Step Function Response

The step function response consists of applying an instantaneous change in the control means and then recording graphically the result of the change on the measured variable. This graphical record obtained in response to the step excitation is called the Process Reaction curve. Two typical process reaction curves are shown in Fig. 21.2.

Curve A shown in Fig. 21.2, describes a simple process in which the measured variable begins to change as soon as the step change occurs in the position of the final element. This means that there is minimum or no resistance to an energy change in the process. At the start, the rate of change is very rapid, but as the process continues, the rate of change slows down.

Hence, the process stores up a part of the energy change. This characteristic of a process is called its capacity. The rate of change of the process reaction curve is called the process reaction rate.

Curve B describes a more complicated process in which there is resistance, more than one capacity and another process characteristic called the dead time.

Dead time is defined as the time gap between the instant deviation or error occurs and the instant when the corrective action starts the first time.

Curve B, indicates, that at the start and for a short time thereafter, no action or nothing happens to the value of the measured variable due to the effect of the dead time. When the variable does begin to change, it does so slowly at first, then speeds up until it approaches the final value and then again it slows down. This change in reaction rate is due to the combination of capacity and resistance, into another capacity. This characteristic is called the transfer lag.

A process having a reaction curve similar to curve A can be controlled by the simplest form of controller. A process produces a reaction curve similar to curve B presents some difficulties. Hence, to select the appropriate controller for a process, an understanding of the control action and of their ability to handle the process characteristics is essentially required.

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