magnetic interference

jawnlooi

hi dudes,
my robot has a motor mounted very close to my HM55B compass sensor (and a H48C accelerometer), and to my knowledge, the HM55B works by sensing the magnetic field. the problem is that my motor is an electric motor (irreplacable, and the components have to be in the position that they are in), and the motor generates an (dynamic?) electric field i assume, which disrupts my compass readings. how do i shield the fields?

with research (and memory of mythbusters), ive come across faraday cages; i was wondering if i could use the gold netting of a faraday cage to COVER the motor only or the electronics only. there are several questions i have:

1) does a faraday cage even prevent magnetic fields, or just RF fields? (if not, i have to use conducting material right? how thick? whats the best material?)
2) can i put a flat (3 inch diameter) netting just above the electronics, instead of covering the whole thing? would the magnetic fields 'wrap' around the netting and get to the electronics either way?
3) would this insulation, whether conductive material or faraday cage also shield the earth's magnetic field? (and thus defeat the whole purpose of a compass)

also, when i tilt the compass forward/backward/sideways, the compass reading changes as well, is there a way to rectify this?

any suggestions?
thanks folks.

Post Edited (jawnlooi) : 11/8/2006 11:19:57 AM GMT

allanlane5

Faraday cage prevents propagation of RF fields by shunting the energy to 'ground'. There IS a 'magnetic' component to electric fields, but it is tiny compared to even a small magnet.

There does exist some stuff called 'mu-metal' which blocks magnetic fields. Google for it. I think Edmund Scientific has some. And yes, it would shield the earth's magnetic field too, so you probably want to put it between the motor and the HM55B -- in other words, wrapping the HM55B in it would be 'bad'.

jawnlooi

awesome, thanks. do you know whats an estimate of how thick material is needed? if my robot has
a normal aircraft motor is 3 inches away and/or 8 inches away (i'm trying to displace my compass)

also, would it help if i had some sort of pattern on the material? (like how soundwaves are eliminated by
wavy patterns like in soundproofing)

thanks again, i'll try to source for a local supplier of mu metal

Post Edited (jawnlooi) : 11/8/2006 2:38:52 AM GMT

The Fat Kid

I've not used any of the compass parts yet, but I have done compass swings in airplanes and boats.· Is there a way to make these adjustments on an electronic compass?

latigerlilly

Use this stuff to shield from electromagnetic radiation:
3M Velostat, Part number 1706, .006" 6 mils thick (.006") or
3M Velostat, Part number 1704, 4 mils thick (.004" )

Vendor:
www.mpsupplies.com/3mstaticfilm1700series.html

I use it to shield sensitive electronic equipment from a powerful electromagnetic coil that I built. It works really good. None of my electronics has busted even though it is near the coil.

I saw the material first here: www.stopabductions.com/ . Kinda creepy website & I don't believe in aliens, BUT I looked at the specs of Velostat and I thought, "This would make a perfect faraday shield for electronics projects!" Also, I have a friend that lives next to a power line & transformer. She has terrible migraines, but after wearing a hat made of this stuff at home, her migraines disappeared.

Shameless Plug
Please (please, please) answer my questions;

I need a part number for a dual coil dpdt relay for the following schematic. I want to use a BOE to switch the device -on+, off, +on-. In other words, on, off, and reversed polarity on, off, etc. I want the switching to be controlled by the BOE. If alternate design is necessary because the dpdt relay I need doesn't exist, then please post detailed schematic and part numbers as I am a novice. Thanks.
http://forums.parallax.com/showthread.php?p=615420

Also, please check my schematic for me and give me a yay or nay for the following:
http://forums.parallax.com/showthread.php?p=615389

Thanks,
Lilly.

allanlane5

"Velostat" will shield RF and EMI -- but the OP was asking about strong magnetic fields. Velostat probably won't do much against those.

I like the hat idea, though.

Bruce Bates

Folks -

I concur with Allan 100%. The ONLY substance of which I'm aware that will block electro-magnetic radiation is Mu metal. Sometimes you can salvage the Mu metal sleeve that's wrapped around the picture tube of older color, and most black and white TVs if you can find one. It is generally quite dull gray in color, and quite soft, due primarily to the high nickel content. The entire alloy is composed as follows: 75% nickel, 15% iron, plus copper and molybdenum in smaller quantities.

Regards,

Bruce Bates

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