"Martin Brown" wrote in message
...
On 18/04/2020 21:41, larkim wrote:
Recently got a dose of laziness and bought a robot mower. Works
really nicely

One thing I haven't got clear in my head is how it knows whether it
is inside or outside of the charged boundary wiring.

When inside the boundary loop it gets a slightly better signal and there
is a null and then a phase reversal as it crosses the boundary wire. It
isn't dissimilar technology to a current based hearing loop. Except that
the lawnmower can afford to use a fair sized loop antenna.

I can power down the whole system, move the robot to just outside the
boundary loop, and when it is all powered back up it instantly
detected as being outside the wire.

Anyone know how this works? What goes down the low voltage cable that
is the boundary wire to allow the robot unit to detect where it is in
relation to being inside or outside?

I think the crucial part of the OP's question was what happens if the mower
is turned on outside the loop. IN this case, it has no phase-reversal event
to detect when it is going outside. If the mower can detect that it is
*already* outside (as opposed to having just *gone* outside) that suggests
that it knows what the normal phase of the signal is, to know that it is
receiving a 180 degree phase-shifted version of it.