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Any way to shield compass from motors? — Parallax Forums

Any way to shield compass from motors?

Don FrenchDon French Posts: 126
edited 2005-09-20 16:39 in General Discussion
My application for the compass module involves placing the compass near some running·DC motors. It is above and behind the motors, but they definitely have an effect strong enough to make the compass readings·useless.· There are some·less desireable options·in which I can move the compass·far away, but·I was wondering if there is there any way to shield the compass from the magnetic effect of the motors but not from the magnetic effect of the earth?· Lead?· Enriched plutonium?

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2005-09-19 23:11
    Short of shielding the motors with "mu metal" (<-Think that's correct)..... but the analogy is the same as someone standing right in front of the TV when you are trying to watch something.

    Is there anyway to re-position your compass so that it is away from the motors? (i.e. get the person in front of the TV to move)

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Don FrenchDon French Posts: 126
    edited 2005-09-19 23:21
    Yes, as I said, it is an option, but I would rather not if I don't have to.

    Mu metal?
  • Don FrenchDon French Posts: 126
    edited 2005-09-19 23:35
    http://digilander.libero.it/paeng/what_mu_metal.htm

    I googled for it. Know of a source of metallic glass?
  • Ken GraceyKen Gracey Posts: 7,401
    edited 2005-09-20 02:53
    Don,

    We dealt with this exact issue on the Penguin robots, but the problem was with the batteries (not the servos). Two solutions worked evenly well to solve our problem:

    (a) A 0.050" steel shield between the batteries and compass solved it
    (b) Moving it an inch away made it work even better.

    Ken Gracey
  • diafysaldiafysal Posts: 92
    edited 2005-09-20 09:47
    Also, keep wiring with high currents away from the compass.

    (some mathematics:
    http://forums.parallax.com/showthread.php?p=544957)
  • Don FrenchDon French Posts: 126
    edited 2005-09-20 15:10
    I don't have any high current wiring to worry about, just the motors.·

    I just got a quote for mumetal from Mushield and it ain't cheap.· There is a $150 lot charge for 1-5 square feet for .14 inch thick sheets, or 1-20 feet of .002 inch foil.· Now, I wonder why they can't cut me a one foot square piece of foil·with some tin snips and send it out for, say $10?

    I really don't want to move my compass far away from the motors for this application, but I may have to.· Or maybe a chunk of cheap steel will work.
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-09-20 15:42
    Well, you can't shield the COMPASS and still read the earth's magnetic field, but you can surely shield the MOTORS. Then the compass will be free to read the earth's magnetic field without distortion.

    And almost by definition, any motor IS a 'high-current' beast. Unless they have fixed magnets -- which has the same effect on your compass.

    Depending on the size of the magnetic field generated by the motors, you can get large reductions in field strength with small movements of the compass. Try Ken's suggested solution using steel, mu-metal may be overkill.
  • Don FrenchDon French Posts: 126
    edited 2005-09-20 16:39
    Right, it is the motors I am looking to shield. mumetal is only overkill because of the price, as mumetal foil might do the trick very nicely and without adding any appreciable weight or bulk. It was used to shield CRTs in the past, but I don't know if that is still true.

    Oh, here is a site that offers foil without a $150 minimum charge. $18.95 a linear foot:

    http://www.berk.com/~lessemf/wiring.html

    And look! They also have 1" wide metal glass tape for $2.50 a linear foot. Hmm. I only need about 2 feet!
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