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Compass chips

Discussion in 'Embedded' started by Tim Wescott, Apr 3, 2014.

  1. Tim Wescott

    Dave Nadler Guest

    Depends on your application! In a car its usually an OK
    assumption. The bigger problem, as others mentioned above,
    is even with a perfect sensor the compensation and/or
    environmental interference may yield unusable results.
    We had a test aircraft where raising the landing gear
    swung the compass ~20 degrees!

    Let us know how you make out,
    Best Regards, Dave
     
    Dave Nadler, Apr 7, 2014
    #21
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  2. You haven't been for a drive with me :)
     
    Clifford Heath, Apr 7, 2014
    #22
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  3. Tim Wescott

    Dave Nadler Guest

    When you're sliding, you're probably not concerned that the
    GPS display has the wrong orientation ;-)
     
    Dave Nadler, Apr 7, 2014
    #23
  4. Tim Wescott

    Tom Gardner Guest

    Or a flight with me :) I've got to do this again soon,
    where the "aircraft departs from controlled flight".

    So do 14 year old pilots, before they are allowed to go solo.

    Tom
    Takeoff is optional, landing isn't.
     
    Tom Gardner, Apr 7, 2014
    #24
  5. Tim Wescott

    Tim Wescott Guest

    GPS + IMU + some lateral vehicle acceleration will give you your heading.

    As soon as the kalman filter sees some lateral acceleration on the GPS
    that it can correlate with accelerometer readings, it knows which way
    you're pointed.

    (Assuming you're in a gravity field)
     
    Tim Wescott, Apr 7, 2014
    #25
  6. Tim Wescott

    Paul Rubin Guest

    Almost once a week I wish that my car GPS had a built-in compass. The
    usual situation is I want to drive to some address, so I enter it into
    the GPS while the car is parked. The GPS figures out the route from
    where I am, but because it doesn't know which way the car is facing, it
    can't tell me which way to start driving. Garmin made some hiking GPS's
    with built-in compasses but I've never seen a car GPS with one. I'd buy
    it if they made it.

    Regarding sensors: http://www.precisionnav.com/ has some kind of compass
    sensor that they say is better than a fluxgate. They used it in a line
    of car compasses and instrumentation boards a while back, and I wrote
    some code that ran one of the instrumentation boards.
     
    Paul Rubin, Apr 7, 2014
    #26
  7. Tim Wescott

    Jasen Betts Guest

    Steer one direction with the rudder and the opposite with the
    elevators and you can get a dead-air heading several degrees off straight,
    and off vertically too.
     
    Jasen Betts, Apr 7, 2014
    #27
  8. Tim Wescott

    Tom Gardner Guest

    True, but you don't need to sideslip.
    Try ridge flying in a glider :)
    Or even just uncoordinated flying, using the rudder without ailerons :)
    Or just a strong 90 degree crosswind approx the same as your airspeed :)
     
    Tom Gardner, Apr 7, 2014
    #28
  9. Tim Wescott

    Tom Gardner Guest

    I question the accuracy without a compass; the alternative
    is to determine the angle by integrating the angular rotation.
    That is, of course, highly sensitive to noise, drift and
    zero offset.

    Solid measurements would be appreciated.

    Gravity isn't required - you need angular rotation.
     
    Tom Gardner, Apr 7, 2014
    #29
  10. Tim Wescott

    Tom Gardner Guest

    Interesting to see how you would negate the field distortion
    due to being surrounded by a large hunk of iron! Even then it
    wouldn't be accurate near Kiirunavarra, even neglecting the
    magnetic dip at high latitudes
    http://en.wikipedia.org/wiki/Kiirunavaara

    Vaguely what kind of measured performance were you able to achieve?
    Did you rely on sensor fusion, i.e. use other instruments as well?
     
