strimmer powered electromagnet

Discussion in 'The Projects Forum' started by jamie_grieve, Apr 29, 2008.

  1. jamie_grieve

    Thread Starter New Member

    Apr 29, 2008
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    Hey there folks, new guy here with a question on electromagnets. Here's the deal: Am in Angola working for a charity clearing landmines and we're bothered by metal contamination on the minefields. We use strimmers to clear the ground prior to demining then use detectors to find the mines. Sounds easy but the mines contain tiny amounts of metal so every signal needs a thorough investigation which is an operation that takes time. Bit of a problem when there's metal all over the place. We've got lots of dead strimmers/brushcutters with good engines, lots of dead cars and honda 2.1 KVA generators about the place to yield parts. My idea is to stick a car alternator onto the strimmer engine and an electromagnet down where the blade would normaly be. Questions: What would be the most efficient way of using the above mentioned parts to turn them into a man portable electromagnet?Would the pressure plate from a clutch make a good magnet even though it's cast and has high carbon content? I envisage the magnet to be a disc about 20-30cm.What, where and how would be the best way to wind armature winding wire round a bit of car/truck/machine/tank to make best use of the power available?Would the 12V from the alternator be the way forward or is there some way of getting a higher voltage from a conventional alternator? They're 3 phase 110V inside right?We're also in a remote area in Africa so KISS applies!Many thanks in Advance.Jamie.
     
  2. flubbo

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    Apr 21, 2008
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  3. SgtWookie

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    Jul 17, 2007
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    Interesting problem, in an extremely dangerous work environment.

    Driving an alternator will require a fair amount of horsepower. I think a typical string trimmer engine has between 1/10 and 1/4 HP.
    1 HP is roughly equal to 745 Watts. So, if you had a 70 Ampere output alternator that you wished to drive at full capacity, it would require 14Volts x 70Amps = 980 Watts = approximately 1.32 HP. However, that is assuming 100% efficiency, and no power being consumed for cooling. It might be realistic to assume roughly 55% efficiency - but I don't know offhand.

    Let's say you had a big string trimmer engine with a 1/4 HP motor.
    0.25 HP x 55% efficiency x 745 = 102 Watts of power.
    102 Watts / 14V = 7.3 Amperes
    So, you could drive a small-ish alternator and get some power out of it.

    Alternators require a small excitation voltage to get them going, as otherwise they won't produce any output. There are no permanent magnets in them. You'll have to use a capacitor on the output to keep the rotor winding supplied after the jump start. If the load is too heavy for the alternator to sustain current flow, the output will rapidly drop off and cease to generate current because the rotor winding will no longer have current.

    But back to the basic idea of using a magnet to pick up these bits of metal lying around. A magnet will only attract items that contain ferros metals; ie: iron/steel. While shrapnel fragments from various pyrotechnic devices such as aerial-dropped bombs will contain iron/steel, many other bits such as older ball ammunition will not; it was made of copper-jacketed lead.

    Other types of ordinance laying about could be more problematic. I can only imagine what would happen if a "dud" hand grenade were given a "whack" by being picked up on your magnet. :eek: You would need to construct your magnet so that if live ordinance were accidentally picked up, that the blast would be vectored safely away from humans. I'm only speculating on this, as I have very little formal training in this area.

    Also, it is my understanding that some mines made years ago were sensitive to the proximity of large ferrous objects or magnetic fields; basically designed to ignore personnel but destroy vehicles. These kinds of mines would be immediately tripped by your envisoned pressure-plate electromagnet, with a very lethal effect on anyone within the blast radius. I have absolutely no idea if such mines might have been deployed in the area you're attempting to clear.

    It seems to me that you need an EOD expert on board your team. I am not that person.
    [eta]
    You might review this patent application:
    http://www.patentstorm.us/patents/5886283-fulltext.html
    There is a "how they work" discussion of a prior-art circuit of a magnetic mine's detector function.
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
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    The full article is pay to read, contact the author to see if this may help.
    http://www.theregister.co.uk/2007/07/30/landmine_sniff_on_the_cheap/

    I've also seen a free software sonar system that was very good at spotting even small boats. I wonder if it could be made to work in a similar fashion to that article.

