Design Considerations

It is probably apparent that most overrides are really feedback control loops. They therefore are subject to stability considerations. In many cases they are also subject to truly hard constraints, as, for example, maximum column-base pressure. Since any feedback control system must have some room within which to work, the overrides must be so designed that the process does not normally approach the hard constraints too closely.

In the case of overrides with proportional-only action, we can visualize, as shown in Figure 9.18, a zone between hard and soft constraints. The width* of this zone is determined by the override loop gain, which is limited by stability considerations. With a proportional control loop designed for dead-beat response, there is a unique relationship between the value of the manipulated variable and the distance between the process variable and its hard constraint. The manipulated variable always reaches its maximum (or minimum) value before the process variable exceeds its hard constraint. The soft constraint will correspond to the minimum (or maximum) value of the override output. The takeover point between normal and override controls will be at a variable position (depending on operating conditions) somewhere between the hard and soft constraints.

* It is very helpful to think of this zone as a fraction of the measured variable transmitter span.



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