    Tom Gardner, Apr 7, 2014
    #30
  11. Tim Wescott

    Paul Rubin Guest

    Car compasses correct for that. When you first install the compass, you
    press a calibrate button, then drive the car around in a circle. The
    changing part of the field is due to the earth, and the fixed part is
    due to the car. So it can subtract out the fixed part. I've had some
    compasses that did that (Precision Navigation Wayfinder), and they
    worked pretty well.
    Of course with a compass and gps in the same box, it could know that you
    were near Kiirunavarra and make appropriate corrections.
     
    Paul Rubin, Apr 7, 2014
    #31
  12. Tim Wescott

    Tom Gardner Guest

    OK, thanks.

    I very much doubt there the device contains sufficient
    localised knowledge. Supposedly Kiirunavarra was
    discovered when someone hiking put his knife down, and
    found it surprisingly difficult to pick up.
    The mountain is now being systematically removed, and
    IIRC will eventually turn into a very big hole!

    What I don't understand is given that the heavier
    elements were all formed in supernovae, how come
    there is a concentration of them? Surely they should
    be uniformly distributed.
     
    Tom Gardner, Apr 7, 2014
    #32
  13. Tim Wescott

    Nils M Holm Guest

    It is not a uniform distribution, but a random distribution,
    which by its very nature has "concentrations" (clusters) and
    "holes". It is like a PRNG returning 2 five times in a row.
    It is improbable, but it still may happen.
     
    Nils M Holm, Apr 7, 2014
    #33

  14. Having an engine out also often results in some amount of slip,
    particularly at lower speeds. Assuming a multi-engine aircraft, of
    course...
     
    Robert Wessel, Apr 7, 2014
    #34
  15. Tim Wescott

    Walter Banks Guest

    I once flew a track in the eastern arctic that magnetic error
    was briefly exactly -90 degrees. (Straight east of the magnetic
    pole) I was close enough to the magnetic pole that the
    magnetic error changed >1 degree / hour in a PBY.

    w..
     
    Walter Banks, Apr 7, 2014
    #35
  16. Tim Wescott

    Phil Hobbs Guest

    My father worked for a mining company called Cominco. At one point in
    the late 1970s they were developing a lead-zinc mine on Little
    Cornwallis Island, which is about 100 miles north of the magnetic pole,
    so the compass variation would have been 180 degrees!

    http://en.wikipedia.org/wiki/Polaris_mine

    He used to joke about renaming it "Palm Island" to improve recruiting. ;)

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
    http://electrooptical.net
     
    Phil Hobbs, Apr 7, 2014
    #36
  17. The truly heavy elements are, indeed, distributed almost uniformly.
    That's what makes them so bloody hard to mine for.

    This is, among other things, the reason why it use to make sense to base
    currencies on gold: the occasional gold rush set aside, gold is actually
    distributed quite thinly, yet evenly all over the place. This means
    that to dig up a certain amount of gold takes essentially the same
    amount of work, no matter where in the world you do it. This meant the
    value of gold stayed relatively fixed over time --- until industrialized
    mining blew up that concept.

    Another example are the so-called "rare earths" (chemically:
    lanthanides), which are currently mined almost exclusively in one place
    in China --- with some nasty political consequences to boot. Yet it
    doesn't really matter much where you open such a mining operation: the
    yield will be pretty similar basically everywhere.

    So why is there almost no other mine which might give the Chinese some
    competition? Well, that's because to get useful amounts of these ores
    involves going through incredibly large amounts of dirt, and using some
    seriously dangerous physical and chemical processing to first isolate
    the extremely diluted rare-earth ores from the rest of the dirt, then to
    separate individual ores them from each other. So you can mine for rare
    earths wherever you want, but you'll get serious amounts of pollution
    coming from some fricking _huge_, ugly holes in the ground if you do.
    This eventually got this kind of mining choked off by environmental
    protection rules, or flat-out closed by the government, almost everywhere.
     
    Hans-Bernhard Bröker, Apr 7, 2014
    #37
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