PSN-L Email List Message

Subject: Re: tin cries - dithering
From: Charles Patton charles.r.patton@........
Date: Tue, 16 Aug 2011 10:59:09 -0700

An open musing about the problem of measuring the spring creep and in 
particular Brett’s thought that it would take days.I’m an impatient man 
and was trying to think how one might speed up this process.

How about bonding a piece of Kynar piezoelectric film to a spring 
candidate and use a lot of high frequency gain on it?The film is 
available in sub-thousandths thicknesses and frequency response is in 
the MHz range.My conjecture is that the micro-slips of the grain 
structure will show up as very high frequency sound pulses in the spring 
that this film will pick up – the sonic equivalent of the Barkhausen 
steps of magnetic domain steps in a ferromagnetic substance undergoing 
changing magnetization.So here we would be sensing the sound of the 
slips directly and this would certainly speed up the process as the 
necessity of all the other apparatus of a force balance seismograph 
would not be necessary – just a simple jiq to place strain on the spring 
candidate and a counter to measure the noise pulses.Low count – good 
candidate, high count – bad candidate.The seismometer measuring method 
is really integrating the stored energy change of a grain slip thereby 
lowering the pulse height.So it occurs to me that the piezoelectric S/N 
could be quite good and by hi-passing the film output, we side step the 
spurious outputs from room vibration and temperature change.

My question for Randall is, are you aware of anybody doing this or is it 
not feasible for some other reason?

Charles R. Patton

On 8/16/2011 10:10 AM, Brett Nordgren wrote:
> Hi Chris,
> Thanks for your suggestion. I understand your approach, though 
> implementing it will be a bit difficult.
> Normally, to get a decent idea of long-period noise we have to record 
> for at least a day, and preferably several. And to separate instrument 
> noise unambiguously from true ground noise we would need to record 
> three instruments and use correlation techniques to distinguish their 
> individual noise contributions.
> See:
> Sleeman, Wettum and Trampert
> "Three-Channel Correlation Analysis: A New Technique to Measure 
> Instrumental Noise of Digitizers and Seismic Sensors"
> BSSA v96 n1 p258
> As it stands, at the lowest frequencies, the noise spectrum we 
> normally see rises smoothly as 1/f and looks almost exactly like the 
> best instruments except, of course, it's not as low. It is entirely 
> possible that some of the noise we see may be true ground noise. We 
> just don't have a quiet enough location to tell without playing the 
> correlation game, which could be a pain.
> If anyone could suggest what is the magnitude of the effect on the 
> spring characteristics, I could at least model the instrument response 
> to such stepwise force changes. Someone must have studied this and 
> come up with some real numbers.
> Wouldn't the six-second microseisms be an effective dither mechanism? 
> They're usually large compared with everything else.
> Thanks,
> Brett
> At 10:01 AM 8/16/2011, you wrote:
>> Hi Brett,
>> I would suggest a purely practical approach. Add another small 
>> winding to force feed
>> back coil. Drive it from a variable AC sinewave current source at 5x 
>> to 10x the cutoff
>> frequency of your Low Pass fllter, so that you can't see any 
>> oscillatory response. Then
>> monitor how the very long period background noise detected changes, 
>> as the drive
>> current is increased ?
>> Regards,
>> Chris
>>> From: Brett Nordgren 
>>> brett3nt@.............
>>> Subject: Re: tin cries
>>> Randall,
>>> What amount of dither do you think would be adequate? I suppose it
>>> could best be expressed as variation in the spring strain, or in
>>> whatever other way you suggest.
>>> Thanks,


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