PSN-L Email List Message
Subject: Re: Simple pendulum response
From: Larry Cochrane lcochrane@..............
Date: Wed, 06 Dec 2006 18:39:17 -0800
Hi Pete,
You are correct, it should have read "velocity
sensor will always outperform a position sensor"
Regards,
-Larry
Pete Rowe wrote:
> Hi Larry
> thanks for forwarding this good paper. There is a
> confusing typo in the 9th paragraph. Here is the
> sentence:
> For frequencies above the
>> natural frequency of the pendulum, a velocity sensor
>> will always outperform a
>> velocity sensor.
>
> I believe it should say "a velocity sensor will always
> outperform a position sensor"
>
> Pete
>
> --- Larry Cochrane wrote:
>
>> Hi Everyone,
>>
>> Dr. Randall Peters asked me to forward the following
>> message to the list.
>>
>> Regards,
>> Larry Cochrane
>> Redwood City, PSN
>>
>> I've been following with interest the discussions
>> concerning instrument
>> characteristics. Now that my schedule is easing
>> somewhat, I felt that I should get
>> involved. Should it happen that any of you respond
>> to these comments and don't hear
>> back from me for a while, it's because I will be
>> away for about a week to the Amer.
>> Geophys. Union Fall Conference in San Francisco
>> (starting 11 Dec.). There I will give
>> a 15 minute oral presentation titled "State of the
>> art Digital Seismograph" . The
>> abstract is posted at
>>
> http://www.agu.org/cgi-bin/sessions5?meeting=fm06&part=S14B&maxhits=400
>>
>> The instrument which will be described (and also
>> demonstrated at one of the booths)
>> uses a "simple" compound pendulum with a natural
>> frequency of 0.92 Hz. It employs my
>> fully differential capacitive detector as a
>> displacement sensor (array form), with
>> electronics based in Analog Devices' new award
>> winning capacitance to digital
>> converter integrated circuit (AD7745). Kudo's to
>> our own Larry Cochrane as the
>> brains behind all of (i) the electronics hardware
>> necessary to do the I2C logic
>> operations required of the chip, and (ii) the
>> software operating system in the form
>> of WinSDR and WinQuake.
>>
>> For those of you who have been monitoring
>> Larry’s instruments at
>> http://seismicnet.com/quakes/images
>> you may have noticed two real-time helicord
>> records generated by the
>> single-pendulum instrument (N-S orientation) that he
>> placed online. The
>> raw-data-train is lctst.gif, which has been
>> high-pass filtered (corner frequency of
>> 10 mHz) before display. The unfiltered waveform is
>> available via download upon
>> request from Larry. This lctst is best suited to
>> the real-time display of
>> earthquakes local to the Redwood City, CA site.
>>
>> For registering teleseismic earthquakes
>> real-time, Larry has also provided
>> lctst1.gif, which is the numerical integration of
>> lctst after first doing a high-pass
>> filter. This operation on the VolksMeter’s output
>> provides a display similar to what
>> is provided by ‘bandwidth extension’ using
>> electronic means in other instruments such
>> as geophones.
>>
>> I was pleased to see John Lahr provide links
>> on his webpage describing (i)
>> transfer function differences between velocity and
>> position sensing, and (ii)
>> discussion of the zero-length spring that was
>> invented by physicist Lucien LaCoste in
>> the early part of last century.
>>
>> There are some things that need seriously to
>> be clarified concerning theory of
>> seismometers, since there is so much confusion; not
>> only among amateur seismologists,
>> but also even many professional geoscientists.
>> Ultimately, the ONLY source of
>> seismograph excitation (no matter the instrument
>> design) is ENERGY. Additionally,
>> the ONLY thing that delivers energy to the
>> seismometer is Earth’s ACCELERATION at the
>> site of the instrument. This is true not only for
>> the instrument’s response to
>> earthquake waves whose periods are shorter than
>> about 300 s, but also for earth ‘hum’
>> in which the instrument responds mainly to tilt,
>> when the periods are greater than
>> about 300 to 1000 s.