    I realize it's not low tech or cheap, just thinking out loud.
     
  5. SgtWookie

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  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Practically speaking, I would want to mount the magnet or electromagnet on a cart and pull it from one location to another. By going back and forth over lanes, you could be reasonably sure of having thoroughly covered a well-defined area.

    I would want to experiment on a test plot with lots of bullet & grenade fragments to see if the idea has real merit. I seriously wonder if any magnet can pull steel fragments out of more than a few millimeters of soil cover. Still, if the cart idea works, it might be possible for it to have a spring harrow attached to loosen some soil, perhaps a few cm.
     
  7. SgtWookie

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    Beenthere, I was thinking something along the same lines.

    String a "clothesline" (a tough and strong line) between two vehicles. The path ahead and between the two vehicles must be manually cleared for blast radius. But a sensor or magnet could be passed back and fourth on pulleys over the "clothesline" to clear the area between the two. This isn't a completely new idea; it's an adaptation of a technique used by Navy minesweepers in WWII to clear ocean minefields in the Pacific.

    A gasoline-powered magnet in today's markets could be very expensive to run. But, that's up to the OP to tell us - if it's cheap enough, I might move there! ;)
     
  8. jamie_grieve

    Thread Starter New Member

    Apr 29, 2008
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    Hello again, and thanks for the replies.
    To give you a better idea of the scenario we are working in fairly dense anti-tank minefields where the particular mines in question have a thin metal casing. At a guess, I'd say 16SWG. Different techniques are employed for battle area clearance so non ferrous metals such as ball ammunition & casings not much of a problem. We are contaminating our own minefields when we conduct demolitions. All mines are destroyed in situ, pulling and disposal is not an option. We've looked at alternative techniques for deflagration etc but we're talking about thousands of mines here.
    Flubbo, thanks, but no thanks. I should have said in my original post that touching the ground is not an option. There is a mixed threat with some mines taking only 2Kg and others needing almost 200Kg. Sensitivity can change wiith time so no ground engagement is possible.

    Sgt Wookie, our strimmers are 1.5 hp so ought to be able to drive something usefull. I didn't give the excitation current much thought but we have some small 6v radio batteries I was going to use. I doubt I can use the magneto on the strimmer but I don't know for sure.
    Your fears for twating the magnet with a hand grenade are well founded but before the strimmer sweep, a preliminary sweep of the area is done to identify any large metal objects like POMZ, grenades, trip wires or anything else of interest. I doubt anything we could build from a strimmer is going to pull anything subsurface. We're reasonably familiar with all the fuze mechanisms we're going to encounter and how they're going to respond to magnetic fields. We're not going to encounter proximity fuzes on this task.
    We've been operating in Angola since 1994 clearing mines, battlefield clearance, EOD and weapons and ammunition disposal so not complete greenhorns but as I'm sure you know, always learning and looking to improve things and make it safer for our guys.
    The alternative is full manual excavation of the entire area 30cm deep in the African sun.
    mrmeval: We're looking at ground penetrating radar so you're on the right track but it's a long way off and still relies on a metal signal to trigger it.
    beenthere, as above, ground engaging not an option. Passing a magnet over the surface in a similar fashion to a detector would be simple and safe to implement. To reitterate, almost all the contamination is on the surface and is thin mild steel.
    Petrol costs not an issue, it's cheaper than water and as easily available. Any costs would be more than made up on increased efficiency.
    Thanks for your thoughts so far, keep 'em coming!
     
  9. SgtWookie

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    Jul 17, 2007
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    Ok - at what RPM do these 1.5HP strimmers run at WOT? (wide-open throttle). I would not be surprised if they're running in the neighborhood of 8,000 - 12,000 RPM, perhaps faster.

    I'm thinking that you will likely need to fashion a speed reduction unit. Having lots of parts sources in the way of disabled vehicles, this shouldn't pose much of a problem; you'll have a selection of V-pulleys and belts to tinker with. But the speed reduction won't be 100% efficient, either.

    How are you set for wire? Do you have spools of various gauges around, or would you be stripping it from derelict autos?

