The Autopilot is basically used when a ship has to steer a set course for a long time without alteration because any deviation from the set course is controlled electronically and automatically.
3 Types of controls:
a) PROPORTIONAL CONTROL The effect on steering when only proportional control is applied causes the rudder to move by an amount proportional to the off-course error from the course to steer and the ship will oscillate on either side of the required course-line.
b) DERIVATIVE CONTROL
The rudder is shifted by an amount proportional to the rate of change of ship’s deviation from the course. The ship will make good a course which is parallel to the required course and will continue to do so until the autopilot is again caused to operate by external force acting on the ship.
There are certain errors due to design parameters of the vessel which have to be corrected. Data signals are produced by continuously sensing heading error over a period of time and applying an appropriate degree of permanent helm is used for this purpose. The permanent helm acts as mid-ship.
The output of these three controls is combined and the net resultant drives the rudder. This type of autopilot is also called as PID Auto Pilot.
The output from a gyro or magnetic compass is coupled to the comparator, in the control unit , along with the input signal from manual course setting control. Any difference between the two signals causes an output error signal whose magnitude is proportional to the difference between the two signals and hence the comparator is also referred to as proportional control. In addition to the proportional control, the control unit also consists of derivative and integral controls which analyse the signals from the gyro or magnetic compass and the course selector
A summing amplifier is used to obtain a resultant error signal from these controls. This error signal is fed to the error amplifier which also gets feedback signals from the rudder, consisting of the rudder position and its movement. The output of this error amplifier is fed via telemotors to the steering gear unit and in turn operates the rudder. The telemotor has two units, i.e. Transmitter and Receiver situated on the bridge and steering gear compartment respectively. There will be no output from the control unit when the difference between the two signals is zero and hence no movement of the rudder results.