> mowing neighbor's flowers

Maybe you could consider using a 433 MHz Tx/Rx pair as a beacon and have
the robot stop dead if the signal becomes too weak. You could also
consider two or more beacons and DF equipment on the robot for navigation.
It strikes me that RF is not disturbed by water, dirt, sunlight etc. The
position where you would mount the beacons would be indifferent (i.e. not
in the center, preferrably spaced out to give good angle resolution and
coverage). The multiplexed whip antenna (artificial doppler) would be
usable I think, using PIN diodes switched by a PIC, that would also
evaluate the standard 433MHz receiver output. The transmitters would be
sequenced (also using PICs) so they use the same frequency but different
IDs. Combined with a mechanical restrainig scheme (bricks on border or
cable on short tent stakes 10 cm above ground delimiting perimeter) this
would be reasonably safe I think.

Since the mower is already round putting the doppler whips on in a circle
on top will be esthetical ;-). Just be sure to put some english
inscription on it so nobody takes it to SETI by mistake.

This could work with the learning scheme. The robot could weave
deliberately to cover a swath slightly wider than the DF accuracy in each
pass, to avoid leaving uncut areas.

There is a firm called Friendly Robotics which makes robotic lawn mowers.
I know nothing of their system.

Peter

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.