude. These include
electrolytic tilt sensors, soil moisture blocks, water-conductivity sensors, and
wetness-sensing grids. The use of single polarity dc excitation with these sensors
can result in polarization of sensor materials and the substance measured.
Polarization may cause erroneous measurement, calibration changes, or rapid
sensor decay.
Other sensors, for example, LVDTs (linear variable differential transformers),
require ac excitation because they require inductive coupling to provide a signal.
Dc excitation in an LVDT will result in no measurement.
CRBasic bridge-measurement instructions have the option to reverse polarity to
provide ac excitation by setting the RevEx parameter to True.
Note Take precautions against ground loops when measuring sensors that require
ac excitation. See Ground Looping in Ionic Measurements (p. 109).
8.1.2.4.2 Resistance Measurements — Accuracy
Read More Consult the following technical papers at www.campbellsci.com/app-
notes (http://www.campbellsci.com/app-notes) for in-depth treatments of several
topics addressing voltage measurement quality:
• Preventing and Attacking Measurement Noise Problems
• Benefits of Input Reversal and Excitation Reversal for Voltage Measurements
• Voltage Measurement Accuracy, Self- Calibration, and Ratiometric
Measurements
• Estimating Measurement Accuracy for Ratiometric Measurement Instructions.
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