## PSN-L Email List Message

Subject: Re: capacitive sensors
From: Christopher Chapman chrisatupw@.......
Date: Thu, 11 Aug 2011 20:54:03 -0400 (EDT)

Randall Peters
To: 'psnlist@...............
Sent: Thu, 11 Aug 2011 14:47
Subject: capacitive sensors

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 =E2=80=98spring=E2=80=99.  The latter is a consequence =
of the fact that there are no perfect macro-scale springs; i.e., ones that =
obey Hooke=E2=80=99s law, which we routinely teach to physics students as t=
hough they describe the real world.  My friend and colleague, Tom Erber, wh=
o recently retired from Illinois Tech University=E2=80=94demonstrated exper=
imentally that only with =E2=80=98spring displacements=E2=80=99 approaching=
atomic dimensions, could a Hooke=E2=80=99s law approximation be possibly a=
chieved.  In other words, by looking at incredibly small motions using an a=
tomic force microscope.  So a seismometer responds not only to changes (acc=
elerations) of its external environment, but ALSO (which so few folks reall=
y appreciate) to changes that happen in the spring itself.  These are espec=
ially troublesome when one wants to look at earth motions that are =E2=80=
=9Clow and slow=E2=80=9D. =20
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=E2=80=99s.  The sensor was one that o=
perated on gap spacing change of parallel plates that had been polished opt=
ically flat to better than a half-wavelength of visible light, and spaced n=
ominally about 5 micro-meters (microns) apart.  Motion of one plate, relati=
ve to the other, was due to 30 and 60 MHz longitudinally excited (pulse ech=
o) ultrasound waves in a  single crystal.  By measuring the 2nd harmonic (r=
elative to the fundamental), I could thus determine as a function of temper=
ature the =E2=80=9Celastic anharmonicity=E2=80=9D of copper (due to the 3rd=
order elastic constants) along each of the three principal axes.   In rece=
nt years I have recognized the importance of =E2=80=9Cdamping anharmonicity=
=E2=80=9D as well as the elastic anharmonicity that I studied as a student.=
I was greatly satisfied to have finally published something significant o=
n this topic in the 10th edition of the McGraw Hill Encyclopedia of Science=
and Technology.  It has gained acceptance by at least the chemistry commun=
ity, as seen by  the =E2=80=98Chem-wiki=E2=80=99 page online at http://chem=
wiki.ucdavis.edu/Physical_Chemistry/Quantum_Mechanics/Quantum_Theory/Trappe=
d_Particles/Anharmonic_Oscillator
If you type into Google the following keywords (without the tick mark=
s causing a literal search):
=E2=80=98anharmonic oscillator access science=E2=80=9D
link of the first page that gets displayed. =20
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=E2=80=99t take my word for it.  The gre=
at physicist R. V. Jones (who died in the late 1990=E2=80=99s) is still quo=
ted by the most knowledgeable modern =E2=80=98artisans=E2=80=99 as the ulti=
mate 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 latte=
r is finally beginning to be used by a small number of university research =
types, at least in California.  Their optics approach is the same as used b=
y the Laser Interferometer Gravitational Wave Observatory (LIGO) people; i.=
e., measure the displacement (not the velocity) by means of a Michelson int=
erferometer.    Optical fibers allow such instruments to be placed way belo=
w ground with minimal difficulties of the type otherwise encountered.
I have done enough with capacitive sensors over the last four decades =
to learn a few things through the =E2=80=98college of hard knocks=E2=80=99,=
in contrast with the work of my degree from the University of Tennessee (i=
nvolving the aforementioned ORNL experience).  It should come as no surpris=
e to anybody with very much electronics experience, that considerable benef=
it is gained (when possible) by operating capacitive sensors in a different=
ial mode.  The improvement in SNR, especially because of better common mode=
electronic rejection can be dramatic.  Also, if the electronics can be con=
figured to involve phase sensitive detection, still more significant gains =
are possible.  My research that has impacted seismology most significantly =
involves the fully differential capacitive sensor that I patented.  I remai=
n amazed at the level of resistance I experienced in trying to publish pape=
rs related to this patent.  One of my papers was actually reviewed by R. V.=
Jones (as was told me by editor Tom Braid of the Review of Scientific Inst=
ruments).  Of my sensor, Jones stated that =E2=80=9Cthe device was twice as=
sensitive as the conventional (half) differential capacitive types=E2=80=
=9D (for equivalent sized electrode areas total), and he liked its symmetry=
, even though he was at that time =E2=80=9Ctoo old to take the time to do a=
detailed theoretical analysis of it=E2=80=9D.
I have written all this to hopefully encourage more folks to =E2=80=
=98think outside the box=E2=80=99 of conventional wisdom.  One person who h=
as done so with some successes is Allan Coleman.  A paper of his from sever=
al 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=E2=80=99s most highly respected seismolo=
gists (Erhard Wielandt).  He notes that the =E2=80=9Cmodern=E2=80=9D force =
balance instrument (of type perfected by his =E2=80=98sidekick=E2=80=99 Gun=
ar Streckeisen, maker of the legendary STS instruments) was made to functio=
n by means of force-feedback so as to behave like the earlier instruments (=
such as the original form of the Sprengnether vertical that I own).  I modi=
fied 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 years ago, titled =E2=80=9CImprov=
ing seismometer performance at low frequencies using newly discovered physi=
cs=E2=80=9D.  It is online at
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 =E2=80=98mechanical common mode rejection=E2=80=99 that=
eliminates the =E2=80=98curse worthy to students=E2=80=99 pendulous swingi=
ng 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 t=
he same AD7745 capacitance to digital converter (Analog Devices) that is us=
ed by the VolksMeter that I created.
=20
Randall=20
=20

