metal dector for barbed wire fences

On a sunny day (Mon, 18 Jun 2012 07:55:09 -0400) it happened "Michael A. Terrell" wrote in :

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Just run an insulated wire around the place. I have done it.

Tytus

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The place? That doesn't tell us anything.

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I assume this is for some sort of automatic vehicle? Because when I'm out in the country, I detect fencing with my eyeballs, or -- if unlucky -- my crotch when I step over it. For me the tip off is a row of fence posts.

UC Davis has been working on machine vision for ag vehicles such that they can detect a weed in, say, a tomato field, and hit it with Roundup. Maybe you can use machine vision to detect a line of fence posts, and stop just short via triangulation.

Um..., Dont you guys read Aviation Week? In the US, Crop Dusters use augmented GPS while airborn at centimetric acuracy. There are many, many, patents to John Deer, etc for augmented GPS using a variety of techniques. Some are free,some are pay to play.

Differntial GPS is cheap, and the Coast Guard/FAA/Private Industry transmits correction signals, on LF beacons, via satellite, and commercial FM stations, as well as dedicated, local, land mobile band transmitters.

I wont get into helicopter wire detection in a open forum, but suffice it to say, commercial and mil systems exist. Some are radar based, some IR, some passive.

Steve

Off the shelf solution: Get some microwave type proximity detectors used on luxury cars for parking assists.

If they false alarm too much on shrubs/weeds etc. one fix might be to install two (on each side). One with a horizontal polarization and one vertical. I'm thinking (some experimentation would be needed to confirm this) that fence wire, being horizontal, is going to give a stronger return to the horizontal polarized sensor then the vertical (which would respond more strongly to vertical stuff like plants).

There are probably some microwave proximity detectors that provide not only distance but signal strength outputs, making the quantitative H/V polarization analysis easier to do. But the auto units are probably cheaper (due to quantity).

I worked on a Vietnam-era mine detector that had tubes in it, I think. The basic idea was a big square drive coil with 4 round coils within the outer coil. The 4 small coils were connected in series, upper left and lower right additive, then lower left and upper right additive with repect to each other, but reversed polarity from the first two. So, with no metal nearby, the circuit nulled out any field from the drive coil. if metal was nearby, it would unbalance the nulling. It could detect a tiny nail a foot away! The drive coil was about 12 x 18" as I recall, but it was 40 years ago. There was a circuit that fed some signal from the drive coil to the sense coils with a nulling pot to compensate for tolerances in the coils.

The performance of this simple circuit blew me away!

Jon

What about mapping your field with GPS first? If you need more accuracy, get a receiver with a differential GPS input and set up a temporary DGPS base station on an established survey marker on your property.

In addition to fence (boundary) detaction, you can mark pretty much anything you need to avoid. And you can use the system for data collection. Some farmers survey their crop yields and soil conditions down to very small areas using GPS.

Robert - that's impressive resolution. What frequency are you using?

Hul

Robert Macy wrote:

wire,

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| fence

Are cows reactive or resistant loads? :-) They sometimes behave both ways...

On a sunny day (Mon, 18 Jun 2012 10:21:21 -0700) it happened Bill Martin wrote in :

I dunno, we did it with a couple of thousand people. It did not seem to effect my field strength measurements a lot. Cows are not generally magnetic I think.

s...

They 'feed' cow magnets to them so all the bits of iron they eat stays in one place.

George H.

On a sunny day (18 Jun 2012 19:18:39 GMT) it happened notbob wrote in :

Interesting, did not know that.

Eating magnets can be very dangerous. Especially the small powerful magnets sometimes used for facial 'art'. When two such magnets attract each other in opposing intestines they can press and kill tissue, causing holes.

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People have experienced severe trauma from those small magnets. The cows have a different digestive system and do not seem affected.

That was a product made by a now defunct sister company of one of my old employers.

Originally cow magnets were shiny alnicos; when for cost savings people went to ferrite, they were encapsulated in plastic.

press

the magnets tend to stay in one section of the cow's stomach. The relevance here is that fence wire and nails are some of the bits of iron that cows can pick up while grazing in a field.

s...

Almost everything in nature abhors a magnetic field and tries to get OUT of it, except for iron, etc.

View the video of the live frog 'floating' in a 3T field. The next time someone says that a space creature inside a saucer cannot survive the motion described, just remember that frog. All it would take is one humongous mag field and move that field as fast as the vehicle and critters inside would not only survive extreme turns, but wouldn't even notice the accelerations.

Cows, being flesh, try to get out of a field, too. Animals actually look more like a big bag of salty water, about as conductive as seawater. To run tests on proximity fence setups we used to fill a basketbal with saline solution and roll it up to the fence. Could actually adjust the sensitivity to NOT detect basketballs, but detect those larger 'medicine' ball sizes which was about the best setting.

Jon - from the era you mention, Don Lancaster wrote an excellent article describing some various ways of detecting metals. He mentions that type, 1 driving coil & 2 sensing coils that null the driving coil. He didn't provide any dimensions, though. The dimensions and sensitivity you mention will serve as a handy reference.

Hul

J> > The detector is intended for a NC tractor generally but more

With the best one being an ALERT PILOT. Sheesh.

Maybe a couple of insulating arms poking out, with steel cables or maybe chains hanging down. Apply a reasonable voltage to the cables and look for the big spikes that will happen when a chain hits the wire. Vegetation or wood fencing will make softer spikes, so can be filtered out.

Or extended balloons. They hit the barbed wire and pop.