ABB IMASI13, Инструкция по эксплуатации

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Automatic Adjustments and Corrections
WBPEEUI240766B0 2 - 7
Point Value Calculation
The ASI module maintains a set of adjustment values for each
input channel. These values correct for offset and gain errors
in the input channel. The raw analog-to-digital converter
count value is converted to an actual input signal value using
the calibration data.
For thermocouple inputs, an adjustment is made for the cold
junction temperature of the thermocouple. An additional,
user-specified adjustment is then applied, if one has been
defined with FC 217. The final corrected input reading is then
converted to engineering units using either thermocouple or
RTD conversion tables, or the engineering unit zero and span
values specified for the input.
The following sections describe the various types of input value
adjustments.
Input Calibration
Each input channel is calibrated at the factory. During the cal-
ibration procedure, any offset and gain errors are identified
and required correction factors are calculated and stored in
the analog input module's nonvolatile memory.
These factors are used when the input channel is scanned to
correct the reading. A calibration adjustment is applied to
active channels and the cold junction reference input.
Cold Junction Compensation
Thermocouple input channels are adjusted for cold junction
temperature. The cold junction reference can either be the
built-in reference available on the I/O module's termination
unit or a value originating from any other function code block
output anywhere in the system. The cold junction reference
supplied by the controller is assumed to be in degrees C. The
ASI module converts this value to millivolts and adds it to the
value from the analog-to-digital converter.
Thermocouple channels identified to be cold junction reference
inputs (FC 216, specification S4) use the built-in cold junction
reference on the termination unit for their cold junction com-
pensation.