The autopilot is equipment used to maintain the ship’s set course electronically and electrically for a long period of time when alteration of course is not required.
This is achieved by comparing the course to steer with the ship’s heading obtained by gyro or magnetic compasses. Any difference between the two will cause an error signal voltage to be created and accordingly correcting helm will be applied to the rudder to bring the ship back to the set course.
The error signal is fed to the error amplifier which also gets feedback signals from the rudder regarding the rudder’s current position and it’s movement. The output of the error amplifier is fed via telemotors to the steering gear unit which in turn operates the rudder. The telemotor has two units- transmitter (situated on the bridge) and receiver (in the steering compartment).
When the difference between the two signals, i.e., the course to steer and present gyro (or magnetic) course, is nil, there will be no output from the control unit and hence no movement of the rudder.