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Why sensing distance in inductive sensors depends more onferromagnetic features than conductivity of

Discussion in 'Electronic Basics' started by [email protected], Sep 27, 2012.

  1. Guest

    As I understand, the principle of operation of inductive sensors relies on Eddy currents, and those induced currents are stronger on more conductive metal (ref. http://en.wikipedia.org/wiki/Eddy_current).

    So, why in practice (and datasheets) sensing distance of these sensors is larger in more ferromagnetic metals? (i.e., it is easier to detect iron than copper).

    Thanks for helping.
     
  2. OK first this is a bit hand-wavy (I'm not doing all the math). The
    eddy currents will depend on the strength of the B field in the
    material (along with other things.) As long as the B field is low
    enough such that it doesn't saturate the iron then the B field in iron
    will be bigger than in copper. And that 'wins' over the conductivity
    difference. It's for a similar reason that iron has a shorter skin
    depth than copper. And a 1/16" sheet of steel is better at shielding
    EM fields than a 1/16" sheet of copper.

    George H.
     
  3. Phil Allison

    Phil Allison Guest

    "George Herold"

    OK first this is a bit hand-wavy (I'm not doing all the math). The
    eddy currents will depend on the strength of the B field in the
    material (along with other things.) As long as the B field is low
    enough such that it doesn't saturate the iron then the B field in iron
    will be bigger than in copper. And that 'wins' over the conductivity
    difference. It's for a similar reason that iron has a shorter skin
    depth than copper. And a 1/16" sheet of steel is better at shielding
    EM fields than a 1/16" sheet of copper.


    ** IOW, iron & steel absorb magnetic energy while most other metals let it
    pass it right through.

    Accounts for the greater damping effect on an EM source that is within
    coupling effect distance.

    Beware - non simple math.


    .... Phil
     
  4. Hi Phil, Well if we restrict the discussion to changing B fields and
    not static ones. Then there is a reduction of the B field in any
    metal. I'm not sure if it's 'more correct' to think about the B field
    being absorbed, or just reflected by the conductor. For the non-
    existent 'perfect' conductor, the changing B field sets up currents on
    the surface and that looks like a reflection (no absorption or
    transmission).
    Yeah, if you want to ask what happens in iron/ steel, I'll throw up my
    hands and call in an engineer, domains, hysterisis, saturation.

    George H.
     
  5. Fred Bartoli

    Fred Bartoli Guest

    George Herold a écrit :
    On ferromagnetic materials the skin depth is way lower, thanks to the
    relative permeability. The sheet resistivity is also much higher than
    for copper, leading to greater losses.

    I once designed a 2.5MHz resonant converter having by design highish
    leakage inductance, i.e. magnetic field to contain. A steel shield had
    big losses and a copper shield just solved that.
     
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