measurement instructions include the RevEx parameter that provides the option to program a
second set of measurements with the excitation polarity reversed. Much of the offset error
inherent in bridge measurements is canceled out by setting RevDiff, RevEx, and MeasOff
to True.
Measurement speed may be reduced when using RevDiff, MeasOff, and RevEx. When
more than one measurement per sensor is necessary, such as occurs with the BrHalf3W(),
BrHalf4W(), and BrFull6W() instructions, input and excitation reversal are applied
separately to each measurement. For example, in the four-wire half-bridge (BrHalf4W()),
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 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,mV200,U1,U3,1,2500,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 Resistance measurements with current excitation
U terminals can be configured to supply precise current excitation for use with resistive bridges.
Resistance can be measured by supplying a precise current and measuring the return voltage.
6. Measurements 72
To Next Page
Loading...
