ComNav P4 Installation and Operation
Document PN 29010100 V1r0 - 20 -
The final part of the picture is the Steering System. In order for the autopilot to steer the
boat,
there must be a steering system capable of moving the rudder independently of the
ship’s
helm. This might be a hydraulic ram that is connected to the rudder post or tiller
quadrant, or
an electric motor connected to the steering cables.
However, no matter what type of steering system the boat uses, electronic signals from the
SPU tell
the system to move the rudder, when needed – how far and in which direction.
Your autopilot may also have a Rudder Follower Unit (also called a Rudder Feedback Unit, or
just “RFU”), a device that tells the SPU what position the rudder is in at any given time.
Autopilot Operation
Maintaining a Heading: AUTO Mode
With a ComNav Autopilot, following a Track or Course that you want to be on is very simple:
•
Set the autopilot into AUTO mode.
•
Using the Control Head, input the Heading you want the autopilot to follow.
•
The autopilot then moves the rudder (as required) to initiate and keep the
vessel on that Heading … just like you would if you were steering manually.
Here’s how it works:
The steering control algorithms running in the firmware on the SPU’s microcomputer are
constantly comparing – multiple times per second – the boat’s actual Heading (as indicated
by
the Compass) to the desired Heading you have input.
If there is a difference, the algorithms then calculate how much to move the rudder, to
bring
the boat back onto the desired Heading. The amount of corrective rudder
movement is
based on:
•
How much the Headings differ.
•
How long there has been a difference and by how much.
•
How rapidly the difference is changing.
Here is an example which illustrates how the autopilot maintains a Heading in AUTO
mode.
Imagine the boat is a few degrees off-course to Port:
•
The SPU will signal the steering system to move the rudder to Starboard,
by a few
degrees.
How many degrees the rudder moves depends on the type of vessel, and its
dynamics
(which you have specified and tested when you set up the autopilot).
Note: On a P4 Autopilot with an RFU, the SPU verifies that the
rudder
has moved, by reading its position from the RFU.
•
The boat begins to turn to Starboard.
•
As the boat turns, the difference between the desired and actual
Heading gets
smaller, and as it does, the SPU then starts to ‘back off” the
rudder – i.e., move it
back to centre – so that the boat does not overshoot the
desired Heading.
•
If the boat isn’t coming back on-course very quickly, the SPU’s
algorithms will not
back the rudder off right away, and might even move the
rudder a bit farther to
Starboard for a while.
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