Honeywell 51204160-175 MC-TDIY22

Override control strategies are primarily used for multivariable constraint control. In such astrategy, control of a single process variable is based on one of two or more PVs. Thesemultiple PVs are referred to as "constraints." A selector algorithm selects the PV to beused for control. When a different PV is selected, the former PV has been overridden—the new PV has constrained the others.

Consider the following examples:

In a typical boiler-control strategy, the constraint on the fuel flow (SP of the fuel-flow

PID controller) can be that it cannot exceed the actual airflow, multiplied by a ratio

(which may be computed by the O2 controller). In this case, the fuel flow is the primaryPV to be controlled and the actual air-flow PV is the "constraint."

In some heating applications, it may be desirable to control the temperature of the feed(the primary PV) as well as possible, without ever letting the temperature of the hottestpart of the pot (or the heat exchanger) exceed a safe limit. In this case, the safe limit onthe pot temperature is the "constraint."

In an oven temperature-control application, the temperatures can be measured atseveral places in the oven, and it may be desirable to control all these PVs bycontrolling one valve. One PID controller can be used for each PV, and the PIDcontroller representing the PV that is farthest from its SP can be allowed to control thevalve. In this case, there is no primary PV, because all the PVs have the sameimportance. Each PID controller represents a constraint on the other PID controllers.

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