On Tue, 17 Dec 2002, Stewart Cobb wrote:
> ...Rate noise density (max spec) is quoted
> as 0.05 degrees/sec/sqrt(Hz). In other words, in a 100 Hz
> bandwidth (reasonable for flight control on small objects)
> you'll see 0.5 degrees/sec of random rate error. You can
> reduce that by filtering the bandwidth down to, say, 1Hz,
> but you can't use a slow signal for flight control.
Mind you, it's quite routine in big rockets for the *control* gyros and
the *guidance* gyros to be different hardware. The high-bandwidth control
gyros are used to keep the thing flying in the desired direction, and the
low-bandwidth guidance gyros are used to decide just which direction that
is. For example, the shuttle has rate gyros separate from its IMU.
You'd still need some external drift-compensation sensor for substantial
periods of powered thrust, though.
> ...Under "temperature calibration"
> it says "overall accuracy of 70 degrees an hour or better
> is possible" using an elaborate temperature calibration
> algorithm. The chip provides a temperature output to assist
> calibration...
Alternatively, one could perhaps do what's done for high-precision timing
crystals: keep them at a constant temperature, by putting them in an
insulated enclosure with a temperature-controlled electric heater. Not
actually all that hard to do, although the power requirement is annoying.
> Presumably the acceleration constants can be calibrated
> per unit, in a centrifuge.
Centrifuge calibration of *gyros* may be a bit tricky! Some careful
thought, and perhaps a query to AD, would be needed; it might be possible
to do it on some axes but not others. What you really want is a way to
produce a precise acceleration *without* accompanying rotation, but that's
harder.
Henry Spencer
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