PSN-L Email List Message
Subject: Re: pivots vs bearing structures
From: tchannel1@............
Date: Mon, 18 Feb 2008 08:21:16 -0700
Hi Charles and Others, I have a small shop and love to build new things,
some work, some don't, but I always learn in doing.
I can not picture your idea, could you send me a sketch? I have made a
couple of the Folded Pendulums sensors and found the concept very promising.
If I can I would like to try your idea in the shop.
Ted
----- Original Message -----
From: "Charles Patton"
To:
Sent: Sunday, February 17, 2008 10:08 PM
Subject: Re: pivots vs bearing structures
> Randall,
> I understand the folded pendulums you mention, but I want to touch on
> several related subjects. Back of the napkin pendulum length for 10 secs
> is about 1000 inches. A one inch swing would be a ½ milli-inch rise.
> This gives me a bit of feel/insight on possible error mechanisms. It
> strikes me that one general problem with flexures is that they are not a
> pivot in the sense of having a known axis like a bearing does. I haven’t
> totally worked out the ramifications, but I’m sure this is the reason many
> amateurs have problems taking Lehman style instruments to long periods.
> Even if they’re not using flexures, pivot points are a round point that
> also may or may not have a constant point of rotation, depending whether
> it is rotating in a pocket or rolling on the surface of its pivot support,
> so the length may well be getting shorter as it rotates and a shorter
> length on the beam equates to the weight dropping, not rising as is
> necessary for stability and so the distance to un-stability is around ½ a
> milli-inch.
>
> So the way I perceive it, a big problem is having a system where the axis
> of rotation remains constant, quite accurately. Unfortunately the only
> solutions I keep coming back to are bearing style things. So then the
> question becomes, “Can a bearing be made that has low loss?” But a
> concurrent question is do I really need a very low amount of loss? I know
> recent discussions have experimented with crossed pivots of extremely low
> loss. Why? The immediate next step will be to add a damper to get to
> something close to critical damping. My understanding is that the only
> reason to have low loss is to be able to use lots of feedback to lengthen
> the period. But if the period can be achieved directly, and it includes
> some damping, so what? In my mind, the important item is
> hysteresis/stiction. As bearings and bearing surfaces can easily be
> ground to a ten-thousandth or even better, 10 or 20 second period
> structures should be in reach.
>
> Back to possible structures. The structure I originally presented is
> probably not possible geometrically. But one that is obviously possible
> is as follows. Imagine a hollow cylinder (like a pipe) that has been
> centerless ground to be round. Now take a high density rod like lead or
> tungsten and center it down the axis of the cylinder with fine adjustment
> screws so you can offset the center of gravity by a fraction of a
> thousandth. (The hollow cylinder construction is to reduce the rotational
> moment of inertia.) Now place this cylinder on a surface plate (again a
> commonly available object that can be obtained flat to fractions of a
> ten-thousandth.) that is level better than a ten-thousandth per inch. Use
> very fine steel (a few thousandths) wire as Rollamite bands. The cylinder
> should roll to center the mass down. So lets assume a three inch dia.
> pipe. That’s roughly 10 inches circumference, or 2.5 inches to 90
> degrees, and raising the mass by the amount of the off-center that could
> be easily set to 1 mill. Easily greater than 10 seconds rotation period?
> Once you have that structure in mind, chop off ¾ of the cylinder not in
> contact with the surface plate. As long as the center of mass is below
> the center of rotation this has become an upside down pendulum that is
> stable on the surface place and the rotational inertia has been reduced to
> a minimum. The position sensor is placed to monitor the mass at the ‘top’
> of this pendulum.
> Just some more idle musings.
> Regards,
> Charles R. Patton
>
>
> Randall Peters wrote:
>> Charles,
>> In effect, what you have described, is to take advantage of the same
>> property that is used by the folded pendulum, which
>> comprises both a `regular' pendulum and also an 'inverted pendulum.
>> Separated from each other and connected by a rigid
>> horizontal boom, their relative influence ('restoring' from the one, and
>> 'destoring' from the other) is determined by how close
>> the inertial mass is placed to one or the other.
>> Because the folded pendulum can be made to have a very long period,
>> upper valuve being limited by mesoanelastic complexity,
>> it appears clear then, that the feedback drive of the primary pendulum by
>> an inverted secondary one is capable (for ideal
>> meaterials) of very long period indeed, and therefore very great
>> sensitivity. Moreover, since the adverse effects of material
>> problems can be essentially eliminated by means of the feedback, I see
>> this as a really attractive idea to try and demonstrate!
>> Are there any takers? (meaning folks like Brett who know how to make
>> control systems work right).
>> Randall
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