Maker Pro
Maker Pro

Decent Gyro chips?

D

Doc

Jan 1, 1970
0
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
 
R

Roger Hamlett

Jan 1, 1970
0
Doc said:
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
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
 
T

Tim Wescott

Jan 1, 1970
0
Doc said:
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
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
 
D

Doc

Jan 1, 1970
0
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.
 
N

Nico Coesel

Jan 1, 1970
0
dalai lamah said:
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

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.
 
R

Rich Grise

Jan 1, 1970
0
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.

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
 
J

John Woodgate

Jan 1, 1970
0
dated Wed said:
Is that where the drift comes from?

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.
 
D

dalai lamah

Jan 1, 1970
0
Un bel giorno Nico Coesel digitò:
You are probably referring to the products of this company:
www.xsens.com

Yep, of course, Leane is just the italian distributor; I had their link
handy and used it. :)
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.

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. ;-)
 
P

Phil Hobbs

Jan 1, 1970
0
John said:
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.

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
 
P

Phil Hobbs

Jan 1, 1970
0
Luhan said:
Nice! Any idea what one of those puppies cost?

Luhan

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
 
S

Sjouke Burry

Jan 1, 1970
0
Phil said:
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
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.
 
R

Richard Henry

Jan 1, 1970
0
Sjouke said:
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.

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.
 
R

Rich Grise

Jan 1, 1970
0
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.

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
 
T

Tim Wescott

Jan 1, 1970
0
Doc wrote:

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.

Artificial horizons have to sense the down vector, which _is_ an
acceleration, or they don't stay accurate for very long.
This has nothing to do with Einstein.

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).
I don't need any major precision here...probably +-2 degs.

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

Tim Wescott

Jan 1, 1970
0
Rich said:
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?
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
 
D

Doc

Jan 1, 1970
0
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
 
Top