Furuno 520 - Manual (Page 10)

1. PRINCIPLE OF THE AUTOPILOT
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Principle of Operation
In the AUTO modes, the heading information from a heading sensor is
continuously compared with the course that is set on the autopilot’s controller. (in
the NAV mode, the course to the waypoint is received from a plotter connected
to the NAVpilot-511/520). With the boat on course, these two signals are equal.
If the boat drifts off course, the difference between the primary heading and the
set course will change proportionally and there will be an imbalance at a
comparator. The comparator’s output will then move up or down depending on
whether the course error is to the left or right of the set course.
The rudder will continue to turn the vessel until a balanced condition is obtained
at the comparator, at which point the drive to the rudder stops.
The rudder position is determined by a rudder reference unit and continuously
monitored by the processor unit.