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Why sensing distance in inductive sensors depends more on ferromagnetic features than conductivity of metals?
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rmend...@gmail.com
Sep 27, 2012, 10:49:53 AM9/27/12
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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.
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.
George Herold
Sep 28, 2012, 9:07:22 AM9/28/12
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On Sep 27, 10:49 am, rmendoz...@gmail.com wrote:
> 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).
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.
> 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.
OK first this is a bit hand-wavy (I'm not doing all the math). The
> 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.
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.
Phil Allison
Sep 28, 2012, 9:45:46 AM9/28/12
to
"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.
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
George Herold
Sep 28, 2012, 11:36:10 AM9/28/12
to
On Sep 28, 9:45 am, "Phil Allison" <phi...@tpg.com.au> wrote:
> "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.
Hi Phil, Well if we restrict the discussion to changing B fields and
> "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.
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).
>
> Accounts for the greater damping effect on an EM source that is within
> coupling effect distance.
>
> Beware - non simple math.
hands and call in an engineer, domains, hysterisis, saturation.
George H.
>
> ... Phil
Fred Bartoli
Sep 29, 2012, 6:57:11 AM9/29/12
to
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.
--
Thanks,
Fred.
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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