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 |
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