IMC Function Block Configuration

Follow these steps to create a basic IMC configuration.
  1. Starting with the default configuration, configure the following parameters.
    Parameter
    Description
    PVEUMax
    Maximum scaled value for PV.
    PVEUMin
    Minimum scaled value for PV.
    SPHLimit
    SP high limit value, scaled in PV units.
    SPLLimit
    SP low limit value, scaled in PV units.
    CVInitValue
    An initial value of the control variable output.
  2. If you have the process model available, you can intuitively tune the IMC control variable by entering the following four parameters.
    Parameter
    Description
    Model Gain
    A nonzero number (negative for direct acting control variable. positive for reverse acting control variable).
    Model Time Constant
    Always a positive number.
    Model Deadtime
    Always a positive number.
    Response Time Constant
    Always a positive number - used to tune the response of the IMC control variable. A smaller number gives a faster response.
    At this point, you have completed the basic configuration. You did not configure the built-in tuner. The control variable is ready to be put online in either Auto or Manual mode. For tuning, use the default settings. Refer to IMC Function Block Tuning.
  3. If you do not know the process model, you need to identify the model and tune the control variable by using the built-in tuner (modeler) for the control variable to operate correctly in the Auto mode.
    The control variable uses a first order lag with deadtime internal process model and a first order filter (total of four tuning parameters) to calculate the CV. The CV is calculated such that the process variable (PV) follows a first order lag trajectory when approaching the setpoint value.
    Speed of response depends on the value of the response time constant. The smaller that the response time constant is, the faster the control variable response will be. The response time constant should be set such that the PV reaches the setpoint in a reasonable time based on the process dynamics. The larger that the response time constant is, the slower the control variable response will be, but the control variable also becomes more robust. Refer to IMC Function Block Tuning.
    In the Manual mode, the CV is set equal to the operator-entered or program-generated CVOper or CVProg parameter.
    For the Manual to Auto mode bumpless transfer and for safe operation of the control variable, the CV rate of change limiter is implemented such that the CV cannot change from its current state any faster than the rate of change limit parameter specified.
  4. Set the CVROCPosLimit and CVROCNegLimit to limit the CV rate of change.
    Rate limiting is not imposed when the control variable is in Manual mode unless CVManLimiting is set.
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