>>
>> Keep in mind that it is very difficult to see a 300
>> to 1000 s periodic signal with a
>> velocity sensor. It is equivalent to trying to look
>> at a very low frequency signal
>> with an oscilloscope using a.c. coupling. Only d.c.
>> coupling (position sensing) is
>> appropriate in this case.
>>
>> There is a dramatic difference between the
>> forcing functions of tilt as
>> contrasted with horizontal ground acceleration. The
>> tilt response is independent of
>> frequency, whereas the response to earthquakes
>> (horizontal acceleration devoid of
>> significant eigenmode oscillatory components) is the
>> classic response given by John
>> Lahr at the following website:
>> http://jclahr.com/science/psn/response/index.html
>>
>> If you look at John’s six transfer function
>> plots provided at
>> http://jclahr.com/science/psn/response/plots.jpg
>> it is the right-most pair (response to acceleration)
>> that ‘summarize the physics’ of
>> how a seismometer operates. Yes, one can configure
>> an instrument to plot data
>> according to any one of the six possibilities John
>> has indicated, but the response to
>> acceleration is what ‘tells the story’ of
>> performance. For frequencies above the
>> natural frequency of the pendulum, a velocity sensor
>> will always outperform a
>> velocity sensor. On the other hand, for frequencies
>> below the natural frequency, a
>> position sensor will always outperform a velocity
>> sensor (all things otherwise
>> identical).
>>
>> I don’t know about you, but I’m not
>> particularly interested in frequencies
>> above 1 Hz. Our Volksmeter easily picks up dynamite
>> blasts and other local
>> disturbances that are nearly always manmade.
>> Because the earth is so large, motions
>> it exhibits in response to dynamic changes
>> (earthquakes, tidal forces, ….) are at low
>> frequencies (not high).
>>
>> At low frequencies where everybody seems
>> increasingly interested in going
>> (reason for bandwidth extension) there is no
>> question of the superiority of position
>> sensing over velocity sensing. Why this obvious
>> fact is so muddled in the minds of
>> so many is a great mystery to me. Maybe it’s
>> because even classical physics is
>> difficult for most everybody to understand.
>>
>> I have placed a paper on my webpage which
>> speaks to this matter, titled
>> ‘Seismometer design based on a simple theory of
>> instrument-generated noise equivalent
>> power:
>> http://physics.mercer.edu/hpage/inep/inep.html
>>
>> For those of you who want to ‘escape the rut’
>> of velocity detection that has
>> held folks captive for way too long—Larry and my
>> other business partner, Les LaZar
>> are positioned to provide you with reasonably-priced
>> essential components to build
>> your own version of the VolksMeter. Probably most
>> of you will prefer to do this
>> rather than pay the present $1000 ‘turnkey’ price
>> for our single-pendulum instrument.
>>
>> I want to point out something that is the
>> result of recently discovered
>> physics—why small-mass instruments don’t perform
>> well. Although conventional wisdom
>> says that it’s because of Brownian motion (larger
>> for smaller masses), this is not
>> really the culprit. The performance limitation is
>> really the result of internal
>> friction problems that science is only beginning to
>> understand. The smaller the
>> seismic mass, the smaller the spring that supports
>> it. The smaller the spring, the
>> more significant is the internal friction associated
>> with the ‘snap, crackle, pop’ of
>> defect structural changes in the spring (processes
>> that operate at the mesoscale).
>> For decades we’ve recognized the all-important
>> properties of defects in
>> semiconductors (basis for p and n material of which
>> devices are made), but until
>> recently very little was understood concerning the
>> importance of defects to internal
>> friction that regulates the low-frequency
>> performance of seismometers.
>>
>> The influence of defects is worse in
>> instruments with springs than in those
>> that use a pendulum, which is more inherently
>> stable. Until better electronics came
>> along, we were stuck with trying to improve
>> low-frequency performance by going to
>> lower natural frequencies of the mechanical
>> oscillator. That is no longer the only
>> viable solution. Although the pendulum lost favor
>> years
> === message truncated ===
>
>
>
>
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