PSN-L Email List Message

Subject: capacitive sensors
From: Randall Peters PETERS_RD@..........
Date: Thu, 11 Aug 2011 09:47:35 -0400


Brett,
     Your equivalent displacement threshold resolution of 40 nm is quite im=
pressive, since you are measuring the motion of a large inertial mass.  But=
 this is probably not a limit imposed by your capacitive sensor.   In gener=
al, seismometer limitations are the result of one or the other of (i) elect=
ronics, or (ii) the 'spring'.  The latter is a consequence of the fact that=
 there are no perfect macro-scale springs; i.e., ones that obey Hooke's law=
, which we routinely teach to physics students as though they describe the =
real world.  My friend and colleague, Tom Erber, who recently retired from =
Illinois Tech University-demonstrated experimentally that only with 'spring=
 displacements' approaching atomic dimensions, could a Hooke's law approxim=
ation be possibly achieved.  In other words, by looking at incredibly small=
 motions using an atomic force microscope.  So a seismometer responds not o=
nly to changes (accelerations) of its external environment, but ALSO (which=
 so few folks really appreciate) to changes that happen in the spring itsel=
f.  These are especially troublesome when one wants to look at earth motion=
s that are "low and slow".
     A capacitive sensor with a large electric field between the plates (th=
e parameter that ultimately determines its threshold sensitivity) can measu=
re down to 0.1 nm, even at room temperature.  This was the displacement thr=
eshold of the capacitive sensor that I used as a student while at Oak Ridge=
 National Laboratory in the late 1960's.  The sensor was one that operated =
on gap spacing change of parallel plates that had been polished optically f=
lat to better than a half-wavelength of visible light, and spaced nominally=
 about 5 micro-meters (microns) apart.  Motion of one plate, relative to th=
e other, was due to 30 and 60 MHz longitudinally excited (pulse echo) ultra=
sound waves in a  single crystal.  By measuring the 2nd harmonic (relative =
to the fundamental), I could thus determine as a function of temperature th=
e "elastic anharmonicity" of copper (due to the 3rd order elastic constants=
) along each of the three principal axes.   In recent years I have recogniz=
ed the importance of "damping anharmonicity" as well as the elastic anharmo=
nicity that I studied as a student.  I was greatly satisfied to have finall=
y published something significant on this topic in the 10th edition of the =
McGraw Hill Encyclopedia of Science and Technology.  It has gained acceptan=
ce by at least the chemistry community, as seen by  the 'Chem-wiki' page on=
line at http://chemwiki.ucdavis.edu/Physical_Chemistry/Quantum_Mechanics/Qu=
antum_Theory/Trapped_Particles/Anharmonic_Oscillator
      If you type into Google the following keywords (without the tick mark=
s causing a literal search):
'anharmonic oscillator access science"
   You should then be able to read the full article by clicking on the 2nd =
link of the first page that gets displayed.
     Even though my ultrasonic system sensitivity was impressive at 0.1 nm,=
 some Russians were able with cryogenic electronics to do a hundred times b=
etter, approaching the size of atomic nuclei.  So thus I am confident that =
the ultimate resolution of a seismograph does not derive from the limitatio=
ns of a capacitive sensor.  But don't take my word for it.  The great physi=
cist R. V. Jones (who died in the late 1990's) is still quoted by the most =
knowledgeable modern 'artisans' as the ultimate authority on sensors of the=
 type used in modern seismology.  He noted that two sensing types will (for=
 reason of the physics used) outperform all others; they are (i) capacitive=
, and (ii) optical.  The power of the latter is finally beginning to be use=
d by a small number of university research types, at least in California.  =
Their optics approach is the same as used by the Laser Interferometer Gravi=
tational Wave Observatory (LIGO) people; i.e., measure the displacement (no=
t the velocity) by means of a Michelson interferometer.    Optical fibers a=
llow such instruments to be placed way below ground with minimal difficulti=
es of the type otherwise encountered.
     I have done enough with capacitive sensors over the last four decades =
to learn a few things through the 'college of hard knocks', in contrast wit=
h the work of my degree from the University of Tennessee (involving the afo=
rementioned ORNL experience).  It should come as no surprise to anybody wit=
h very much electronics experience, that considerable benefit is gained (wh=
en possible) by operating capacitive sensors in a differential mode.  The i=
mprovement in SNR, especially because of better common mode electronic reje=
ction can be dramatic.  Also, if the electronics can be configured to invol=
ve phase sensitive detection, still more significant gains are possible.  M=
y research that has impacted seismology most significantly involves the ful=
ly differential capacitive sensor that I patented.  I remain amazed at the =
level of resistance I experienced in trying to publish papers related to th=
is patent.  One of my papers was actually reviewed by R. V. Jones (as was t=
old me by editor Tom Braid of the Review of Scientific Instruments).  Of my=
 sensor, Jones stated that "the device was twice as sensitive as the conven=
tional (half) differential capacitive types" (for equivalent sized electrod=
e areas total), and he liked its symmetry, even though he was at that time =
"too old to take the time to do a detailed theoretical analysis of it".
        I have written all this to hopefully encourage more folks to 'think=
 outside the box' of conventional wisdom.  One person who has done so with =
some successes is Allan Coleman.  A paper of his from several years ago is =
posted on my webpage at
http://physics.mercer.edu/hpage/mkxx1.pdf
         The capacitive sensor, unlike the Faraday Law (coil/magnet) veloci=
ty sensor of a previous generation (World Wide Standard Seismograph Network=
--WWSSN) is fundamentally a displacement sensor.  I encourage you to look i=
n the writings of one of the world's most highly respected seismologists (E=
rhard Wielandt).  He notes that the "modern" force balance instrument (of t=
ype perfected by his 'sidekick' Gunar Streckeisen, maker of the legendary S=
TS instruments) was made to function by means of force-feedback so as to be=
have like the earlier instruments (such as the original form of the Sprengn=
ether vertical that I own).  I modified my Sprengnether to function more na=
turally as a displacement-measuring instrument, using my patented sensor.  =
For those of you who are interested, there is a paper that I wrote about si=
x years ago, titled "Improving seismometer performance at low frequencies u=
sing newly discovered physics".  It is online at
http://physics.mercer.edu/hpage/broad.pdf
     Anyone interested in following Allan Coleman in the use of my sensor f=
or seismic purposes, may want to look first at a pedagogical description of=
 how it works.  This is located on the Tel-Atomic webpage at http://www.tel=
atomic.com/mechanics/sensor.html
The Cavendish balance takes advantage, not only of electronics common mode =
rejection, but also 'mechanical common mode rejection' that eliminates the =
'curse worthy to students' pendulous swinging modes that made this classic =
experiment much more difficult in the past.  It may also be of interest to =
note that the heart of the electronics is the same AD7745 capacitance to di=
gital converter (Analog Devices) that is used by the VolksMeter that I crea=
ted.