    As you're likely already aware, automotive alternators have 3-phase delta stators which are connected to a 3-phase full-wave bridge rectifier. For your application, you could actually use a delta or wye configuration winding around the stripped pressure plate, and connect the three phases to the delta or wye windings on the pressure plate. You would still need the bridge rectifier and regulator hooked up, just so that you can maintain proper current to the rotor windings.

    You might actually be able to use the strimmer's magneto just to "jump start" the alternator. Once a little current went through the rotor windings with no load on the output except for a capacitor, it would be self-sustaining very rapidly. Until/unless you managed to overload the output, of course.

    But being a solid chunk of cast iron, you would have a lot of losses due to eddy currents. It might be easier to use an old steering wheel; wrap turns of wire around the rim. It certainly would be much lighter in weight than a pressure plate.

    Still in the brainstorming phase here.
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you use DC to make the field, then there won't be eddy current losses. Alternators put out rectified 3 phase, so a bit of a capacitor would remove most of the ripple - probably not a biggie for DC in coils anyway.
     
  11. jamie_grieve

    Thread Starter New Member

    Apr 29, 2008
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    Thanks again guys for the advice. I think we're getting somewhere now. To give you some idea of my background, I trained as a mechanical engineer and anything I know about electromagnets, eddy currents and delta stators you could comfortably write with a marker pen on the back of a small stamp on a small day at a small convention.

    Ok - at what RPM do these 1.5HP strimmers run at WOT? (wide-open throttle). I would not be surprised if they're running in the neighborhood of 8,000 - 12,000 RPM, perhaps faster.
    I reckon around 8000 is about right. They're 4 strokes so not just as fast as the 2 strokes. Direct drive is how it's going to be. I just want a working prototype to prove the idea in field conditions and to justify further development by people who know what they're doing. There's plenty of cars out there spinning alternators faster than this bearing in mind the size of crank pulley versus that on the alternator.

    As you're likely already aware, automotive alternators have 3-phase delta stators which are connected to a 3-phase full-wave bridge rectifier.
    I'm trawling distant memories to recall something of the sort!
    For your application, you could actually use a delta or wye configuration winding around the stripped pressure plate, and connect the three phases to the delta or wye windings on the pressure plate.
    Erm, um how does one of them look?
    So I understand, would I be winding the coil around the pressure plate or would I be winding it aroind a metal bar which is then placed accross the pressure plate?
    If it makes any difference I can cut any shape from either mild steel plate or armour in thickness from 1/4" to 1 1/16" using oxy acetylene.

    You might actually be able to use the strimmer's magneto just to "jump start" the alternator. Once a little current went through the rotor windings with no load on the output except for a capacitor, it would be self-sustaining very rapidly. Until/unless you managed to overload the output, of course.
    Could you explain this a little bit better please? There's also a small wire going to the stop control which has 1.5 to 3.5V. This is what I was going to try first. It's the self sustaining element I'm not so clear on. I understand that some alternators can be damaged by running them with batteries disconnected and some are OK about it.

    You would still need the bridge rectifier and regulator hooked up, just so that you can maintain proper current to the rotor windings.
    How would this look and what would be the advantages were it to be wired as you suggest with the wye configuration?
    How are you set for wire? Do you have spools of various gauges around, or would you be stripping it from derelict autos?
    I'll most probably be stripping the wire from (lots of) dead 2.1 KVA generators but dead alternators are available too.

    If you use DC to make the field, then there won't be eddy current losses. Alternators put out rectified 3 phase, so a bit of a capacitor would remove most of the ripple - probably not a biggie for DC in coils anyway.
    So do I want to be using DC or 3 phase and I'd be finding a capacitor from an old television I suppose. Erm, which one?
    __________________


    Still in the brainstorming phase here.
    Once I've got a design to work to it won't take very long to actualy make the thing. I'm also using it as a classroom lesson for my generator guys. It's amazing what they can do given that they were all refugees with almost no education a few years ago.
    I'll post some photos of the build as it progresses. we've got a satelite link for the e-mail running from a generator. It also shows them the whole deal about when you don't know, ask. And if we can get a response from the other side of the world then even better!
    Cheers,
    Jamie.
     
