Page 329 - Introduction to Marine Engineering

Instrumentation and control 303

Open

Closed ~

Time
Figure 15.29 Two-step or on-off control

again and the temperature will cease to fall and will rise again. This form
of control is acceptable where a considerable deviation from the desired
value is allowed.

Continuous action

Proportional action
This is a form of continuous control where any change in controller
output is proportional to the deviation between the controlled condition
and the desired value. The proportional band is the amount by which the
input signal value must change to move the correcting unit between its
extreme positions. The desired value is usually located at the centre of
the proportional band. Offset is a sustained deviation as a result of a load
change in the process. It is an inherent characteristic of proportional
control action. Consider, for example, a proportional controller
operating a feedwater valve supplying a boiler drum. If the steam
demand, i.e. load, increases then the drum level will fall. When the level
has dropped the feedwater valve will open. An equilibrium position will
be reached when the feedwater valve has opened enough to match the
new steam demand. The drum level, however, will have fallen to a new
value below the desired value, i.e. offset. See Figure 15.30.

Integral action
This type of controller action is used in conjunction with proportional
control in order to remove offset. Integral or reset action occurs when
the controller output varies at a rate proportional to the deviation