Sample rate selection for digital control project

Tim Wescott wrote:

   ...

> As Rune mentioned Roy's problem is probably simpler, or at least less 
> subtle.
> 
> I suspect that if you know the boat's speed, position and direction, and 
> if you know the wind speed and direction relative to the boat, that you 
> have all the information that you need to at least competently handle the 
> boat -- doing it with real flair would take either lots of programming, 
> or a really good human skipper.

Maybe we're using the same word to mean different things. I don't think 
of an autopilot as creating autonomy. A system that changes course from 
waypoint to waypoint and avoids unanticipated obstacles is nearly there.

As for wind speed and direction relative to the boat, that changes with 
the boat's speed through the air. On most tacks, a sail's function is 
redirecting air that would pass over the side over the stern instead. 
(When before the wind, a sail slows air that passes over the bow.) 
Efficient sails are traditionally analyzed as airfoils, but I think of 
them as turbine blades. Their leading edges should face into the 
apparent wind and their trailing edges should be just to windward of the 
stern. Their curves are intentional, not evidence of the difficulty of 
pulling them flat.

Nevertheless, one could sail adequately with a perfectly flat sail. For 
every real fore-and-aft sail, there is a flat model that acts similarly. 
  That flat sail nearly aligns with the boom. I think it's easy to see 
that the best set for that sail bisects the angle between the apparent 
wind and the centerline of the boat. You might not win races by setting 
the main boom to bisect the angle between the centerline and the 
telltale, but you'll get where you want to go and pretty handily at that.

My (analog) RC model boat had radio control for the rudder, but servoed 
the sail according to the principle above. To keep it simple the boat 
was cat rigged (one sail, no jib) and to make it simpler yet, the sail 
was trimmed by rotating the mast-boom assembly with a worm gear located 
below deck. No sheet, no "ropes" to tangle or go slack. As long as the 
wind was up. it would go within 45 degrees of it; any closer, and I had 
to tack to get there. *But the rudder was in my hand.* It may have had 
an autonomous sail, but it was not an autonomous boat.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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