ComNav Commander P2 & P2VS Installation & Operation How Autopilots Work
Document PN 29010074 V4.1 - 20 -
Autopilot Operation
Maintaining a Heading: AUTO Mode
Following a Track or Course you want to be on is simple, with a ComNav Autopilot:
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Put the autopilot into AUTO mode.
•
Tell the autopilot the Heading you want to follow
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, using the Control Head.
•
The autopilot then moves the rudder from side to side as required, to put & keep the
boat on that Heading … just like you would if you were steering by hand.
But as you might expect, inside the autopilot it’s a bit more complicated than that!
Here’s how it works:
The steering control algorithms running in the firmware on the SPU’s microcomputer are
constantly comparing – many times per second – the boat’s actual Heading (as indicated by
the Compass) to the desired Heading you have given it.
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:
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How much the Headings differ.
•
How long there has been a difference, & by how much.
•
How rapidly the difference is changing.
Here’s an example which illustrates how the autopilot maintains a Heading in AUTO mode.
Let’s say 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 boat, and its dynamics
(which you have specified & tested when you set up the autopilot).
Note: on a Commander P2, or P2VS with an RFU, the SPU verifies that the
rudder has moved, by reading its position from the RFU. On a P2VS without
an RFU, the SPU estimates the rudder position by computing it, based on
various factors determined during setup.
•
The boat begins to turn to Starboard.
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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.
•
On the other hand, if the boat is coming back too quickly, the rudder will be moved
quickly back to dead-ahead – or maybe even a bit to Port for a few seconds.
•
Eventually, the actual Heading will match the desired Heading.
•
If (when!) the boat falls off-course again, the same sequence will be repeated.
In summary, if you look back at page 18, you can see that this is all very much like steering
by hand. But it’s all happening many times per second, much more precisely & rapidly than a
human can manage – and without coffee breaks or naps, either!
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ComNav Autopilots have an important safety feature: when you first enter AUTO mode, the autopilot will use the boat’s
current Heading as the desired Heading, so there’s not a turn right away … until you order one, with the Control Head.
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