ComNav Commander P2 - Power Steer Mode Operation

ComNav Commander P2 & P2VS Installation & Operation How Autopilots Work
Document PN 29010074 V4.1 - 23 -
Following a Track: AUTO/ALC Mode
Some ComNav autopilots (including the P Series), provide another way to keep your boat on
a specific Track : Automatic Leeway Correction.
This feature can be turned on in AUTO mode.
ALC is similar to NAV mode, in that it uses NMEA 0183 Latitude & Longitude data to sense if
the boat is following the desired Track.
Unlike NAV mode, however, a ComNav autopilot with the ALC feature only needs Latitude &
Longitude data in AUTO/ALC mode. It does not need Cross-Track Error data from the
external Navigation System, nor to have any waypoints set in the system. The ALC feature
calculates the Cross-Track Error on its own, using the desired Heading active at the point in
time when AUTO/ALC mode is first engaged.
Power Steer: “Drive by Wire”
Most ComNav autopilots have a POWER STEER mode, which provides a way for the user to
control the rudder directly. In effect, the autopilot acts as a sort of “electronic steering wheel”,
allowing the operator to steer the boat manually, in a similar way as when using a standard
manual steering wheel or tiller – but instead steering the boat with the autopilot’s Control
Head, or any remote controls it is equipped with.
Steering, when using the autopilot in this mode, is done with two buttons and/or a knob on
the Control Head. One button, and/or a clockwise turn of the knob, will move the rudder to
Starboard, causing the boat to turn that way; the other button, and/or a counter-clockwise
turn of the knob, will move the rudder to Port so that the boat turns that way.
Using the buttons/knob, the rudder can be placed in any desired position where it will remain.
This feature is useful when docking or manoeuvring, because the person steering the boat
can take their hands off the controls between rudder movements.
More Information (for the Technically Inquisitive)
Mathematically, the autopilot’s steering algorithms are based on a control method known as a
“PID Loop” … a mnemonic for a “Proportional, Integral, Derivative” error-correction technique.
The amount of rudder movement to correct for any Heading error (or to make a turn), is
continuously re-computed, in a processing loop, as a value that is:
• Proportional to the error (or how much more of the turn is still to be done).
• An Integral of the error/turn … how big for how long.
• A Derivative of the error/turn … how fast it is changing.
Many references can be found on PID Loops, on the Internet & elsewhere, if you want to
know more! And ComNav occasionally holds training seminars – check with your Dealer.