Section 8. Operation
By supplying a precise and known voltage to a resistive-bridge circuit and
measuring the returning voltage, resistance can be calculated.
CRBasic instructions for measuring resistance include:
BrHalf()
— half-bridge
BrHalf3W()
— three-wire half-bridge
BrHalf4W()
— four-wire half-bridge
BrFull()
— four-wire full-bridge
BrFull6W()
— six-wire full-bridge
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Available resistive-bridge completion modules are listed in the
appendix
Signal Conditioners
(p. 647).
The CR1000 has five CRBasic bridge-measurement instructions. Table
Resistive-
Bridge Circuits with Voltage Excitation
(p. 338)
shows ideal circuits and related
equations. In the diagrams, resistors labeled R
s
are normally the sensors and those
labeled R
f
are normally precision fixed (static) resistors. CRBasic example
Four-
Wire Full-Bridge Measurement
(p. 340)
lists CRBasic code that measures and
processes four-wire full-bridge circuits.
Offset voltages compensation applies to bridge measurements. In addition to
RevDiff
and
MeasOff
parameters discussed in the section
Offset Voltage
Compensation
(p. 323),
CRBasic bridge 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
,
MeasOff
, and
RevEx
to
True
.
Measurement speed can be slowed when using
RevDiff
,
MeasOff
, and
RevEx
.
When more than one measurement per sensor are necessary, such as occur 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. Further, the results of measurement
instructions (X) must be processed further to obtain the resistance value. This
processing requires additional program execution time.
338
Summary of Contents for CR1000
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