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In <1991Jun20.192334.24623@auto-trol.com> bridon@auto-trol.com (Brian Donahue) writes:
>i have this (maybe ridiculous) idea for a 'automatic' lawn mower.
>The idea would be to mount a mulching lawn mower on a 'computer-guided'
>platform of sorts. ... use an NC-type algorithm to guide your mower and 

Maybe I'm just brain-dead today... what's an NC-type algorithm?

>Being a robotics neophyte, i have no idea how to approach this problem
>or what complications may make the problem harder than it sounds.  
>For example, would i need some sort of tracking system?  or can i assume
>that for every turn of the wheel the lawn mower goes a certain distance?
>(how accurate can i get w/ no tracking system?)

You definitely need some kind of tracking system.  Consider:  1) Wet grass
is slippery.  2) The yard is bumpy, so the wheels actually travel slightly
different distances.  I doubt anyone could build an unguided system that 
would hit a 1-meter target across a 10-meter yard, without some massive 
sensing/feedback.  Maybe a walking-type system would work, but I don't think
it can be done with wheels.  (If I'm wrong, someone please tell me, but we 
had problems making a synchronous-drive base roll straight on carpet, and 
this seems like a much harder problem.)

>or is this totally ridiculous & not worth pursuing?

It's not totally ridiculous.  It'll be a great learning experience. :-)

Seriously, it depends on how much time you want to spend on it, and how 
general you want the solution to be.  My first inclination was to advise you
to build some sort of position-and-orientation sensor, that could tell the
platform where it was well enough to use a general path-planning algorithm.
 I've been in school too long.
So, I'd advise building a really simple sensing system, consisting of 
reflectors all around the yard and a machine that only knows enough to head
for a reflector and turn when the PC tells it to.

Mark off the yard with a grid, making each "strip" (crossing strips form
the grid) the width of the mower's blade.  Where each strip intersects the 
edge of the yard, put a reflector.  If there is an obstacle in the middle
of a strip that obscures the strip's reflector, put a reflector on each side
of the obstacle as well.  Now sit down with graph paper and draw your yard,
and then draw a path that covers each square of graph paper without an
obstacle in it.  Put hardware on your mower to do two things:  1) Move toward
a reflector that is in front of it.  2) Turn 90 degrees.  3) Sense when it 
rolls past a reflector to the side of it.  Program your PC to count the 
reflectors it passes and turn at the appropriate points to follow your path.

Another possibility:  Bury a wire under the yard, and make your machine follow
the wire.  This is simplest--you don't even need a computer.  But it takes a 
lot of wire for a big yard, and you have to worry about something breaking the
wire.  (Of course, you have to worry about something removing the reflector,
too.)  I don't know what kind of circuitry you need to make this work--I would
guess an RF generator you hook up to the wire, two antennas on the machine,
compare the signal strength from the antennas to decide where to turn.

If anyone has any feedback on anything I've said, please tell me, either in
e-mail or on the net.  Thanks!

-- 
Chris Phoenix			cphoenix@csli.stanford.edu
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bomb detonate assasinate Libya Bush nuclear PCP <- Just to waste the CIA's time