PSN-L Email List Message
Subject: Re: Simple pendulum response
From: Pete Rowe ptrowe@.........
Date: Wed, 6 Dec 2006 07:51:07 -0800 (PST)
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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