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inside/outside perimeter detection at lawn center

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purplelobster

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Hi all,
I'm new to ardumower and I am making plans to build a robot lawn mower.

It is unclear to me if the robot is able to detect that the perimeter sender has stopped working (due to a defect or a grid blackout).
Is the perimeter signal also detectable in the center of the lawn? (in normal use)
Commercial robot lawn mowers are able to do this, I'm wondering how.
 
Hi Alexander,
Thank you for the links and confirming that is works in the center as well, I understand the method.
The magnetic flux seen by the receiver coil will reduce when the distance to the perimeter wire is increased. Do you require an adjustable amplifier gain in hardware or is the 'signal to noise' ratio after ADC sampling large enough to pick up the 'small' signal in the center of the lawn?

I assume the same holds when the robot 'tracks' the perimeter wire? In that case the vertical 'looking' coil will hardly pick up any flux if the tracking error is zero. (the better positioned above the wire, the smaller the signal).
 
Signal correlation with long code bits comes to the rescue - even if a signal is below the noise floor level (dB) you can get a good SNR after correlation - if the signal is long enough. That's how GPS works and how the Ardumower perimeter works. So, that's why the correlated SNR is large enough in the center of the lawn (assuming no other signal from DC/DC converters etc. is near the receiver coil). Here's a simulation of the principle: http://www.grauonline.de/alexwww/ardumower/filter/filter.html
For the tracking you are right, exactly above the perimeter is no flux. So tracking actually is some kind of zick-zack motion following the perimeter but that's ok and can be reduced by some dead time in the PID tracking controller. It's not a real problem.
 
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