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Decent Gyro chips?

Discussion in 'Electronic Design' started by Doc, Aug 16, 2006.

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  1. Doc

    Doc Guest

    Anyone know of any decent gyro chips in a hand-mountable package...e.g.
    DIP, SMT DIPs. I'm trying to measure angles irrespective of
    acceleration, so "rate gyros" and inclinometers won't cut it.
    Thanks
    -Doc
     
  2. I don't think what you want exists.
    To get a 'static' angular displacement readout, really requires at heart,
    a 'real' gyroscope. The IC's, all use devices like crystals (or processes
    like laser interferometry for some 'up market' devices), to give angular
    rate detection. You can integrate the output from an angular rate sensor
    over time, to give angular displacement (some chips even do this for you).
    You make no mention of the sort of timescales/accuracies involved, but
    small gyroscopes, or the integrated output from a rate sensor, _will_
    drift significantly over time. About the smallest 'true' gyroscope I know
    of, is the Northrop G-2000 DTG. If your accuracies can be achieved by time
    integrating the output from a rate gyro, this will be the cheapest, and
    lowest power solution...

    Best Wishes
     
  3. Tim Wescott

    Tim Wescott Guest

    Huh?

    You appear to want an absolute angle sensor -- but absolute angles don't
    exist, at least not in a universe governed by Einstein's Relativity.

    Any gyro will drift. A 'position' gyro will read out an offset from
    it's frame, but it'll still drift. A rate gyro will read out a rate
    that you have to integrate, giving you more opportunities to decrease
    the drift performance, but all else being equal a rate gyro can be as
    good as a position gyro if you support it with the right signal
    processing electronics.

    For that matter, rate gyros or any other gyros are built to be
    insensitive to acceleration, although they always are sensitive to it to
    some extent.

    If you're going to succeed with applying a gyro you have to expect it to
    drift a bit, and have some scaling errors. You have to determine just
    how much drift and other errors you can stand, then see if you can find
    a gyro that fits your application.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  4. Doc

    Doc Guest

    What I'd like to do is spin a gyro up with one degree of freedom
    perpendicular to it's spin axis and measure the angular change in the
    gimbal. Much like the artificial horizons that have been in airplanes
    for the last 70 years. This has nothing to do with Einstein. I don't
    need any major precision here...probably +-2 degs.

    As far as "rate gyros", the only reason they work is based on
    acceleration. Tim, "rate gyros" are quite different than actual
    gyroscopes.
     
  5. dalai lamah

    dalai lamah Guest

    Un bel giorno Doc digitò:
    You need a full-blown inertial platform, i.e. three accelerometers, three
    giroscopes and usually three magnetic sensors. This is not bad:

    http://www.leane.it/categorie/195/1479/files/MTi_leaflet-l.pdf
     
  6. Nico Coesel

    Nico Coesel Guest

    You are probably referring to the products of this company:
    www.xsens.com. I have been using these devices for several projects at
    work, but I must say they are not very suitable for use on a moving
    vehicle. When the vehicle accellerates, the sensor wil detect a
    (false) tilt.
     
  7. Rich Grise

    Rich Grise Guest

    I've just done a little Gedankenexperiment here. When I was a kid, the
    family had a toy gyroscope which was, of course, a real gyroscope, just
    cheap, and I'm wondering "what makes it drift?" Well, there's friction,
    that could introduce some torque into the gimbals, but mostly, I was
    visualizing starting a friction-free gyro on a gimbal, hold it in my
    hands, and start walking. When I've walked from, say, 33 degrees north to
    53 degrees north, the gyro would have "drifted" twenty degrees, no? And
    then there's Earth's rotation - if I'm standing at the equator and spin up
    a gyro with its axis vertical, six hours later the axis would be
    horizontal, right?

    Is that where the drift comes from?

    Thanks,
    Rich
     
  8. It's the irreducible drift that's left after you've eliminated all the
    controllable influences. Of course, it's then the Earth that is
    drifting; the gyro stays put. See Foucault's pendulum.
     
  9. Luhan

    Luhan Guest

    Nice! Any idea what one of those puppies cost?

    Luhan
     
  10. dalai lamah

    dalai lamah Guest

    Un bel giorno Nico Coesel digitò:
    Yep, of course, Leane is just the italian distributor; I had their link
    handy and used it. :)
    Are you sure it was false? It is normal that a vehicle (also a sports
    vehicle, even if it is much more rigid) varies its pitch angle of some
    degrees.

    One evident advantage of these sensors is that they don't exhibit any angle
    drift, probably thanks to the magnetic sensors. One disadvantage that we
    noticed is that the correction algorithms aren't very good when the device
    is in motion and then suddenly stops in a static position; it takes tenths
    of seconds (and more!) before the angles stabilize. I suppose it is related
    to the problem you noticed during the accelerations, probably they use some
    kind of predictive algorithm (kalman filters, etc...). Maybe there is some
    way to tweak the algorithm parameters to adapt it to different applications
    (static, quasi-static, motion under 1g etc, just like GPS); I don't know
    because we haven't studied these sensors very thoroughly, it was just part
    of a feasibility study and the answer of the study - NO! - stopped any
    further investigation. ;-)
     
  11. dalai lamah

    dalai lamah Guest

  12. Phil Hobbs

    Phil Hobbs Guest

    Foucault's pendulum precesses because the earth's rotation exerts a
    torque on the pendulum by moving its mounting point. A Foucault
    pendulum has to follow the earth, but a gimballed gyro doesn't, so it
    doesn't exhibit the precession.

