Page 118 - Electronic Navigation Cyber Book

10. Important Limitations: The auto pilot system must be such that
the preset heading cannot be altered by intentional intervention of
onboard personal and the heading control system should change the
course to preset heading without overshooting its position

As we have stated above, auto-pilot is an undeniable boon in modern
navigation. It is the responsibility of the officers to ensure that they are
completely aware of the equipment and its features and controls to
make a proper and efficient use of it. Despite the fact that auto-pilot
systems varies in model from ship to ship, it’s working principle and
features will be the same. Deck officers making use of the equipment
are strongly recommended to read the manufacture’s operating
manual to get a thorough understanding of the equipment.

Automatic steering principles
Whatever type of system is fitted to a ship, the basic principles of
operation remain the same. Before considering the electronic aspects of
an automatic steering system it is worthwhile considering some of the
problems faced by an automatic steering device.
In its simplest form an autopilot compares the course-to-steer data, as
set by the helmsman, with the vessel's actual course data derived from a
gyro or magnetic repeating compass, and applies rudder correction to
compensate for any error detected between the two input signals. Since
the vessel's steering characteristics will vary under a variety of
conditions, additional facilities must be provided to alter the action of the
autopilot parameters in a similar way that a helmsman would alter his
actions under the same prevailing conditions.
For a vessel to hold a course as accurately as possible, the helm must
be provided with data regarding the vessel's movement relative to the
course to steer line. 'Feedback' signals provide this data consisting of
three sets of parameters.

• Position data: information providing positional error from the course
line.
• Rate data: rate of change of course data.
• Accumulative error data: data regarding the cumulative build-up of
error.
Three main control functions acting under the influence of one or more of
the data inputs listed above are: proportional control, derivative control
and integral control.

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