Process Control Instructions
182 Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
The function block behaves in a defined way for any combination of CV Active
and Target lists and CV Auto-Manual modes. The function block attempts to
accomplish these goals in the following order of priorities:
1. Control PV to SP
2. Control Target1stCV to its target value
3. Control Target2ndCV to its target value
If any CV is put in Manual mode, the CC function block gives up the goal with
priority 3. If two CV's are in Manual mode, the CC function block is reduced to
an IMC, (single input, single output) control variable controlling the PV to its
setpoint.
In addition to this, however, the control variable reads the Manual CV values from
the CV's that are in Manual mode as feedforward signals. Then, the CC function
block predicts the influence of the Manual CV values on the PV by using the
appropriate internal models, and calculates the third CV that remains in Auto
mode.
For integrating process types (such as level control and position control), internal
nonintegrating models are used to approximate the integrating process. The
Factor parameter is used to convert the identified integrating process models to
nonintegrating internal models used for CV calculation. This is necessary to
provide for stable function block execution.
A model initialization occurs:
• During First Scan of the block
• When the ModelInit request parameter is set
• When DeltaT changes
You may need to manually adjust the internal model parameters or the response
time constants. You can do so by changing the appropriate parameters and setting
the appropriate ModelInit bit. The internal states of the control variable will be
initialized, and the bit will automatically reset.
For example, modify the Model Gain for CV2 - PV model. Set the ModelInit2
parameter to TRUE to initialize the CV2 - PV internal model parameters and for
the new model gain to take effect.
Initialization
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