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Author Topic: electronic scale and microwave - and Magnetic Fields shielding  (Read 3730 times)
bernard
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« on: July 18, 2009, 05:09:28 PM »

hi,

I was using an electronic scale in the kitchen and it was fluctuating like crazy for no apparent reason... until I discovered the culprit:  my microwave was cooking something!  Stopped the microwave and everything was like normal, started it again, and scale going bezerk again.  This occured over a meter away from the microwave.

My first thought: My microwave is an old one so it might be leaking RF. But there is another possibility: Magnetic fields.  Electronic scales, it seems, can be very sensitive to magnetic fields - I was reading this here: http://www.jscale.com/support.html. I would not be surprised that the microwave emit strong magnetic fields given the high power they consume (not sure how to test that, maybe if I find a compass?).

I had been reading on magnetic fields -- shielding in particular -- http://www.cvel.clemson.edu/emc/tutorials/Shielding02/Practical_Shielding.html -- to see what could be done to fight jitter since the Wacom sensor use magnetic fields.  Magnetic fields are strange beasts and faraday cage techniques (like the foil / cling wrap thing) has near zero effect on this. You need a thick material -- one that a magnet is attracted too.  Also shield the source in a circle has zero effect either(!) -- you have to shield the sensitive device or put some flat wall to "intercept" the field.

BTW, at the back of my Wacom sensor board, there is a sheet of metal and magnets are attracted to it -- so this seems to be meant to affect magnetic fields.

 
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Gem
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« Reply #1 on: July 18, 2009, 08:16:43 PM »

RF shielding just needs a highly conductive material (copper would be ideal).  I was under the understanding that as the waves hit the material they more or less ground out and become a current inside it.

Should we be dealing with RF shielding or magnetic shielding?
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bernard
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« Reply #2 on: July 18, 2009, 08:46:02 PM »

Looking at the technology description from Wacom on their "component" web site, they talk about some "magnetic resonance" (not so sure what it means exactly) --  my gut feel is that magnetic fields plays an important role when sensing the pen position.  Maybe I should find the patent text for the wacom -- it might give more indication as to how it works.

Also, I am not sure that the frequency of AC magnetic fields can be just "changed" so to avoid "interference" -- I am really not sure -- but I think that in a given space you have a field or you do not have a field: frequency does not matter.  Actually, some tricks to control fields is to create other fields to counteract or redirect.

Still reading on that subject. Quite strange stuff.
« Last Edit: July 19, 2009, 05:14:53 AM by bernard » Logged
Drewid
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« Reply #3 on: July 19, 2009, 05:22:39 PM »

I remember reading that the Wacom shield is lead foil, though I can't remember where I heard it.
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