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Discrimination of Ferric Targets on higher friquencies. Inspired by Induction Cooking article.

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  • Discrimination of Ferric Targets on higher friquencies. Inspired by Induction Cooking article.

    High frequency signals (24 kHz) will make 56 times more resistance for eddy currents in STEEL then in COOPER. (See chart below.)

    So use of different frequencies will give more information about target. Probably multi-frequency detectors like Minelab BBS (Broad Band Spectrum) and Fisher CZ line (dual frequency: 5 kHz and 15 kHz) already using that.

    If you read how induction cooking works you will see how it discriminates iron.
    You can use only magnetic cookware to let eddy currents to cook your food because in magnetic materials high frequency current flows near the surface and create high resistance but in colored metals it goes deep and resistance is low. (Cookware will stay cool.)

    We can use VLF detector with different frequencies to measure resistance (how quick eddy currents dissipate) and get more info from target or probably PI detector with different pulse width if it will work. (Or PI-IB detector hybrid)
    Also thin aluminum foil can create high resistance in high and low frequency so can be recognizable too.

    I think it will work this way:

    Material --- Low frequency, High frequency
    -------------------------------------------
    Ferric Target: Low resistance, High resistance
    Aluminum Foil: High resistance, High resistance
    Colored metals: Low resistance, Low resistance
    -------------------------------------------

    From Wikipedia:
    "The cooking vessel is made of stainless steel or iron. The increased magnetic permeability of the material decreases the skin depth, concentrating the current near the surface of the metal, and so the electrical resistance will be further increased. Some energy will be dissipated wastefully by the current flowing through the resistance of the coil. To reduce the skin effect and consequent heat generation in the coil, it is made from litz wire, which is a bundle of many smaller insulated wires in parallel. The coil has many turns, while the bottom of the pot effectively forms a single shorted turn. This forms a transformer that steps down the voltage and steps up the current. The resistance of the pot, as viewed from the primary coil, appears larger. In turn, most of the energy becomes heat in the high-resistance steel, while the driving coil stays cool." http://en.wikipedia.org/wiki/Induction_cooking

    Also picture of Induction Cooking coil below painfully recalls spiral coil for PI detector.
    Attached Files
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