= Randall Peters <PETERS_RD@..........>
To: 'psnlist@............... <psnlist@..............>
Sent: Thu, 11 Aug 2011 14:47
Subject: capacitive sensors

Brett,
Your equivalent displacemen= t threshold resolution of 40 nm is quite impressive, since you are measurin= g the motion of a large inertial mass.  But this is probably not a lim= it imposed by your capacitive sensor.   In general, seismometer l= imitations are the result of one or the other of (i) electronics, or (ii) t= he =E2=80=98spring=E2=80=99.  The latter is a consequence of the fact = that there are no perfect macro-scale springs; i.e., ones that obey Hooke= =E2=80=99s law, which we routinely teach to physics students as though they= describe the real world.  My friend and colleague, Tom Erber, who rec= ently retired from Illinois Tech University=E2=80=94demonstrated experiment= ally that only with =E2=80=98spring displacements=E2=80=99 approaching atom= ic dimensions, could a Hooke=E2=80=99s law approximation be possibly achiev= ed.  In other words, by looking at incredibly small motions using an a= tomic force microscope.  So a seismometer responds not only to changes= (accelerations) of its external environment, but ALSO (which so few folks = really appreciate) to changes that happen in the spring itself.  These= are especially troublesome when one wants to look at earth motions that ar= e =E2=80=9Clow and slow=E2=80=9D.
A capacitive sensor wi= th a large electric field between the plates (the parameter that ultimately= determines its threshold sensitivity) can measure down to 0.1 nm, even at = room temperature.  This was the displacement threshold of the capaciti= ve sensor that I used as a student while at Oak Ridge National Laboratory i= n the late 1960=E2=80=99s.  The sensor was one that operated on gap sp= acing change of parallel plates that had been polished optically flat to be= tter than a half-wavelength of visible light, and spaced nominally about 5 = micro-meters (microns) apart.  Motion of one plate, relative to the ot= her, was due to 30 and 60 MHz longitudinally excited (pulse echo) ultrasoun= d waves in a  single crystal.  By measuring the 2nd ha= rmonic (relative to the fundamental), I could thus determine as a function = of temperature the =E2=80=9Celastic anharmonicity=E2=80=9D of copper (due t= o the 3rd order elastic constants) along each of the three princ= ipal axes.   In recent years I have recognized the importance of = =E2=80=9Cdamping anharmonicity=E2=80=9D as well as the elastic anharmonicit= y that I studied as a student.  I was greatly satisfied to have finall= y published something significant on this topic in the 10th edit= ion of the McGraw Hill Encyclopedia of Science and Technology.  It has= gained acceptance by at least the chemistry community, as seen by  th= e =E2=80=98Chem-wiki=E2=80=99 page online at http://chemwiki.ucdavis.edu/Phys= ical_Chemistry/Quantum_Mechanics/Quantum_Theory/Trapped_Particles/Anharmoni= c_Oscillator
If you type into Goog= le the following keywords (without the tick marks causing a literal search)= :
=E2=80=98anharmonic oscillator access science=E2=80= =9D
You should then be able to read the ful= l article by clicking on the 2nd link of the first page that get= s displayed.
Even though my ultraso= nic system sensitivity was impressive at 0.1 nm, some Russians were able wi= th cryogenic electronics to do a hundred times better, approaching the size= of atomic nuclei.  So thus I am confident that the ultimate resolutio= n of a seismograph does not derive from the limitations of a capacitive sen= sor.  But don=E2=80=99t take my word for it.  The great physicist= R. V. Jones (who died in the late 1990=E2=80=99s) is still quoted by the m= ost knowledgeable modern =E2=80=98artisans=E2=80=99 as the ultimate authori= ty on sensors of the type used in modern seismology.  He noted that tw= o 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 used by a small number of university research ty= pes, at least in California.  Their optics approach is the same as use= d by the Laser Interferometer Gravitational Wave Observatory (LIGO) people;= i.e., measure the displacement (not the velocity) by means of a Michelson = interferometer.    Optical fibers allow such instruments to = be placed way below ground with minimal difficulties of the type otherwise = encountered.
