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Rate-gyro accuracy

Discussion in 'Electronic Design' started by Marco Trapanese, Sep 26, 2010.

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  1. Hello,

    in the ADXRS614 datasheet I read:

    "CALIBRATED PERFORMANCE
    Using a 3-point calibration technique, it is possible to calibrate
    the null and sensitivity drift of the ADXRS614 to an overall
    accuracy of nearly 200°/hour. An overall accuracy of 40°/hour
    or better is possible using more points."


    What does that mean?
    I don't understand the 200°/hour and 40°/hour values.

    I'm going to integrate the rate to obtain an heading. But I need an
    accuracy better than 2° per hour.
    Does the sentence above tell me that if I integrate the rate (let's
    assume with no noise...) after 1 hour I may have an error of about 200° ?

    I can't use a compass because there is a lot of iron in the nearby. I've
    already tried with a (calibrated) HMR-3000 with no chance.

    So I'm going to use a rate-gyro to have a relative heading, but I'm not
    sure if I can get the required accuracy over the time.

    Thanks for any hint.
    Marco
     
  2. Il 26/09/2010 18:58, Vladimir Vassilevsky ha scritto:

    :-/
    Other options?


    I can't. It's an underwater equipment.
    Anyway, even if I was on the surface I don't know how to get the heading
    from the GPS. The platform doesn't move, just rotates.

    Marco
     
  3. Tim Williams

    Tim Williams Guest

    Magnetometer.

    Tim
     
  4. Il 26/09/2010 19:28, Bill Sloman ha scritto:

    Good ideas, but I can't use anything off-board :(

    Marco
     
  5. Il 26/09/2010 19:19, Tim Williams ha scritto:

    Hi Tim,

    as far as I know, the solid-state compasses (like the HMR3000) contain 3
    magnetometers.
    Because you suggest me to use them I'm pretty sure I'm wrong.

    So, why magnetometers won't be affected by iron?

    Marco
     
  6. Il 26/09/2010 20:27, Richard Henry ha scritto:

    Ok. Very bad :(


    The wrong thing is that there is no contact :)
    It's an underwater vehicle.

    Marco
     
  7. Il 26/09/2010 20:49, Jan Panteltje ha scritto:

    Zero visibility.


    yes, it works. But it also affected by the iron around.

    Marco
     
  8. Tim Williams

    Tim Williams Guest

    Oh, OP said iron. Nevermind.

    Not entirely impossible, as gigantic steel ships have compasses. But if
    you don't know where the iron is (say you're swimming around the metal
    roots of a pylon, or something), it would be rather difficult to
    compensate for.

    Can you grab some of those ex-mil gyros from Electronic Goldmine?
    Or...talk to whoever they got them from if you need quantity. Or if you
    need to go to the manufacturer...well, that's going to be pricey...

    Tim
     
  9. Il 26/09/2010 21:08, Tim Williams ha scritto:

    Yep.
    I have a *lot* of iron just under the bottle. Then, the platform will be
    very close to such a pylon.


    mmm... I think it will be difficult, but I can try.

    Marco
     
  10. Il 26/09/2010 21:06, Jan Panteltje ha scritto:

    Good idea!
    Anyway, it requires a quite complex firmware, I bet.

    Marco
     
  11. Il 26/09/2010 21:38, Muzaffer Kal ha scritto:

    Eh, if I could pay $10k I don't ask here :)

    Marco
     
  12. Rich Webb

    Rich Webb Guest

    If it's for a personal or educational project, you might try fusing
    several independent yaw-rate sensors.

    If it's commercial project then something like this
    http://www.kearfott.com/content/view/185/130/ is what your competition
    is using.
     
  13. RoV

    RoV Guest

    Hi Marco,
    to get that accuracy you need a gyro based on FOG or RLG technology.
    The market is rich of equipment of that kind (look for IMU, AHRS, INS), but
    they are quite costly. True heading gyros are capable of giving heading
    respect to geographic North (they measure the earth rotation rate).
    That's exactly what most underwater vehicles use.

    P.S. it's not sufficient to integrate yaw rate to get heading. You must have
    all three rate axes.

    Ciao,
     
  14. Il 26/09/2010 22:04, RoV ha scritto:

    I know the FOG technology. I go to read something about RLG.
    A true heading is good but not mandatory. Also a relative heading with a
    quite good accuracy is ok.


    If the vehicle doesn't change much its attitude (say 1-2° for pitch and
    roll) the error on yaw axis should be very small, if I'm not wrong.

    Anyway, I could integrate all three axis, but the FOG technology is too
    much expensive for me.

    Ciao
    Marco
     
  15. Il 26/09/2010 22:04, Rich Webb ha scritto:

    Using two gyros, one upside down, may compensate something? They will be
    at the same temperature so the drift should be equal. But it should sum
    in opposite directions.


    I used VRUs on ships. For this projects I think they are too big and
    expensive.

    Thanks to you too
    Marco
     
  16. Rich Webb

    Rich Webb Guest

    What are the potential liability costs if a pointing error causes
    Something Really Bad to happen? The up-front cost of a proper FOG or RLG
    IMU or AHRS with good drift numbers may be a small fraction of that.
     
  17. Rich Grise

    Rich Grise Guest

    So, can you, while setting it up, use a separate gyro or compass on the
    service boat, and turn the platform, take readings, compare, and
    compensate for the iron with some kind of look-up table or something? The
    iron isn't expected to change, is it?

    Thanks,
    Rich
     
  18. Rich Grise

    Rich Grise Guest

    A gyro shouldn't be affected by metal at all - it doesn't rely on any
    magnetic field, just the gyroscopic effect. Of course, then you'd have
    to account for drift and precession - the gyro will point the same way
    relative to the Universe, even as Earth rotates on its axis and revolves
    around the Sun.

    But they seem to have figured out how to correct for that on, say,
    airliners. :)

    Good Luck!
    Rich
     
  19. dalai lamah

    dalai lamah Guest

    Un bel giorno Marco Trapanese digitò:
    Thermostat the sensor; most of the offset and sensitivity error of MEMS
    sensors is due to temperature drift.
    You won't probably get down to 2° per hour though; we are talking about
    fiber optic gyro performances here, and if it was enough to thermostat a
    10$ MEMS sensor nobody would pay thousand $ for a FOG. :)
     
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