PSN-L Email List Message

Subject: Re: profound instrument differences
From: "Charles R. Patton" charles.r.patton@........
Date: Sun, 17 Feb 2008 13:03:42 -0800


There is another possibility rather than the moving pivot as you 
describe.  Keeping in mind that the basic pendulum period is due to the 
change in height of the bob during the swing that sets the period, then 
if we flatten the swing, the period will increase.  Therefore starting 
with the concept that the upper pivot, rather than the customary shape, 
a point on a flat supporting surface, is a flat rolling on a curved 
surface.  If this curved surface is such that the height of pendulum is 
constant over the swing, then the period is infinite.  Obviously a bit 
much.  It also has the problem that the surface is not round, but 
increasingly steep off the center, a recipe for slipping.  So we marry 
that with the old Rollamite bearings, to prevent side slip, and put on 
(immerse in?) lots of lubricant to prevent stiction.  Of course this 
then comes back to the current discussion about macroscopic metal 
hysteresis, but I suggest that the Rollamite bearings in this case will 
be very fine wires just sufficient to prevent side slip, not large 
springs supporting the mass of the pendulum, so the macroscopic 
properties will not intrude.


Charles R. Patton

Brett Nordgren wrote:
> Randall,
> At 08:56 AM 2/17/2008 -0500, you wrote:
>> It appears I haven't been clear (or emphatic) enough in stating the 
>> profound differences between a vertical seismometer and a
>> pendulum operating as (i) typical horizontal seismometer and/or (ii) 
>> tiltmeter.
>>     Unlike the vertical that has become standard (influenced greatly 
>> by LaCoste), an ordinary pendulum is not capable of simple
>> mechanical 'period lengthening' by means of structural 
>> rearrangement.  But what makes any pendulum superior to any vertical
>> seismometer--is its ability to look at REALLY low frequencies in a 
>> way that will ALWAYS be impossible for a vertical.  The
>> bottom line is that we need to finally understand that different 
>> frequency regimes call for different instruments!
>> There never will be a single instrument labeled the SEISMIC-DO-ALL; 
>> since the physics refuses to cooperate.
>>  Randall
> Randall,
> This morning I had an idea which might possibly be an approach to 
> improving pendulum performance by using feedback, and I think it would 
> act in a way which would meet with your approval.
> Start with a 300mm pendulum which is hung from a pivot which can be 
> moved horizontally by electronics. ('noisless' motor and leadscrew? or 
> possibly black magic might be necessary)  Measure the pendulum angle 
> relative to reference vertical (SDC sensor with static plates attached 
> to the moving pivot?).  Then apply feedback to move the pivot so as to 
> keep the measured angle as close to zero as possible.
> Clearly, there are a multitude of practical issues to solve before 
> you'd ever have something useful, but I believe that a design based on 
> such a concept could possibly have a number of properties which are 
> consistent with the performance characteristices you have set out, and 
> which, in addition, might provide significantly improved linearity.
> I've only thought about this for a few minutes, so please let me know 
> what fundamental errors you see.
> Brett
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