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.
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.
n Eddy currents, and those induced currents are stronger on more conductive metal (ref.http://en.wikipedia.org/wiki/Eddy_current).
larger in more ferromagnetic metals? (i.e., it is easier to detect iron th an copper).
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.
"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
t
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.
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.
-- Thanks, Fred.
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