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