Offset voltage compensation applies to bridge measurements.
RevDiff
and
MeasOff
parameters are discussed in
(p. 147). Much of the offset error inherent
in bridge measurements is canceled out by setting
RevDiff
and
MeasOff
to True.
CRBasic Example 1: 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,mV2500,1,Vx1,1,2500,False,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
14.3.2 Strain measurements
A principal use of the four-wire full bridge is the measurement of strain gages in structural stress
analysis.
StrainCalc()
calculates microstrain (µɛ) from the formula for the specific bridge
configuration used. All strain gages 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 gage 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
dashed code when the bridge is configured so the output increases with increasing strain. A
dashed code sets the polarity of V
r
to negative.
14. Measurements
75