NavList:

   

Reply

Paul Hirose's contributions are always full of interest and knowledge, and
his latest is no exception.

However, I wonder whether there's a bit of misunderstanding between us.

I had written-

| > Although inertial navigation systems always show some drift, as a result
of
| > having to integrate (twice) the accelerations, which always have some
| > zero-error, I doubt (from my inexpert perspective) whether any of that
drift
| > derives from the pseudo-gyros, which sense the orientation.

And Paul replied, in [4459]-

| Actually, I've read that gyros have always been the limiting
| factor in INS performance. The B-1 navigator's manual says, "The
| navigation accuracy of the INS inertial platform is most dependent on
| gyro performance. Accelerometer accuracy is less critical unless major
| malfunctions occur."

But I wonder whether Paul and I are discussing the same type of gyro, here.
My words referred to that modern development, which I christened the
"pseudo-gyro", without spinning parts, and relying on the interactions of
two counter-rotating light-beams. I suspect, though I can't be sure, that
the words Paul quoted from the manual were from the days of the real-gyro,
with spinning rotor, and I would readily agree that in that situation, an
important source of drift would arise in the orientation-sensing of those
gyros. Perhaps Paul will confirm whether or not I have got that right.

Frank Reed mentioned that "RLGs in real navigation systems are equipped with
little vibrating devices to keep them lively. These tend to prevent the
counter-rotating beams from locking into the same state."

Indeed, a "dithering" mechanism is required to avoid the tendency of the two
light-beams to lock into phase together at very low rotation rates, and that
dither has to be averaged out or allowed for, and provides a possible source
of some short-term error. Nevertheless, isn't it the case that there is
effectively zero long-term drift in the orientation sensing of these
devices; quite different from the behaviour of their mechanical
predecessors?

==========================

Paul added the following interesting information, some of which I had read
about elsewhere-

| The B-52 SPN/GEANS INS had the most interesting gyro I've encountered.
| The rotor was a hollow beryllium sphere roughly the size of a golf ball.
| It rotated at about 650 revolutions per second, suspended by
| electrostatic force in a spherical vacuum chamber. The gap between
| sphere and wall was on the order of .002 inch, as I recall. Spin-up
| torque was applied only at startup. After that the ball coasted in
| vacuum with almost zero friction. To power down, the system braked the
| sphere, then shut off the suspension voltage (the electrodes were on the
| inner wall of the chamber). A power interruption would crash the sphere,
| so a "rotor support battery" ensured backup power long enough for a
| proper shutdown.

| Since there was no contact between the rotor and its envelope, an
| optical sensor observed a mark on the ball's "north pole" to keep the
| INS platform aligned with the spin axis.
|
| A couple of these spheres are depicted here:
| http://periodictable.com/Elements/004/index.html

With a spacing of only .002 inch, and the ball held there in place against
its weight entirely by electrostatic forces, I wonder how "stiff" that
positioning could be, and how well it could resist any sudden accelerations
caused by vibration, or by buffeting as the aircraft flew through
turbulence. I bet the assembly had to be carefully cosseted within
spring-mounting and padding.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply