Sensor for tractor implement that detects contact with galvanized wire

[laudrup]

Hi,

I am looking to buy/build a basic sensing system that could go on a tractor implement and detect when part of that implement is touching a galvanized trellis wire, and turn on a warning light. I was thinking to do it based on the ability of the galvanized wire to conduct electricity (better than wooden posts, leaves, plastic, etc. that it might also encounter but are not a problem do). The tractor has ~12V power available via an auxiliary plug (a 25A continuous current outlet).

At a basic level I was thinking of having leads from the + and - of the 12 V plug with 30cm metal rods at the end of each and when both rods touch the galvanized wire, that completes the circuit. A relay in one of the leads could be used to turn on a light.

I am not sure on the approach though - safety, blowing fuses, risk of starting fires if the metal rods touch dead grass, etc.

Detecting contact with a metal (galvanized wire in this case) based on its ability to conduct electricity better than wood/plastic/leaves, seems like it is a problem that must have been encountered before.

I wondered if anyone might have any experience/ideas on systems/circuits that could detect contact with a metal, or other advice on what I might be able to try?

Many thanks in advance for any help.

[BeBuLamar]

If you can post a picture of your tractor and show the likely location on the tractor that may touch the wires then we have a better understanding of the problem you try to solve. If you use low 12VDC I do not see much of a danger but how can you be sure the 2 probes will touch the wires when the tractor touch the wires?

[laudrup]

Thanks BeBuLamar for the quick reply.
Please see attached an image of the tractor implement where galvanized wire has wrapped around a tiller. I am thinking about having 2 robust conducting metal rods in some shape attached to the tiller but electrically insulated from it, so that if wire got caught and started to wrap around like this, it would bridge the 2 metal rods and complete the circuit. I haven't worked out the exact design/shape of the metal rods, but if I am able to come up with a design where the wire would bridges the 2 rods when it is getting caught, I wanted to understand if there could be some electrical setup to detect this bridging of the 2 rods by the wire.   
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[jpanhalt]

I suspect something could be done to detect contact, but that would probably not be much help in avoiding the mess.  I run a 90" JD flail mower on a rough field to keep weeds down. PTO speed is 540 rpm.  On the rare occasion I have contacted wire or plastic string, it was too late to stop getting it tangled, and the mower does take awhile to stop turning.  Fortunately, after about 2 passes, I had "found" all of the loose wire thoughtless people had dumped in the field.  That was about 10 years ago.

I suspect avoidance is the best cure.

[BeBuLamar]

I haven't figure out how to detect the wire before the tractor tangles with it. However, for you 2 probes idea what do you want to tractor to do when the 2 probes are bridged with the wire?

[tlovie]

So you can make these 2 rods that are electrically insulated from each other.  You could connect your +12v supply in series with a 1a fuse, then a 680 ohm resistor, then a led, then to one rod.  The other rod would be connected to ground.  The first rod needs to be electrically insulated from the tractor chassis and ground, you could maybe hold it in a plastic gromet or something.

[brucehoult]

Perhaps you can do something that is more along the lines of a non-contact metal detector. Very close proximity (or contact) with a long wire can change a resonant frequency, or perhaps you use a pulse and look for echoes with longer delay than normal (and above some threshold strength).

[janoc]

Perhaps you can do something that is more along the lines of a non-contact metal detector. Very close proximity (or contact) with a long wire can change a resonant frequency, or perhaps you use a pulse and look for echoes with longer delay than normal (and above some threshold strength).

That's not likely to work given the bouncing machinery on rough ground (and thus wildly changing distances) and the hulking mass of metal of the tractor/implement nearby that is going to drown out any signal from the wire. One could mount a metal detector on some sort of arm in front of the tractor but again - not likely to work well/at all given the constant bouncing and thus changing distance. And now you would have another thing sticking out in front to snag debris with (and rip it off) ...

A contact based system could work but it isn't likely to be reliable enough to be useful - poor contact, dirt, difficulty of ensuring the wire actually hits both of the sensing electrodes simultaneously, the electrodes getting snagged and damaged on weeds, brush ... Moreover, the operator is very unlikely to notice a fleeting contact - it would need some sort of latching circuit so that even a short touch would trigger it.

If all you want to is to detect a wire snagging then you could point a camera at the device and have some machine learning/computer vision setup trained to recognize the wire getting caught - but that would be really complex to do (e.g. how to get enough training data? Can't exactly go around and feed wire into the machine on purpose ...) and difficult to make reliable in "field conditions" (pun intended).

I would agree on this with jpanhalt - avoidance is probably the best cure here. Buy a cheap metal detector and walk the property with it before going there with a tractor. Pick up any loose discarded wire and any other debris that could get snagged.

[Infraviolet]

Personally I wouldn't put too much faith in a contact based sensing system, all too easy for slight misalignments to let the tractor "implement" (what is it precisely, what shape, conductive all over or only at certain points?) to let it hit the metal trellis you want to avoid bashing and yet do so with one or no contacts making good enough contact to conduct.

I'd be much more inclined to try making a metal detecting inductive or capacitative sensor calibrated to work at a short range when close to a trellis wire. With correct calibration you should be able to make these immune to the efect of the nearby tractor, even perhaps by putting a metal can around their back to "shield" somewhat against their eing able to detect changes in any direction except "ahead" where you'd be looking for wire detection, and at ranges of mm or less. Inductive and capacitiative type short range metal detectors can be bought as industrial units for about £20 each, shaped like a bolt and should be pretty reasonably ruggedised againt the rigours of raking through rubble. Or even something optical IF it can be kept clean (unlikely for this spinning cutter setup, although atleast it looks like you are somewhere dry and dusty rather than wet and muddy).

EDIT: just saw your picture, if the problem is snagging wire damaging the spinning disc/blade, then what about doing some sort of rpm sensing or motor current reading instead, detect a stall starting to happen and cut the power to the motor to prevent further damage, then manually clean out the wire, then reconnect the motor and start again?

[themadhippy]

motor current reading instead, detect a stall starting to happen and cut the power to the motor

not sure how much current flows in a pto or hydraulic motor

[laudrup]

Thanks so much everyone for the great comments and suggestions. Really appreciate it. I might experiment with a few things, but like many people said avoidance is probably the best cure.