Decent Gyro chips?

[Jim Thompson]

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

If you're so well-informed why did you bother posting ?


...Jim Thompson

 

[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.

That isn't precession, it's just the rotation of the Earth. The
distinction is that when something precesses, its axis moves with
respect to an inertial frame, due to an applied torque component
perpendicular to the rotation axis. Your gyro's rotation axis was
merely remaining stable in an inertial frame--yet another successful
test of the law of conservation of angular momentum. Precession is what
spinning tops do when they aren't supported at their centre of mass.

Take a wheel of a bicycle, and spin it while holding it up vertically by
one end of the axle. It will precess due to the torque of its weight
acting on half the length of its axle. If you hold both ends, it
doesn't precess because there's no torque applied. Torque applied along
the rotation axis also doesn't cause precession--it just changes the
rotation rate.

Cheers,

Phil Hobbs

 

[Nico Coesel]

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.

Yes. I have mounted one in a car for use as an accellerometer and
compass. If I pull up or brake, the sensor thinks it is being tilted.
These sensors are great for head and motion tracking.

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. ;-)

The sensor can be set to include more or less of the earths magnetic
field into its calculations. Xsens is also very keen on supporting
their customers. As far as I know they are also working on a version
of their software which doesn't has the false tilt problem.

 

[Nico Coesel]

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.

Not entirely true, the sensors can be compensated for magnetic
disturbances. I'm using one as a compass inside a car. The error is
less than 5%.

 

[Richard Henry]

Not entirely true, the sensors can be compensated for magnetic
disturbances. I'm using one as a compass inside a car. The error is
less than 5%.

5% of what? Bearing accuracy (18 degrees)? Time (1.2 hours per day)?
etc,etc,etc

And how do you test for it?

 

[Nico Coesel]

5% of what? Bearing accuracy (18 degrees)? Time (1.2 hours per day)?
etc,etc,etc
Bearing.

And how do you test for it?

Drive around with the car using a route on which the same straight
roads are passed in both directions.

 

[dalai lamah]

Un bel giorno Luhan digitò:

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

I'm afraid not, and neither are roubles.

However, it's not that much for a sensor of this kind, someone asked me
twice as much just for a triaxial accelerometer...

 

[Richard Henry]

Drive around with the car using a route on which the same straight
roads are passed in both directions.

How about comparing to GPS bearings (which depend on movement, I know).

 

[Phil Hobbs]

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

But it doesn't maintain its position, it precesses. The plane of the
oscillation rotates at a rate of something like 1 cycle every 36 hours
at mid-latitudes. The period of the precession (360 degree rotation of
the plane of the oscillation) is 24 hours / sin(latitude).

Cheers,

Phil Hobbs