when excitation is reversed, the differential measurement of the voltage drop across the sensor is
made with excitation at both polarities and then excitation is again applied and reversed for the
measurement of the voltage drop across the fixed resistor. The results of the measurements (X)
must then be processed further to obtain the resistance value, which requires additional program
execution time.
CRBasic Example 2: :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 X_1
Public R_1
Public R_2 = 1000 'Resistance of fixed resistor R2
Public R_3 = 1000 'Resistance of fixed resistor R3
Public R_4 = 1000 'Resistance of fixed resistor R4
'Main Program
BeginProg
Scan(500,mSec,1,0)
'Full Bridge Measurement:
BrFull(X,1,mV250,1,Vx1,1,4000,True,True,0,60,1.0,0.0)
X_1 = ((-1 * X) / 1000) + (R_3 / (R_3 + R_4))
R_1 = (R_2 * (1 - X_1)) / X_1
NextScan
EndProg
6.3.2 Strain measurements
A principal use of the four-wire full bridge is the measurement of strain gauges in structural
stress analysis. StrainCalc() calculates microstrain (µɛ) from the formula for the specific
bridge configuration used. All strain gauges supported by StrainCalc() use the full-bridge
schematic. 'Quarter-bridge', 'half-bridge' and 'full-bridge' refer to the number of active elements
in the bridge schematic. In other words, a quarter-bridge strain gauge has one active element, a
half-bridge has two, and a full-bridge has four.
StrainCalc() requires a bridge-configuration code. The following table shows the equation
used by each configuration code. Each code can be preceded by a dash (-). Use a code without
the dash when the bridge is configured so the output decreases with increasing strain. Use a
6. Measurements 66
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