Page 332 - Introduction to Marine Engineering

306 Instrumentation and control

Output
Measured value Desired value
Orifice

.Supply
air
Nozzle

Integral
action Feedback
bellows bellows

Derivative
action —
valve

Integral
action
valve
Figure 15.32 Pneumatic three-term controller

derivative action valve is open and the integral action valve closed, then
only proportional control occurs. It can be seen that as the flapper
moves towards the nozzle a pressure build-up will occur which will
increase the output pressure signal and also move the bellows so that the
flapper is moved away from the nozzle. This is then a negative feedback
which is proportional to the flapper or measured value movement.
When the integral action valve is opened, any change in output signal
pressure will affect the integral action bellows which will oppose the
feedback bellows movement. Varying the opening of the integral action
valve will alter the amount of integral action of the controller. Closing
the derivative action valve any amount would introduce derivative
action. This is because of the delay that would be introduced in the
provision of negative feedback for a sudden variable change which
would enable the output signal pressure to build up. If the measured
variable were to change slowly then the proportional action would have
time to build up and thus exert its effect.
An electronic three-term controller is shown in Figure 15.33. The
controller output signal is subjected to the various control actions, in this