Page 331 - Introduction to Marine Engineering

Instrumentation and control 305

Final condition

Process

Proportional plus integral action

Desired value
Proportional plus integral plus derivative action

Time
Figure 15.31 System response

Controllers

The controller may be located close to the variable measuring point and
thus operate without the use of a transmitter. It may however be located
in a remote control room and receive a signal from a transmitter and
relay, as mentioned earlier. The controller is required to maintain some
system variable at a desired value regardless of load changes. It may also
indicate the system variable and enable the desired value to be changed.
Over a short range about the desired value the controller will generate a
signal to operate the actuating mechanism of the correcting unit. This
control signal may include proportional, integral and derivative actions,
as already described. Where all three are used it may be known as a
'three-term' controller.
A pneumatic three-term controller is shown in Figure 15.32. Any
variation between desired and measured values will result in a
movement of the flapper and change in the output pressure. If the