Decent Gyro chips?

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

Reply to
Doc
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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

Reply to
Roger Hamlett

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.

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Tim Wescott
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Reply to
Tim Wescott

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.

Tim Wescott wrote:

Reply to
Doc

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:

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asd
Reply to
dalai lamah

You are probably referring to the products of this company:

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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.

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Reply to
Nico Coesel

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

Reply to
Rich Grise

In message , dated Wed, 16 Aug 2006, Rich Grise writes

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.

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Reply to
John Woodgate

Nice! Any idea what one of those puppies cost?

Luhan

Reply to
Luhan

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. ;-)

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asd
Reply to
dalai lamah

Un bel giorno Luhan digitò:

Here:

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asd
Reply to
dalai lamah

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

Reply to
Phil Hobbs

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

Reply to
Phil Hobbs

Hmmm. Unless those prices are in Pesos....

Luhan

Reply to
Luhan

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.
Reply to
Sjouke Burry

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.
Reply to
Richard Henry

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

Reply to
Rich Grise

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.

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Reply to
Tim Wescott

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.

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Tim Wescott
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Reply to
Tim Wescott

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

Tim Wescott wrote:

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Doc

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