I would like a robotic lawnmower that is only partially 'autonomous' - I would take it out of the garage and start it, and then it would proceed to do the following.
a.) Follow a trail through the grass. (Pennies / aluminum stakes / magnets)
b.) Basic obstacle avoidance with bump sensors or poximity sensors.
My thoughts were to mount some form of a metal detector to the front of the bot that would follow something in the grass such as round magnets or pennies. The detector would have to be smart enough to only look at or evaluate the objects that I want it to follow. So ignore a bottle cap, but respond to a copper penny.
It would be hard to to discriminate reliably between pennies and caps and you'd have to lay them very close, to get any following out of it (but then discrimination could be based on whether a given piece was logically placed)..
Besides, the mower would not like chopping away at pennies.
The bot would have a compass - and I would know it's exact direction, and I could supplement it's location with a GPS module so that I could keep it in the yard and keep it roughly where it is supposed to go.
GPS is not precise enough to ensure a clean edge (or even a ragged one).
A wire (carrying a signal) surrounding the area could be used for a "fence" that it wouldn't cross.
I do not want to follow a wire. - I want to control it via objects in the ground. Like I aluminum stakes, etc. etc. I have about 2 acres - this bot would run on a gasoline engine and batteries for navigation.
Objects on the ground sounds like a disaster to the mower and in the ground, they'd be even harder to detect.
You might consider IR emitting "towers" in at least 3 corners to triangulate the position.
If the area has got fairly straight sides, a tower in each corner could carry a LASER fence as well.