Function Block Properties Dialog Box - Limits 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
Limits
 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
Limits
tab:
Parameter
Description
Set Point
  • High Limit
Enter a value for the SP high limit, scaled in PV units.
The valid range is
SPLLimit
 to
PVEUMax
.
The default value is 100.0.
  • Low Limit
Enter a value for the SP low limit, scaled in PV units.
The valid range is
PVEUMin
 to
SPHLimit
.
The default value is 0.0.
Control Variables
  • High Limit
Enter the control variable high limit value.
This is used to set the
CVHAlarm
 output. It is also used for limiting a CV when in Auto, CascadeRatio, or Manual mode if
CVManLimiting
 is TRUE.
Valid values are in the range of
CVLLimit
 <
CVHLimit
 LessThanOrEqualTo 100.0.
  • Low Limit
Enter the control variable low limit value.
This is used to set the
CVHAlarm
 output. It is also used for limiting a CV when in Auto, CascadeRatio, or Manual mode if
CVManLimiting
 is TRUE.
Valid values are in the range of 0.0 LessThanOrEqualTo
CVLLimit
 <
CVHLimit
.
  • Positive Rate of Change
Enter the CV rate of change (ROC) limit, in percent per second.
ROC limiting is only used when in Auto, CascadeRatio, or Manual mode if
CVManLimiting
 is TRUE.
A value of zero disables CV ROC limiting.
Valid values are in the range of 0 to the maximum positive floating point value.
The default value is 0.0.
  • Negative Rate of Change
Enter the CV rate of change (ROC) limit, in percent per second.
ROC limiting is only used when in Auto, CascadeRatio, or Manual mode if
CVManLimiting
 is TRUE.
A value of zero disables CV ROC limiting.
Valid values are in the range of 0 to the maximum positive floating point value.
The default value is 0.0.
  • Limit Control Variable in Manual Mode
Select this check box to limit the control variable in manual mode.
If
Manual
 mode and
CVManLimiting
 are TRUE, CV is limited by the
CVHLimit
and
CVLLimit
 values.
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.
Provide Feedback
Have questions or feedback about this documentation? Please submit your feedback here.