  12. SgtWookie

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    Jul 17, 2007
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    This reply is just to sort out who said what (I frequently use the Reply w/Quote, and you have to add your own [ /QUOTE] and [ QUOTE] blocks to keep things straight who said what)

    jamie_grieve wrote:
    SgtWookie wrote:
    jamie_grieve replied:
    SgtWookie wrote:
    jamie_grieve replied:
    SgtWookie wrote:
    jamie_grieve replied:
    SgtWookie wrote:
    jamie_grieve replied:
    SgtWookie wrote:
    jamie_grieve replied:
    SgtWookie wrote:
    jamie_grieve replied:
    Beenthere wrote:
    jamie_grieve replied:
    __________________


    SgtWookie wrote:
    jamie_grieve replied:
     
  13. SgtWookie

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    Jul 17, 2007
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    This is a "stream of thoughts" brainstorming post, more or less. As I think about things, I'll be updating it.

    OK, just to start off, here's a comparison of Delta vs Wye windings:
    [​IMG]

    Note that what looks like straight orange wires in this illustration is actually supposed to represent coils.
    Ignore the "transformer" and "primary/secondary" notations - a delta is a delta, and a wye is a wye. In a delta configuration, current can flow between two phases by travelling between just two points, or one coil. In the wye configuration, current must always pass through at least two coils.

    I've added a generalized schematic of an alternator showing the basic concept of the stator windings, a 3-phase full-wave rectifier bridge, a diode trio which supplies the voltage regulator, the rotor winding which is controlled by the regulator, the FIELD connection which excites the regulator for the "kick start", and the capacitor where the battery usually goes. I just drew this up from memory from what older GM alternators looked like internally, so there may be some inaccuracies - but it's close. Other makes of alternators would be somewhat similar. The "diode trio" seems redundant, but those rectifiers have significantly less loading on them than the rectifier bridge does.

    If you were to use just a DC coil, the windings instead of going on to bars that were attached to the pressure plate, you would go around and around the entire pressure plate - sort of like superimposing a couple of donuts, or lying a coiled-up garden hose on a trash can lid. (Sure, odd mental pictures, but it's easier to write it out than make a God-awful drawing).

    The ideas of using either (delta- or wye-wound AC coils), or (a DC coil), present their own unique problems. As I mentioned earlier, there would likely be significant losses due to eddy currents. Beenthere mentioned that with a DC coil, there would be no eddy current losses.

    However with a DC coil, you will likely run into coil saturation. At a certain point, when the magnetic field has built up as large as it's going to, the coil is saturated and current flow through the DC coil may significantly increase. To combat this, you would need to wind a lot more turns on than you would for a set of AC coils (making the assembly heavier), or perhaps use a series resistor to limit current, which would waste a lot of power.

    An additional disadvantage of using a DC coil would be the losses incurred via the built-in 3-phase full-wave bridge rectifier. These are absolutely necessary to convert the 3-phase AC to DC for automotive electrical use, and for keeping the rotor windings energized. However, the rectifiers are not terribly efficient; there is somewhere between a 0.7v and 1.2v drop (approximate) across each rectifier when in operation. Looking at just one of the three pairs of rectifiers, that is a total of 1.4v to 2.4v "overhead" loss when generating a (roughly) 14v output. You could be getting 15.4v to 16.4v right from the output of the 3-phase windings instead of the 14v DC out. OTOH, the Vf (forward voltage) of the rectifiers may be the key to keeping the alternator alive while a large load is applied.

    While there would be losses from eddy currents, with AC it would be much harder to get the coils into saturation, as the voltages driving the current will be changing continually at a fairly high frequency. Just HOW high the frequency will be depends upon the construction of the alternator and the rotational speed of the rotor. This will mean fewer windings will be required to keep the coils out of saturation, resulting in a lighter weight assembly.

    The actual physical construction of the thing:
    I started out thinking of winding the coils like one would wind a toroid like this:
    [​IMG]
    but that would not do, as any items attracted to the magnet would immediately begin damaging the wiring by thwacking it. Then I thought about attaching bars to the top side of the plate as you mentioned, however that leaves quite a bit to be desired as well.

