I like the earlier suggestion of running a current through a wire and detecting it with a hall-effect sensor or pickup coil. The only problem I see is that the field generated by the loop will be detectable a long distance from the wire.
Ok, assuming that the wire perimeter fence is taped to concrete under a "thin" rug. I suggest getting a lot of the old 300ohm TV signal wire. Tape it down flat under the rug, short one end while powering the other end. Size the supply to run several amps of AC or DC current through this long narrow loop. This will form a magnetic field that only extends 1-2 inches out from the cable with an easy to detect double sign/phase shift when crossing the cable and a clear maximum when on top of the cable. A pickup-coil or hall-sensor should then be able to locate the wire within less than an inch with a simple detection threshold.
Comments
Jim
http://forums.parallax.com/showthread.php/77195-RoboCut-BASIC-Stamp-powered-autonomous-lawnmover
http://forums.parallax.com/showthread.php/86830-Building-a-sensor-to-locate-a-buried-wire-fence
http://forums.parallax.com/showthread.php/133655-Navigate-Robot-Lawn-Mower
Ok, assuming that the wire perimeter fence is taped to concrete under a "thin" rug. I suggest getting a lot of the old 300ohm TV signal wire. Tape it down flat under the rug, short one end while powering the other end. Size the supply to run several amps of AC or DC current through this long narrow loop. This will form a magnetic field that only extends 1-2 inches out from the cable with an easy to detect double sign/phase shift when crossing the cable and a clear maximum when on top of the cable. A pickup-coil or hall-sensor should then be able to locate the wire within less than an inch with a simple detection threshold.
Marty