MMC Function Block Configuration
Starting with the default configuration, configure the following parameters.
Parameter | Description |
PV1EUMax | Maximum scaled value for PV1. |
PV1EUMin | Minimum scaled value for PV1. |
PV2EUMax | Maximum scaled value for PV2. |
PV2EUMin | Minimum scaled value for PV2. |
SP1HLimit | SP1 high limit value, scaled in PV units. |
SP1LLimit | SP1 low limit value, scaled in PV units. |
SP2HLimit | SP2 high limit value, scaled in PV units. |
SP2LLimit | SP2 low limit value, scaled in PV units. |
CV1InitValue | An initial value of the control variable CV1 output. |
CV2InitValue | An initial value of the control variable CV2 output. |
CV3InitValue | An initial value of the control variable CV3 output. |
If you have the process models available, you can intuitively tune the MMC function block by entering the following parameters. At this point, you have completed the basic configuration. You did not configure the built-in tuner. The function block variables are ready to be put on-line in either auto or Manual mode. For tuning, the default settings will be used.
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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