    Now what I'm thinking is that each coil could be broken down into two separate coils, and attached to the upper side of the pressure plate using something like large carriage bolts all the way through to the bottom of the plate. This would mean six separate coils, evenly spaced, attached to the upper side of the pressure plate. This would give six alternating magnetic poles, ie: N, S, N, S, N, S, as you went around the bottom of the plate.

    It would also help with your coil winding, as it's going to be difficult getting the wire out of the existing machines without nicking the insulation or over-flexing it, causing failure by metal fatigue.

    The "spools" for the windings could be simply a couple of large washers on the aforementioned large bolt, with some tape or perhaps a strip of auto upholstery wound around it twice first, and then just wind on the wire, leaving a foot or more on each end. If no washers are handy, you could use your oxyacetelyne torch to burn some out of an automotive bumper or something - but you won't want any sharp edges that might nick the wiring insulation. You might find suitable washers on autos, such as shock absorber bushing washers, torsion bar bushing washers, body mounts, etc.

    Now that I think about it more, you might just weld the heads of some rather large suspension bolts (think A-frame bushing bolts) directly to the top of the pressure plate, making a rather even hexagon pattern. (See the attached extremely crude "ElectromagnetFrame.png" drawing.) A-frame bushing bolts/nuts will be a pain in the keister to get off with minimal equipment, but they might be the best "raw materials" you have for this endeavor. Besides, you will get a nut, a bolt, and two washers that will all fit together for each one you trouble yourself to retrieve. You may wish to roll the auto over (drain the gas/petrol first, of course), and use the torch to cut the springs before removing the bolts, since you probably don't have a spring compressor handy. If you drilled a hole near the edge of every other washer, you could attach three same-length chains or wires to suspend the assembled magnet by, that would tend to keep it level.

    Still, having this flat washer-shaped magnet above ordinance - if it should happen to trigger something, it will tend to cause the detonation to spread in a somewhat evenly horizontal fashion, right up until the pressure plate gets tossed like a coin at very high velocity. I have no clue as to what the pressure wave may look like, but the larger mines will likely cause the pressure plate to fragment into small missiles, most of them going up - but what goes up, must come down, at nearly the same velocity and force that it went up with, so protection from such fallout would be necessary. Thwacking a fragmentation grenade with the magnet would similarily focus the shrapnel in a near-circular horizontal fashion. A couple of vee-shaped angle irons on each side designed to deflect the blast away from personnel may or may not be helpful.

    More about the alternator - do you have several of the same make/model of vehicles around? If you are going to start fabbing up a prototype, it would be nice to have some spare parts about because you're sure to break something along the way.

    Driving the alternator - it'll be impossible to achieve perfect alignment of the shafts in the field, so a solid mechanical coupling is out; besides, the strimmer motor was not designed to drive an item with a large flywheel effect, and the crankshaft would very likely fail due to over-flexing (torsional stresses). However, I was thinking about a reasonably easy-to-fabricate flexible drive for the coupling of the strimmer motor to the alternator. You might salvage the flexible drive from the steering gear of one of the autos, they're usually located close to the steering box. Or you could cut a disk slightly larger than the diameter of the alternator's pulley from the sidewall of a tire.

    Drill three holes spaced 120° apart in the alternator pulley's flange on the same radius, as evenly as you can.
    Drill six holes in the tire sidewall disk spaced 60° apart on the same radius as the alternator pulley holes.

    Fab a small plate (or salvage a pulley from somewhere that has the same ID as the strimmer motor shaft) and drill three 120° spaced holes on that same radius, then weld/bolt/fasten however to the strimmer shaft. Bolt the tire sidewall disk to both the strimmer shaft adapter and the alternator pulley. This will hopefully take care of much of the harmonic vibration and crankshaft torsional loading problems, but no guarantees. Of course, the better you get the thing aligned, the longer the flex coupling will last. Staking the threads would be a nice touch, as I wouldn't want to be anywhere near it if a nut decided to work itself loose.

    It would help a good deal if you could dismantle one of the alternators and take fairly detailed photos of it during dissassembly, so that we can help you identify the components and see what you're dealing with.
     
  14. rwmoekoe

    Active Member

    Mar 1, 2007
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    sarge, you have a sound knowledge and experience in mechanics too... salute!
     
