PSN-L Email List Message

Subject: Re: tin cries - dithering
From: Brett Nordgren brett3nt@.............
Date: Tue, 16 Aug 2011 15:37:39 -0400


I would define the process of perfecting these=20
instruments as an exercise in patience.  Nothing happens fast.

It would be most interesting if someone wanted to=20
try your experiment.  However any 'pops' large=20
enough for piezo detection, we should surely be=20
seeing as steps in the feedback force and the=20
output signal.  These instruments are incredibly=20
sensitive in their ability to observe minute=20
changes in the spring force.  There are=20
definitely some creep-related pops when a spring=20
is first installed whose rate gradually reduces=20
to essentially zero, over a period of weeks or=20
months.  Or they go away after a few hours of baking.

I am assuming what Randall is talking about is=20
something else, more related to spring=20
motion.  Until I can see some data I'd prefer to=20
spend our time solving problems whose effects we=20
are generally able to observe.  Then, once I=20
understand the magnitude and nature of this=20
effect, we can consider what  we might want to do=20
to improve our instruments' performance.  The=20
problem is that, so far as we are aware, we have=20
yet to see any clear evidence of spring granularity.


At 01:59 PM 8/16/2011, you wrote:
>An open musing about the problem of measuring=20
>the spring creep and in particular Brett=92s=20
>thought that it would take days.I=92m an impatient=20
>man and was trying to think how one might speed up this process.
>How about bonding a piece of Kynar piezoelectric=20
>film to a spring candidate and use a lot of high=20
>frequency gain on it?The film is available in=20
>sub-thousandths thicknesses and frequency=20
>response is in the MHz range.My conjecture is=20
>that the micro-slips of the grain structure will=20
>show up as very high frequency sound pulses in=20
>the spring that this film will pick up =AD the=20
>sonic equivalent of the Barkhausen steps of=20
>magnetic domain steps in a ferromagnetic=20
>substance undergoing changing magnetization.So=20
>here we would be sensing the sound of the slips=20
>directly and this would certainly speed up the=20
>process as the necessity of all the other=20
>apparatus of a force balance seismograph would=20
>not be necessary =AD just a simple jiq to place=20
>strain on the spring candidate and a counter to=20
>measure the noise pulses.Low count =AD good=20
>candidate, high count =AD bad candidate.The=20
>seismometer measuring method is really=20
>integrating the stored energy change of a grain=20
>slip thereby lowering the pulse height.So it=20
>occurs to me that the piezoelectric S/N could be=20
>quite good and by hi-passing the film output, we=20
>side step the spurious outputs from room vibration and temperature change.
>My question for Randall is, are you aware of=20
>anybody doing this or is it not feasible for some other reason?
>Charles R. Patton


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