# 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. ### George HeroldGuest

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 AllisonGuest

"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. ### George HeroldGuest

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 BartoliGuest

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.