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Industrial Metal Detection Ideas?

Discussion in 'General Electronics' started by Christopher Ott, Apr 30, 2005.

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  1. Hello folks,

    I'm exploring the feasibility of inline metal detection for horizontal wood
    recyclers. Here is a link to a photo of a typical recycler.

    These are generally 500 to 1000 HP diesel units made by many several
    different companies. They run debris through a rotor/grating assembly and
    produce chips on the other end. They are used for a number of other
    applications (compacting roofing shingles for landfill, chipping railroad
    ties, etc.), but the most typical use is by landscaping suppliers who turn
    yard debris and slash piles into landscaping mulch.

    The metal contaminations occurs when large pieces of steel get into the
    piles. Axes, sledgehammers, rebar, railroad spikes, loader-bucket teeth and
    other large, heavy objects will break the teeth off the rotor and blow out
    bearings. This causes several hours to several days of downtime.

    I'm looking into methods of detecting this contamination before it hits the
    rotor. This is complicated by the fact that the hopper is steel, as is the
    drag chain used to pull the material into the rotor. Non-metallic drag belts
    are far too fragile and are not commonly used on the infeed side. Also the
    loader bucket which dumps material into the hopper does tend to confuse
    traditional metal detection technology.

    Here's what's been tried:

    Traditional metal detection coils. The large amount of steel on three sides,
    and drag chain made the output virtually worthless. This technology could
    not determine a difference between a tin can (which can safely pass through
    the rotor and be picked out with a magnet) and sledgehammer head.

    Vibration sensors on the bearings: This does work to a certain extent.
    Sometimes (like with a railroad spike in a tie) the high frequency ticking
    of the rotor bits hitting the metal can be sensed and the load can be
    reversed and dumped out the back to be sorted. However what more commonly
    happens (with loose metal) is the system simply senses the hit milliseconds
    before the contamination gets pulled in and destroys the bits.

    What I'm curious about is if anyone out there has seen this problem solved
    before. Perhaps in a different application which I can leverage to this one.
    Also, any thoughts on sensors which might work. Other ideas? I'm open to any


    Christopher Ott
    Ott Electronics Corp.
    Chandler, AZ, USA
  2. Robert Baer

    Robert Baer Guest

    At least you have a partial "solution" using sound (the unique noise).

    First dumb thought i had, is to add a "pre-chopper" that uses
    flexible metal "bits" that do not get mangled by anything; their
    function is to create that metallic noise before the junk gets to the
    working chopper.
    Naturally, there is a trade-off here, as the "flexible bits" need to
    be strong enough to dig thru the wood, etc to "find" buried metal, but
    flexible enough for minimal damage when they do hit metal.
    So, they should be easily replaceable in the field, and sufficent
    spares should be on site (presumes cheap "flexible bits").

    Next dumb idea was to use sound (or RF) for determinimg density; an
    engineering challenge.
  3. Andy P

    Andy P Guest

    On the flexible "bit" front, as was mentioned earlier..why not use
    chains? Beat the heck outta them with rotating chains akin to mine
    clearing devices. Chances are that the dense metal of the contaminants
    wont be damaged by the chains, but the wood sure will.

    Another thought is a thermal camera. Run everything through a pseudo
    induction heater (like put a heating coil above the conveyor for a few
    feet) and then after that put a thermal camera. The metal pieces will
    hold heat longer than the wood, and you should be able to pick up the
    temperature differences and stop the feed before anyhting damaging gets
    into the machine.
  4. Actually, flail chains are used in a similar product, a debarker. The chains
    pound the bark off the tree before it's fed into a chipper. The problem with
    flail chains is that they work much, much slower than hardened bits. The
    overall throughput of the system cannot be reduced (or no one would buy it.)

    The temperature profiling idea is do-able, but would be a severe safety
    hazard. Remember there's tons of dried organic material everywhere around
    these machines. barkdust, gasoline, diesel fuel and acres of (quite
    flammable) mulch piles everywhere around this equipment.

    Good ideas, lets keep em coming,

  5. Jim Douglas

    Jim Douglas Guest

    OK, this might be off base but what about some type of XRAY equipment like
    at the airports, have someone sitting there watching the crap going in at
    the beginning. Another idea is a "large magnet" on one side of the box in
    front so that anything
    with metal within it would move to that side??

    I remember seeing a demo movie of a large metal chipper\shredder, they were
    dumping in cars, metal buildings, refridgerators, air conditioners, big
    stuff and this thing chewed it up quicker than a dog that had missed a meal,
    cool technology!
  6. MG

    MG Guest

    A section of the magazine/shoot, far from the chipper blade, could be made
    of wood or aluminum and improve the sensitivity to magnetic field.

    The material could go through or above a coil which is part of an
    oscillator, steel will detune the frequency which can be measured easily,
    usually this method is very sensitive and frequency drift is proportional to
    the quantity of steel, a threshold can be set. Still a window transparent
    to magnetism is necessary, no steel structure near the coil.

    Ultrasound (choose a frequency much above the rattle and vibrations) the
    echo should be stronger from large metal than wood.

    Let us know which solution will finally work.

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