Process Control Instructions
244 Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
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.
Parameter Description
ModelGains Nonzero numbers (negative for direct acting control variable, positive
for reverse acting control variable)
ModelTimeConstants Always positive numbers
ModelDeadtimes Always positive numbers
RespTimeConstants Always positive numbers
Active 1st, 2nd and 3rd CV for PV1 and PV2 Specify the order in which CV's will be used to compensate for PV - SP
error.
TargetCV Specify which CV will be driven to its target value.
CVTargetValues Specify to which values should the control variable drive the
individual CV's if selected as the TargetCV.
TargetRespTC Specify the speed of CV's to approach the target values.
For integrating process types (such as level control and position control), internal
nonintegrating models are used to approximate the integrating process. The
Factor parameters are used to convert the identified integrating process models to
nonintegrating internal models used for CV calculation. This is necessary to
provide for stable MMC execution. The MMC function block can handle any
combinations of PV1 and PV2 that are integrating or nonintegrating process
types.
The function block uses first order lag with deadtime internal process models and
first order filters (total of up to 24 tuning parameters-6 models, 4 parameters each)
to calculate the CV's. Each CV is calculated such that each 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's reach the setpoints in
reasonable time based on the process dynamics. The larger that the response time
constants, the slower the control variable response will be, but the control variable
also becomes more robust.
In the Manual mode, the control variables (CV) are set equal to the operator-
entered Manual CV 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.
Set the CVROCPosLimit and CVROCNegLimit to limit the CV rate of change.
Rate limiting is not imposed when control variable is in Manual mode unless
CVManLimiting is set.
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