Function Block Properties Dialog Box - Model Tab Overview (IMC)

The Internal Model Control (IMC) instruction controls a single process variable by manipulating a single control-variable output. This function block performs an algorithm where the actual error signal is compared against that of an internal first-order lag plus deadtime model of the process. The IMC function block calculates the control variable output (CV) in the Auto mode based on the PV deviation from the set point (SP), internal model, and tuning.
Use the
Model
 tab of the
Function Block Properties
 dialog box to set the values of the parameters for the selected IMC function block.
The following table describes the parameters available on the
Model
tab:
Parameter
Description
Model Gain In
Select whether process gain calculates error as:
  • % of Span
  • Engineering Units
The default value is
% of Span
.
Process Type
Select the process type as:
  • Non-integrating
  • Integrating
The default value is
Non-integrating
.
Model Factor
Enter a value for the non-integrating model approximation factor.
This value only applies for
Integrating
 process type. It appears dimmed when the process type is set to
Non-integrating
.
The default value is 100.
Process Gain Sign
Select Positive or Negative as the sign of the process gain.
  • Negative
     - negative process gain (increase in output causes a decrease in PV).
  • Positive
     - reset to indicate a positive process gain (increase in output causes an increase in PV).
The default value is
Positive
.
Model Gain
Enter a value for the internal model gain parameter in seconds.
Enter a positive or negative value based on process direction.
The default value is 0.0.
Model Time Constant
Enter a value in seconds for the internal model time constant in the range of 0.0 to the maximum positive floating point value.
The default value is 0.0.
Model Deadtime
Enter a value in seconds for the internal model deadtime in the range of 0.0 to the maximum positive floating point value.
The default value is 0.0.
Response Time Constant
Enter a value in seconds for the tuning parameter that determines the speed of the controller action.
The valid range is 0.0 to the maximum positive floating point value.
The default value is 0.0.
Status
Displays status information for blocks containing status bits. If none of the status bits are set, an
OK
 message appears in this field. Otherwise, a list of the status bits that are set appears here. This list is comprised of the parameter names associated with the status bits, and typically explain which parameters were configured with invalid values.
Execution Order
Displays the block's execution number if the routine is in a verified state. If the routine is not yet verified, a
Routine not verified
 message appears. Note that this item is read-only.
Never display description in a routine
Select this check box if you do not want the description to display in the routine.
OK
Click this button to add the selected item in the instruction list to the routine and close the
Function Block Properties
 dialog box.
Cancel
Click this button to cancel any edits you have made and close the
Function Block Properties
 dialog box.
Apply
Click this button to accept your changes and continue editing in the
Function Block Properties
 dialog box.
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