Randall

Brett,

     Your equivalent displace= ment threshold resolution of 40 nm is quite impressive, since you are measu= ring the motion of a large inertial mass.  But this is probably not a = limit imposed by your capacitive sensor.   In general, seismomete= r limitations are the result of one or the other of (i) electronics, or (ii= ) the ‘spring’.  The latter is a consequence of the fact t= hat there are no perfect macro-scale springs; i.e., ones that obey Hooke= 217;s law, which we routinely teach to physics students as though they desc= ribe the real world.  My friend and colleague, Tom Erber, who recently= retired from Illinois Tech University—demonstrated experimentally th= at only with ‘spring displacements’ approaching atomic dimensio= ns, could a Hooke’s law approximation be possibly achieved.  In = other words, by looking at incredibly small motions using an atomic force m= icroscope.  So a seismometer responds not only to changes (acceleratio= ns) of its external environment, but ALSO (which so few folks really apprec= iate) to changes that happen in the spring itself.  These are especial= ly troublesome when one wants to look at earth motions that are “low = and slow”. 

  &nbs= p;  A capacitive sensor with a large electric field between the p= lates (the parameter that ultimately determines its threshold sensitivity) = can measure down to 0.1 nm, even at room temperature.  This was the di= splacement threshold of the capacitive sensor that I used as a student whil= e at Oak Ridge National Laboratory in the late 1960’s.  The sens= or was one that operated on gap spacing change of parallel plates that had = been polished optically flat to better than a half-wavelength of visible li= ght, and spaced nominally about 5 micro-meters (microns) apart.  Motio= n of one plate, relative to the other, was due to 30 and 60 MHz longitudina= lly excited (pulse echo) ultrasound waves in a  single crystal.  = By measuring the 2nd harmonic (relative to the fundamental), I c= ould thus determine as a function of temperature the “elastic anharmo= nicity” of copper (due to the 3rd order elastic constants)= along each of the three principal axes.   In recent years I have= recognized the importance of “damping anharmonicity” as well a= s the elastic anharmonicity that I studied as a student.  I was greatl= y satisfied to have finally published something significant on this topic i= n the 10th edition of the McGraw Hill Encyclopedia of Science an= d Technology.  It has gained acceptance by at least the chemistry comm= unity, as seen by  the ‘Chem-wiki’ page online at http://chemwiki.ucdavis.e= du/Physical_Chemistry/Quantum_Mechanics/Quantum_Theory/Trapped_Particles/An= harmonic_Oscillator