    Cheers,

    Phil Hobbs
     
  13. Phil Hobbs

    Phil Hobbs Guest

    That's a cute gizmo--it nulls out the drift by using magnetic north and
    the local vertical as long-term references. Should work anyplace except
    the magnetic poles, where one rotation will become indeterminate because
    the two directions will be collinear.

    Of course, if you put it on a steel object, all bets are off.

    Cheers,

    Phil Hobbs
     
  14. Luhan

    Luhan Guest

  15. Sjouke Burry

    Sjouke Burry Guest

    Oh yes it does. I modified a big giro from an airplane,
    an on balancing(any imbalance causes it to drift) ,
    i could not stop it from moving.
    Until i found out i was measuring earth rotation.
    To my surprise it was quite accurate !!
    360 degr/day, 15 degr/hour,15arcmin/minute.
     
  16. As I recall from my US Navy inertial nav training, a properly balanced
    gyro-stabilized platform has a natural period of oscillation of about
    84 minutes, the same time as the theoretical orbital period of a
    satellite orbitting at teh surface of the earth.
     
  17. Rich Grise

    Rich Grise Guest

    Just a nitpick, but I don't think that "maintaining its position while the
    Earth rotates under it" counts as "precession." :)

    Of course, I'm probably wrong - I usually am, until somebody enlightens me.
    :)

    Thanks,
    Rich
     
  18. Tim Wescott

    Tim Wescott Guest

    Doc wrote:

    Artificial horizons have to sense the down vector, which _is_ an
    acceleration, or they don't stay accurate for very long.
    No, just with how the universe works. In your original post you said
    "angle" without the qualifier "change". This implied some magical
    absolute angle (which doesn't exist -- see the theory of General
    Relativity) or the angle relative to the Earth (which requires you to
    sense acceleration).
    Well, that's good. But you're leaving out the other part -- 2 degrees
    over what period of time? As I already stated, _all_ gyros drift. The
    less you want them to drift, the more space you have to find for them,
    and the more money you have to pay. But even if you build one as big as
    your house, and spend 10^6 bucks on it, it'll still drift -- just not
    much if you're getting your money's worth.
    A "rate gyro" is any gyro that reads out in rate. The good ones have
    spinning wheels inside of them.

    How is an assembly with bearings, a motor, a spinning wheel and some
    electronics different from an "actual" gyroscope? If an actual
    gyroscope doesn't have a spinning wheel, what does it have?

    Please educate me. I've been using rate gyros (with spinning wheels) in
    aerospace applications for going on 10 years now -- obviously I need my
    ignorance corrected. Perhaps you could call up all the major defense
    suppliers and systems integrators in the world and straighten them out,
    too? I'm sure it'll be taken as a public service.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  19. Tim Wescott

    Tim Wescott Guest

    No, there are other sources of drift. All too many, actually.

    A spinning body precesses because when it experiences a torque on it
    that's perpendicular to its axis of spin. It precesses in a direction
    at right angles to both the torque and its axis of spin.

    Gyros drift because of unwanted torques. That's it. That's all.
    Unfortunately there are gazillions of different ways that the gyro can
    get torqued. The ways that I know of are:

    * The gimbal can be out of balance. This is why that toy gyro precesses
    when you hang one end from a string. In fact, a gyro compass uses a
    gimbal with a specific imbalance to align itself with the spin axis of
    the Earth.

    * The gimbal bearings may not be free enough, so that when the frame
    rotates it torques the gyro ever so slightly.

    * An out-of-balance wheel coupled with a flexible frame can do it.

    * Bad bearings.

    * The motor can exert torque that's not parallel to the shaft (and you
    have to keep it spinning!)

    This is a short list, but I don't grow gyros, I just smoke them.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  20. Doc

    Doc Guest

    First of all, you come off as a pretty confrontational person who seems
    to want some kind of argument to ensue. I find it hard to believe your
    professional life has consisted of anything more than condescension,
    irritating people, and patting yourself on the back.

    Secondly, I thought it was clear that were talking about ICs here. A
    rate gyro chip will not sense position, only accelerations. I'm sure
    you know what the difference is between the two. I was under the
    impression that chips were available now that could sense a change in
    angle relative to an inital angle purely by POSITION even if only for a
    couple of seconds. I have 3 systron-donner BEI GyroChips sitting on my
    desk here (about $2000 each) which are extensively used in airplanes
    and defense applications. Though they are called GyroChips, they are
    nothing more than angular RATE sensors. If you know of a chip I'm
    describing, please let me know. Otherwise, please apply your extensive
    "knowledge" elsewhere. Thanks for sharing.
    -Doc
     
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