PSN-L Email List Message

Subject: Re: Article on gravimeters
From: George Harris gjharris@.............
Date: Fri, 12 May 2000 09:36:28 -0400

ChrisAtUpw@....... wrote:

> Dear Mr.Harris,
>     Thank you for a very interesting letter.
> >  First, as some may know, inertial navigation depends on the use of very
> > precise gyros to stabilise a platform about all three axes.
>     Were they using all air bearings and quartz controlled synchronous
> motors, please?

You would not believe what the gyros looked like!  Each one was a cylinder about 5
inches in diameter and
8 inches long.  The rotor was floated in a can in a dense fluid.  The pivots were
small jewel bearings.

>     Was the angle sensing LVDT, LCDT, optical fringe or what?

The acceleration sensing and precession torquing was done with an item called a
microsyn.  It is a small rotary magnetic assembly similar to the linear ones in

> > Mounted on the platform are two very precise calibrated accelerometers
> >  which measure the two components of the horizontal acceleration.
>     What principle was used for measuring accelerations and were they low
> range absolute units, rather than 'AC' / piezo etc. types, please?
> > The outputs of these accelerometers is doubly
> >  integrated, and used to correct the vertical orientation of the platform by
> >  precessing the gyros.  Back in the 50's, such a platform could be
> stabilised
> > to something like a minute per hour or less.

>     It sounds impressive. Do you know if there are any inertial platforms
> going cheap on the used equipment market? Getting one might be a good start
> to developing a gradiometer!

It is my understanding that the airlines use inertial sensors now.  It should be a
good place to start.  I wrote
the first major military proposal for an inertial system for GM, and I believe
that they made the systems for
commercial use later.

>     Regards,
>     Chris Chapman
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Larry Cochrane <cochrane@..............>