PSN-L Email List Message
Subject: Re: pivots vs bearing structures
Date: Mon, 18 Feb 2008 17:20:29 -0700
Charles, Yes the .jpg helps... Please can you now explain how a pendulum
is attached, or to which part it is attached?
----- Original Message -----
From: "Charles R. Patton"
Sent: Monday, February 18, 2008 10:22 AM
Subject: Re: pivots vs bearing structures
> Hi Ted,
> It's about 350 KB so you can download it at your leisure.
> The "Rollamite" like wires primarily keep the orientation of the cylinder
> under control. They are also likely to make the cylinder less likely to
> hang or stick due to dust and lint ( the relatively high pressure of the
> wires will cut through many of the contaminants. I recommend non-magnetic
> parts, lead, brass, aluminum so that the changing magnetic field of the
> earth is not a factor. (It might not be anyway, but I believe in trying
> to head off some variables from the start.)
> Hope this makes the idea a bit clearer.
> Charles Patton
> tchannel1@............ wrote:
>> 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
>> If I can I would like to try your idea in the shop.
>> ----- Original Message ----- From: "Charles Patton"
>> Sent: Sunday, February 17, 2008 10:08 PM
>> Subject: Re: pivots vs bearing structures
>>> 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.
>>> Charles R. Patton
>>> Randall Peters wrote:
>>>> 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).
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