I have done enough with cap= acitive sensors over the last four decades to learn a few things through th= e =E2=80=98college of hard knocks=E2=80=99, in contrast with the work of my= degree from the University of Tennessee (involving the aforementioned ORNL= experience).  It should come as no surprise to anybody with very much= electronics experience, that considerable benefit is gained (when possible= ) by operating capacitive sensors in a differential mode.  The improve= ment in SNR, especially because of better common mode electronic rejection = can be dramatic.  Also, if the electronics can be configured to involv= e phase sensitive detection, still more significant gains are possible.&nbs= p; My research that has impacted seismology most significantly involves the= fully differential capacitive sensor that I patented.  I remain amaze= d at the level of resistance I experienced in trying to publish papers rela= ted to this patent.  One of my papers was actually reviewed by R. V. J= ones (as was told me by editor Tom Braid of the Review of Scientific Instru= ments).  Of my sensor, Jones stated that =E2=80=9Cthe device was twice= as sensitive as the conventional (half) differential capacitive types=E2= =80=9D (for equivalent sized electrode areas total), and he liked its symme= try, even though he was at that time =E2=80=9Ctoo old to take the time to d= o a detailed theoretical analysis of it=E2=80=9D.
I have wr= itten all this to hopefully encourage more folks to =E2=80=98think outside = the box=E2=80=99 of conventional wisdom.  One person who has done so w= ith some successes is Allan Coleman.  A paper of his from several year= s ago is posted on my webpage at
The= capacitive sensor, unlike the Faraday Law (coil/magnet) velocity sensor of= a previous generation (World Wide Standard Seismograph Network--WWSSN) is = fundamentally a displacement sensor.  I encourage you to look in the w= ritings of one of the world=E2=80=99s most highly respected seismologists (= Erhard Wielandt).  He notes that the =E2=80=9Cmodern=E2=80=9D force ba= lance instrument (of type perfected by his =E2=80=98sidekick=E2=80=99 Gunar= Streckeisen, maker of the legendary STS instruments) was made to function = by means of force-feedback so as to behave like the earlier instruments (su= ch as the original form of the Sprengnether vertical that I own).  I m= odified my Sprengnether to function more naturally as a displacement-measur= ing instrument, using my patented sensor.  For those of you who are in= terested, there is a paper that I wrote about six years ago, titled =E2=80= =9CImproving seismometer performance at low frequencies using newly discove= red physics=E2=80=9D.  It is online at
Anyone interested in follow= ing Allan Coleman in the use of my sensor for seismic purposes, may want to= look first at a pedagogical description of how it works.  This is loc= ated on the Tel-Atomic webpage at http://www.telatomic.com/mechanics/sensor= ..html
The Cavendish balance takes advantage, not only of e= lectronics common mode rejection, but also =E2=80=98mechanical common mode = rejection=E2=80=99 that eliminates the =E2=80=98curse worthy to students=E2= =80=99 pendulous swinging modes that made this classic experiment much more= difficult in the past.  It may also be of interest to note that the h= eart of the electronics is the same AD7745 capacitance to digital converter= (Analog Devices) that is used by the VolksMeter that I created.

Randall