Page 349 - Introduction to Marine Engineering

322 Instrumentation and control

Figure 15.45 Bridge control of controllable-pitch propeller

would supply fuel to the engine to start and continue operation. A
feedback signal of engine speed would shut off the starting air and also
enable the governor to control engine speed. Engine speed would also
be provided as an instrument reading at both control stations.
A bridge control system for a controllable-pitch propeller is shown in
Figure 15.45. The propeller pitch and engine speed are usually
controlled by a single lever (combinator). The control lever signal passes
via the selector to the engine governor and the pitch-operating actuator.
Pitch and engine speed signals will be fed back and displayed at both
control stations. The load control unit ensures a constant load on the
engine by varying propeller pitch as external conditions change. The
input signals are from the fuel pump setting and actual engine speed.
The output signal is supplied as a feedback to the pitch controller.
The steering gear is, of course, bridge controlled and is arranged for
automatic or manual control. A typical automatic or auto pilot system is
shown in Figure 15.46. A three-term controller provides the output
signal where a course deviation exists and will bring about a rudder
movement. The various system parts are shown in terms of their system
functions and the particular item of equipment involved. The feedback
loop between the rudder and the amplifier (variable delivery pump)
results in no pumping action when equilibrium exists in the system.
External forces can act on the ship or the rudder to cause a change in the