Furuno NAVpilot NAVpilot-500 - Manual (Page 9)

1. PRINCIPLE OF THE AUTOPILOT
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Principle of Operation
In the AUTO modes, the heading information from an associated sensor is
continuously compared with the set course (in the NAV mode, the course to the
waypoint set on the plotter connected). With the boat on course, the two signals
are equal.
If the boat goes off course, the difference between the primary heading and the
set course will change proportionally and there will be an imbalance at the
comparator, whose output will move up or down depending on whether the
course error is to the left or right of the set course.
The rudder continues to move until a balanced condition is obtained at the
comparator, at which point the drive switches off.
To set the rudder when the boat is off-course, the rudder signal is generated at
the rudder reference unit, then delivered the processor unit.