I'm designing my lawn mower robot, and I am in the perimeter stage. The electronic part is done, and works quite good, now comes the software.

I have 2 sensors on the front of the robot, left and right, turned 45° with respect to the forward direction. They sense the signal emitted by the wire.

I need an advice on how to manage the approach of the robot to the line. I mean the stage right before it starts following the line..

It can approach the wire from any angle, so the signals are always different and I don't know how to move the robot in the right position in order to start the following...

What's the usual approach?


  • \$\begingroup\$ What signal is transmitted by the wire, and what are the sensors? \$\endgroup\$ – Chu Oct 5 '15 at 13:03
  • \$\begingroup\$ The wire transmits a square wave signal at 34 KHz. The receiver sensor is a coil tuned on that frequency and an amplifier. When one of the sensors is on the wire, the signal amplitude is about 2 V. \$\endgroup\$ – Val Oct 5 '15 at 13:29
  • \$\begingroup\$ Most industrial applications require the robot to start from a known datum position, and to recalibrate regularly. You'd be well advised to adopt a similar protocol. If the robot is allowed to 'hunt', it's out of control! \$\endgroup\$ – Chu Oct 5 '15 at 17:11
  • \$\begingroup\$ Maybe a hint: precision sensors use methods to find where the signal is close to zero. Like fluxgate sensor for current sensors and compass. Perhaps combining two coils togeether you can null the voltage, that would be the exact middle point. \$\endgroup\$ – Marko Buršič Oct 6 '15 at 8:13

If wire is located in the middle, signals from both coils are in phase. You could measure the phase difference with phase comparator. It would require a reference signal that drives a closed loop PLL. This reference signal goes to phase comparator, the output of phase comparator controls the VCO, then the output of VCO is fed back to phase comparator.
Now when VCO outputs the signal wich is in phase with reference signal you can add 2 phase comparators and compare the phase shift for each coil.
The only problem is where to get the reference signal. You could mount a 3rd coil in the middle, or you can add the signals of two coils if they are the same amplitude, perhaps an additional AGC apmlifieris needed for each coil.

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  • \$\begingroup\$ this looks quite complicated... at this point it's easier to use odometry to navigate back to the home :) regarding your other solution, the problem is not follow the wire when it's in the middle, but reach the position in which it is in the middle! it can arrive from any direction, so how I move it exactly on the wire? maybe it's more a software/robotic problem.. \$\endgroup\$ – Val Oct 6 '15 at 14:22

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