Strain Measurements - Campbell CR300 Series Product Manual

Offset voltage compensation applies to bridge measurements. RevDiff and MeasOff
parameters are discussed in
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
Public R_3 = 1000
Public
R_4 = 1000
'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
StrainCalc()
analysis.
configuration used. All strain gages supported by
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
Minimizing offset voltages
'Resistance of fixed resistor R2
'Resistance of fixed resistor R3
'Resistance of fixed resistor R4
calculates microstrain (µɛ) from the formula for the specific bridge
to negative.
r
(p. 147). Much of the offset error inherent
StrainCalc()
use the full-bridge
14. Measurements 75