  15. SgtWookie

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    Jul 17, 2007
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    Thanks, but I'm just "winging it" as best I can here.

    These folks need serious help! From what little research I've done, there are or were in excess of EIGHT MILLION mines deployed in Angola, of at least 70 different types, manufactured in many different countries - and in some areas of Angola, mines are STILL being actively laid!

    Some of the mines are triggered by as little as 2kg of force (around 5lbs) that are designed as antipersonnel mines. This has caused more than 80,000 Angolans to lose limbs, roughly 40% of them women and children.

    Some of the mines require around 200kg (500lbs) of force, designed to destroy/disable vehicles/tanks.

    Some of them are triggered magnetically; they usually are "enabled" when an iron or steel object (such as a tank/truck/bulldozer) passes overhead, and when the magnetic field weakens, the mine detonates.

    Some of them are anti-minesweeper type, which are triggered by the signals put out by metal detectors.

    I can't hope to document them all, as I only know what I've read about it. Details are rather sparse, but there are statistics.

    If someone else has ideas or experience with this kind of thing, please speak up! I'm just brainstorming at this point. I would hate to overlook something that may cost someone (or several people) their lives.

    The DC coil idea certainly isn't out the window; that may indeed prove viable. I simply don't know offhand.
     
  16. jamie_grieve

    Thread Starter New Member

    Apr 29, 2008
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    Hey sarge, tried to send some photos of the story so far but satelite not playing too well today. Will all the different north and south poles on the same plate not cancel each other out? I've got a 28v alternator good for 24A to play with or some 12v 65A ones. I'm gonna make a DC magnet from the 28v one and try to make an AC magnet from the 12v one.
    From the schematics you sent would I be correct in saying I would for a wye join three ends together then simultaneously wrap arround the offending object say a bar at this stage with each of the other ends connected directly to each of the stator windings directly?
    For the delta configuration it looks like we just have three coils joined in a delta shape with each of these connected directly to the stator windings.
    For the 28V alternator I have, what size of capacitor does C1 want to be?
    It's all happening very slowly I'm afraid because I want my local staff to be involved.
    Regards,
    Jamie.
     
  17. hgmjr

    Moderator

    Jan 28, 2005
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    For clarification purposes, I think three mine types have been mentioned thus far.

    1. Anti-personnel (TYPE 1) - triggered when a person steps on or near the mine. This would imply it is sensitive to weight and/or vibration.

    2. Anti-personnel (TYPE 2) - Triggered by the electro-magnetic energy emitted by a metal-detector.

    3. Anti-tank - Triggered by proximity of large metal object and/or vibration.

    Are there any other types other than the ones mentioned above?

    Are you assuming that each of these mine types are equally likely to be encountered?

    hgmjr
     
  18. hgmjr

    Moderator

    Jan 28, 2005
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    I am not at all familiar with this topic but I am interested in contributing if I can.

    Are you looking at a given strategy:

    1. Detect location then once located come back later and detonate mine using an explosive charge or other means in the proximity of the mine.

    2. Deliberately try to induce detonation coincident with detection.

    3. Other?

    hgmjr
     
  19. hgmjr

    Moderator

    Jan 28, 2005
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    You may have already encountered this paper in your research.

    hgmjr
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    Hgmjr,
    At this point, Jamie is just trying to build a strong magnet to pick up shrapnel and other bits of metal that are lying around in the minefields, to reduce the effort that is required of those who are using metal detectors to locate the actual mines. Apparently, there is SO much iron and steel junk lying around that it causes the people with the metal detectors to have to stop and investigate each tiny fragment.

    When a mine is found, it is detonated in place. It is far too risky to attempt digging them up. When a mine is detonated, the metal parts fly everywhere, adding to the problem of detecting the next mines. But if they had a reasonably powerful and portable magnet, they could skim the surface with it to quickly collect the ferros fragments. Apparently, that's all the help Jaime needs from us at the moment; attempting to build an electromagnet from items that they have in ready supply from disabled autos/trucks/tracked vehicles.

    If the mines are fairly old (like perhaps over 7 years) the magnetic-sensitive and minesweeper-sensitive mines may be less of a threat, since their batteries would likely be run down - merely speculation on my part.
     
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