PSN-L Email List Message

Subject: Re: pivots vs bearing structures
From: "Charles R. Patton" charles.r.patton@........
Date: Mon, 18 Feb 2008 09:22:59 -0800

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 
> 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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