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'[OT] Inertial Guidance (was: PIC-based angle senso'
1997\12\05@133944 by Jeff Cesnik

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So I can get away with two accelerometers measuring X/Y acceleration in
the horizontal plane, and a single axis gyro measuring yaw to track
position over a time period, given an accurate temperature reference and
correcting for gyroscopic precession?  Would this redundancy avoid
double integration problems?  What kind of accuracy can I expect?  Where
can I find the integration formulas I need?

I should have been a physics major...

- Jeff Cesnik


Sean Breheny wrote:

{Quote hidden}

<snip!>

{Quote hidden}

1997\12\05@150519 by Sean Breheny

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At 01:31 PM 12/5/97 -0500, you wrote:
>So I can get away with two accelerometers measuring X/Y acceleration in
>the horizontal plane, and a single axis gyro measuring yaw to track
>position over a time period, given an accurate temperature reference and
>correcting for gyroscopic precession?  Would this redundancy avoid
>double integration problems?  What kind of accuracy can I expect?  Where
>can I find the integration formulas I need?
>
>I should have been a physics major...
>
>- Jeff Cesnik
>

Hi Jeff,

I wasn't following this thread from the beginning, could you say a bit more
about your application? Do you need to orient an object in the X-Y plane as
well as its angular orientation?

I guess I rambled a bit too much in my last message, because I am not
really recomending using gyros, unless your device is expected to be able
to handle frequent accelerations.

If you simply want to measure an angle on an otherwise fixed object, I
would use a plumb bob system, i.e. a weight attached to an optical encoder
by a short arm. The weight would always point toward the center of the
earth and you could just calibrate the device once and thereafter, get
excelent accuracy without having to worry about integration.

If your device is going to accelerate sharply on a frequent basis and needs
to be able to measure angular orientation even during periods of
acceleration, then the plumb bob method won't work because the bob will be
shifted by any acceleration. If the device is fixed, moving at a constant
speed, or if you don't care about accuracy during acceleration periods, the
plumb bob is definately the simplest and best solution (as far as I can see).

If you are puting this in an aircraft or missile :) , then a gyro would be
a more appropriate choice. A perfect gyro (one which is perfectly balanced
and has no friction in the bearings) keeps its spin axis always pointed in
the same direction in space, regardless of the orientation of the vehicle
it is in. Because of friction and slight imbalances, though, the gyro will
need to be re-calibrated at intervals because it will slowly align its spin
axis with the way the vehicle is rotating due to friction in the bearings
allowing the bearings to apply a torque to the gyro wheel. For example,
gyroscopic heading indicators in small aircraft (used to compensate for
local magnetic anomalies' effect upon the magnetic compass) cost about $600
(I think) and generally will give a usable reading (less than 1 deg error)
for about a half hour (If I remember correctly) between settings. So, since
you probably don't want to spend $600 on your device, you should expect to
have to reset your gyro at least this often. Very good gyros are used on
missiles (there are even magnetically or air suspended gyros which have
VERY little friction) and this is why they are able to work so well.

The gyros sold for model aircraft I believe are used to help fix an axis
for certain aerobatic manuvers, and therefore do not need to be extremely
accurate for very long periods of time (most RC flights last only about
10-15 mins before refueling).

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
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1997\12\05@151752 by Rob

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On Fri, 5 Dec 1997, Sean Breheny wrote:
>
> have to reset your gyro at least this often. Very good gyros are used on
> missiles (there are even magnetically or air suspended gyros which have
> VERY little friction) and this is why they are able to work so well.

The quality of a mechanical (tuned rotor) gyro is determined by the
spinning mass and the rotation points as well as the sealed enclosure. A
near perfectly balanced gyro mass is the key.  Mass unbalances can cause
errors, and long term drift can result from the pre3cession errors.

Newer technolgies such as ring laser gyros and so forth have their place
in the gyro world, but the best for long term accuracy are form the tuned
gyro type.

Rob

{Quote hidden}

1997\12\05@152825 by Sean Breheny

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At 03:16 PM 12/5/97 -0500, you wrote:
>On Fri, 5 Dec 1997, Sean Breheny wrote:
>>
>> have to reset your gyro at least this often. Very good gyros are used on
>> missiles (there are even magnetically or air suspended gyros which have
>> VERY little friction) and this is why they are able to work so well.
>
>The quality of a mechanical (tuned rotor) gyro is determined by the
>spinning mass and the rotation points as well as the sealed enclosure. A
>near perfectly balanced gyro mass is the key.  Mass unbalances can cause
>errors, and long term drift can result from the pre3cession errors.
>
>Newer technolgies such as ring laser gyros and so forth have their place
>in the gyro world, but the best for long term accuracy are form the tuned
>gyro type.
>
>Rob
>

I must admit to not being familiar with the principles of operation of
non-tuned rotor gyros, but I do remember that some R/C gyros a called piezo
gyros. I'm not sure what if anything the piezoelectric effect has to do
with gyros. I also don't know how good the accuracy of the piezo gyros is
compared to traditional ones.

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
Web Page Under Construction!
.....shb7KILLspamspam.....cornell.edu

1997\12\05@182245 by Robert Nansel
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On Fri Dec 5 Sean Breheny wrote:

>I must admit to not being familiar with the principles of operation of
>non-tuned rotor gyros, but I do remember that some R/C gyros a called piezo
>gyros. I'm not sure what if anything the piezoelectric effect has to do
>with gyros. I also don't know how good the accuracy of the piezo gyros is
>compared to traditional ones.
>
>Sean

Piezo gyros work on the principle of an oscillating beam instead of a
spinning mass. If you set a cantilevered a beam with a mass on the end
oscillating it will tend to want to keep oscillating in the same plane.
Rotation outside of that plane are detectable by measuring the strain
transverse to the oscillation plane. Hence piezos are inherently rate
sensing gyros, not  orientation sensing.

--BN


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1997\12\06@112548 by Andy Kunz

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>transverse to the oscillation plane. Hence piezos are inherently rate
>sensing gyros, not  orientation sensing.

They are widely used in the R/C industry because they are sufficiently
accurate during the flight time (10-20 min typical), they are small, and
because they are very inexpensive.  In fact, some of the more popular gyros
(Arcamax, for instance) use gyros developed for camera stabilization.

Yes, they are rate gyros.  If you want to get a position, it is necessary
to integrate the error outputs (that is, you sum the error value
continuously (each sample)).  To get a heading-hold output, simply apply
servo signals such that you end up with a 0 for the sum.

There are other folks on this list with whom I've corresponded privately
who have made gyros for model helis.  Maybe changing the title would help
generate feedback from them.

More information on what you're trying to do would be helpful, too.

Andy

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         Hardware & Software for Industry & R/C Hobbies
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