PSN-L Email List Message

Subject: Re: pivots vs bearing structures
From: tchannel1@............
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,
> See:
> 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.
> Regards,
> 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 
>> 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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