Seiwa SW AP03 - Manual (Page 7)

CHAPTER 1 SYSTEM DESCRIPTION
1.1 INTRODUCTION TO AUTOPILOTS
The main function of a marine autopilot is to hold the heading of a vessel on a reference
course which is held in the memory of the autopilot. When it is operating, the autopilot
continuously compares the vessel’s heading with a reference course, and if they are
different, it applies helm to bring the vessel back on course. Since there has to be a
compromise between the accuracy of course holding and the activity of the rudder, the
autopilot has controls which let the user set the balance between these two factors.
The four basic components of an autopilot are a compass, an electronic control box, a
rudder angle sensor (transducer) and the steering drive. See Fig 1.1. In a SW AP system,
the electronics are housed in two cases - a Junction Box containing most of the system and
a Controller (Control Head), which is mounted near the steering station.
RUDDER
STEERING DRIVE
RUDDER STEM
CONTROL
POWER
TRANSDUCER
COMPASS
Figure 1.1 Basic components of a marine autopilot.
Modern autopilots perform other functions as well and this introduction explains how these fit
in with the basic function and how they provide a wider range of options for the user.
1.1.1 THE REFERENCE COURSE
When the autopilot is first turned on, it rests in an idle (STANDBY) state in which it displays
the heading, but does not steer the vessel. It is activated by switching it into the PILOT
state. At the moment this is done, the current heading is put into memory as the reference
course and the autopilot starts steering to hold the heading on this reference course. The
user can change the reference course at any time and the heading will swing round to match
the new course.
There are two other ways of setting the reference course. If the autopilot is connected to a
GPS navigation receiver, the heading is then controlled to place the vessel on a direct track
between the origin waypoint and the next waypoint.
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