  &nbs= p;   If you type into Google the following keywords (without the = tick marks causing a literal search):

&#= 8216;anharmonic oscillator access science”

   You should then be able to read the full article by c= licking on the 2nd link of the first page that gets displayed.&n= bsp;

     Even= though my ultrasonic system sensitivity was impressive at 0.1 nm, some Rus= sians were able with cryogenic electronics to do a hundred times better, ap= proaching the size of atomic nuclei.  So thus I am confident that the = ultimate resolution of a seismograph does not derive from the limitations o= f a capacitive sensor.  But don’t take my word for it.  The= great physicist R. V. Jones (who died in the late 1990’s) is still q= uoted by the most knowledgeable modern ‘artisans’ as the ultima= te authority on sensors of the type used in modern seismology.  He not= ed that two sensing types will (for reason of the physics used) outperform = all others; they are (i) capacitive, and (ii) optical.  The power of t= he latter is finally beginning to be used by a small number of university r= esearch types, at least in California.  Their optics approach is the s= ame as used by the Laser Interferometer Gravitational Wave Observatory (LIG= O) people; i.e., measure the displacement (not the velocity) by means of a = Michelson interferometer.    Optical fibers allow such instr= uments to be placed way below ground with minimal difficulties of the type = otherwise encountered.

   = ;  I have done enough with capacitive sensors over the last four decad= es to learn a few things through the ‘college of hard knocks’, = in contrast with the work of my degree from the University of Tennessee (in= volving the aforementioned ORNL experience).  It should come as no sur= prise to anybody with very much electronics experience, that considerable b= enefit is gained (when possible) by operating capacitive sensors in a diffe= rential mode.  The improvement in SNR, especially because of better co= mmon mode electronic rejection can be dramatic.  Also, if the electron= ics can be configured to involve phase sensitive detection, still more sign= ificant gains are possible.  My research that has impacted seismology = most significantly involves the fully differential capacitive sensor that I= patented.  I remain amazed at the level of resistance I experienced i= n trying to publish papers related to this patent.  One of my papers w= as actually reviewed by R. V. Jones (as was told me by editor Tom Braid of = the Review of Scientific Instruments).  Of my sensor, Jones stated tha= t “the device was twice as sensitive as the conventional (half) diffe= rential capacitive types” (for equivalent sized electrode areas total= ), and he liked its symmetry, even though he was at that time “too ol= d to take the time to do a detailed theoretical analysis of it”.=

        I= have written all this to hopefully encourage more folks to ‘think ou= tside the box’ of conventional wisdom.  One person who has done = so with some successes is Allan Coleman.  A paper of his from several = years ago is posted on my webpage at

http://physics.mercer.ed= u/hpage/mkxx1.pdf

   =       The capacitive sensor, unlike the Faraday La= w (coil/magnet) velocity sensor of a previous generation (World Wide Standa= rd Seismograph Network--WWSSN) is fundamentally a displacement sensor. = ; I encourage you to look in the writings of one of the world’s most = highly respected seismologists (Erhard Wielandt).  He notes that the &= #8220;modern” force balance instrument (of type perfected by his R= 16;sidekick’ Gunar Streckeisen, maker of the legendary STS instrument= s) was made to function by means of force-feedback so as to behave like the= earlier instruments (such as the original form of the Sprengnether vertica= l that I own).  I modified my Sprengnether to function more naturally = as a displacement-measuring instrument, using my patented sensor.  For= those of you who are interested, there is a paper that I wrote about six y= ears ago, titled “Improving seismometer performance at low frequencie= s using newly discovered physics”.  It is online at

http://physics.mercer.edu/hpage/broad.pdf

     Anyone interested in following Allan Colem= an in the use of my sensor for seismic purposes, may want to look first at = a pedagogical description of how it works.  This is located on the Tel= -Atomic webpage at http://www.telatomic.com/mechanics/sensor.html

The Cavendish balance takes advantage, not only of electronic= s common mode rejection, but also ‘mechanical common mode rejection&#= 8217; that eliminates the ‘curse worthy to students’ pendulous = swinging modes that made this classic experiment much more difficult in the= past.  It may also be of interest to note that the heart of the elect= ronics is the same AD7745 capacitance to digital converter (Analog Devices)= that is used by the VolksMeter that I created.

 

Randall

 

=

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