Allen-Bradley LOGIX 5000 Reference Manual page 249

Process Control Instructions
Temperature Example Explanation
Manipulating the PV at the setpoint is the top priority. The high pressure steam
and cooling are selected as the most active actuators. At steady state, the same two
controls should assume their target values: CV3 at 10% and CV2 at 0%. CV1 will
assume any value needed to maintain PV at the setpoint; therefore, its target value
is irrelevant since manipulating the PV at the setpoint is a higher priority control
objective. Target CV priorities and target CV values can be changed on-line.
The CC function block calculates CV1, CV2 and CV3 such that the control goals
are accomplished in the following order of importance:
1. Control PV to SP
2. Control CV2 to its target value
3. Control CV3 to its target value
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 on-line in either auto or
Manual mode. For tuning, the default settings will be used. Refer to CC Function
Block Tuning.
If you do not know the process models, you need to identify the models and tune
the function block by using the built-in tuner (modeler) for the function block to
operate correctly in the Auto mode.
The function block uses first order lag with deadtime internal process models and
first order filters (total of up to twelve tuning parameters) to calculate the CV's.
Each 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 constants. The
smaller the response time constants, the faster the control variable response will be.
The response time constants should be set such that the PV reaches the setpoint in
reasonable time based on the process dynamics. The larger the response time
constants are, the slower the control variable response will be, but the control
variable also becomes more robust. See the tuning section for more details.
In the Manual mode, the control variables (CV) are set equal to the operator-
entered or program-generated CVnOper or CVnProg parameters. For the Manual
to Auto mode bumpless transfer and for safe operation of the control variable, the
CV rate of change limiters are implemented such that CV's cannot move from
current states by more than specified CV units at each scan.
To limit the CV rate of change:
•
Set the CVnROCPosLimit and CVnROCNegLimit
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
Chapter 1
249