PSN-L Email List Message
Subject: Re: Combination spring-mag_levitation Anyone Tried This ??
From: ChrisAtUpw@.......
Date: Thu, 6 Dec 2007 22:15:18 EST
In a message dated 06/12/2007, gmvoeth@........... writes:
Has anyone ever used a spring to take up most the mass weight then maybe
apply an ounce or less levitation magnetically to set with electronics the free
period within a geophone.
I'd think you would pump a current limited signal into the geophone and use
some kind of
feedback circuit that might set the free period. I am looking only for a
free period in the 3 to 4 second range.
Hi All,
Trying to provide magnetic 'springs' for seismometers tends to quickly
run into the problem of temperature coefficients and drifts. Alnico has the
lowest Tc, being about -1 or -2 x10^-4 / C Deg depending on the grade. Sm-Co is
about -3x10^-4 / C Deg and NdFeB is about -11x10^-4 / C Deg. Ferrite magnets
are a lot worse.
Sandwiches of thick Sm-Co and a thin opposing sheet of NdFeB have been
used to provide ~high stability quadrupole magnets, but stabilising the
temperature of the magnetic material is probably an easier option.
The torsion coefficient of Steel Springs is about -2.6x10^-4 / C Deg.
Only Elinvar and NiSpanC springs have very low coefficients in the ppm range.
If you consider electromagnets, you have the problem of voltage
reference drifts and amplifier drifts, as well as expansion coefficients and the
thermal convection from heating. The common voltage reference diodes still have
drifts of 20 to 150 ppm / C Deg. The few types with drifts in the low ppm
range like the LM399 and LT1027 are quite expensive. Type 'prec ref' into
_www.digikey.com_ (http://www.digikey.com)
Weak springs were used to extend the period of the 1 to 3 second
Willmore seismometers to about 20 seconds. These were produced in both vertical and
horizontal versions. 'Trimming' of the period by magnetic repulsion has also
been used on horizontal Lehman seismometers, but it can render them sensitive
to stray magnetic field changes. A similar repulsion method has been used on
clock pendulums to correct for period / swing angle timing errors.
Trying to use the magnetic repulsion of pyrolitic graphite also tends to
run into Tc problems as well as giving large stray magnetic fields.
Geophones can have their natural periods very considerably extended (to
<~1/20 natural frequency), if they are fitted with an amplifier with a
negative input impedance roughly equal to their electrical resistance. The current
generated by any motion is fed into a current to voltage converter and this
output is proportional to frequency. The armature is effectively held
stationary with respect to the case. A second amplifier stage with the output
proportional to 1/f gives the normal velocity output. The sensitivity is quite low.
These are described at
_http://www.lennartz-electronic.de/PDF_documents/Seismometers.pdf_ (http://www.lennartz-electronic.de/PDF_documents/Seismometers.pdf)
An alternative method also used by Lennartz is described by Roberts,
P.M. 'A Versatile Equilization Circuit for Increasing Seismometer Velocity
Response Below the Natural Frequency' BSSA Vol 79, no 4, pp 1607-1617 Oct. 1989
See also _http://jclahr.com/science/psn/roberts/index.html_
(http://jclahr.com/science/psn/roberts/index.html) This circuit selectively amplifies the f^2
'tail' of the normal geophone response below resonance and can provide a
period extension of ~x10. Both types require the use of very low noise amplifiers,
preferably with discreet matched pair low noise input transistors.
Aaron Barzilai modified geophones by adding a capacitative position
detector. See
_http://micromachine.stanford.edu/smssl/projects/Geophones/ASMEWin'98FinalDraftSlides.pdf_
(http://micromachine.stanford.edu/smssl/projects/Geophones/ASMEWin'98FinalDraftSlides.pdf) and other references cited at
_http://micromachine.stanford.edu/smssl/projects/Geophones/_
(http://micromachine.stanford.edu/smssl/projects/Geophones/) Unfortunately he seems to have used a
difficult mechanical modification and his digital electronics, also described, was
more noisy than it needed to be.
Regards,
Chris Chapman
In a message dated 06/12/2007, gmvoeth@........... writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" color=3D#000000>Has anyone ever us=
ed a=20
spring to take up most the mass weight then maybe apply an ounce or less=20
levitation magnetically to set with electronics the free period within a=20
geophone.
I'd think you would pump a current limited signal into th=
e=20
geophone and use some kind of
feedback circuit that might set the free=20
period. I am looking only for a free period in the 3 to 4 second=20
range.
Hi All,
Trying to provide magnetic 'springs' for=20
seismometers tends to quickly run into the problem of temperature coefficien=
ts=20
and drifts. Alnico has the lowest Tc, being about -1 or -2 x10^-4 / C D=
eg=20
depending on the grade. Sm-Co is about -3x10^-4 / C Deg and NdFeB is ab=
out=20
-11x10^-4 / C Deg. Ferrite magnets are a lot worse.
Sandwiches of thick Sm-Co and a thin opposing s=
heet=20
of NdFeB have been used to provide ~high stability quadrupole magnets,=20=
but=20
stabilising the temperature of the magnetic material is probably an easier=20
option.
The torsion coefficient of Steel Springs is abo=
ut=20
-2.6x10^-4 / C Deg. Only Elinvar and NiSpanC springs have very low coefficie=
nts=20
in the ppm range.
If you consider electromagnets, you have the=20
problem of voltage reference drifts and amplifier drifts, as well as expansi=
on=20
coefficients and the thermal convection from heating. The common voltag=
e=20
reference diodes still have drifts of 20 to 150 ppm / C Deg. The few types w=
ith=20
drifts in the low ppm range like the LM399 and LT1027 are quite expensive. T=
ype=20
'prec ref' into
www.digikey.com
Weak springs were used to extend the period of=20=
the=20
1 to 3 second Willmore seismometers to about 20 seconds. These were produced=
in=20
both vertical and horizontal versions. 'Trimming' of the period by magnetic=20
repulsion has also been used on horizontal Lehman seismometers, but it can=20
render them sensitive to stray magnetic field changes. A similar repuls=
ion=20
method has been used on clock pendulums to correct for period / swing angle=20
timing errors.
Trying to use the magnetic repulsion of pyrolit=
ic=20
graphite also tends to run into Tc problems as well as giving large stray=20
magnetic fields.
Geophones can have their natural periods very=20
considerably extended (to <~1/20 natural frequency), if they are fitted w=
ith=20
an amplifier with a negative input impedance roughly equal to their electric=
al=20
resistance. The current generated by any motion is fed into a current to vol=
tage=20
converter and this output is proportional to frequency. The=20
armature is effectively held stationary with respect to the case. A second=20
amplifier stage with the output proportional to 1/f gives the normal velocit=
y=20
output. The sensitivity is quite low. These are described at
ht=
tp://www.lennartz-electronic.de/PDF_documents/Seismometers.pdf
An alternative method also used by Lennartz is=20
described by Roberts, P.M. 'A Versatile Equilization Circuit for=20
Increasing Seismometer Velocity Response Below the Natural=20
Frequency' BSSA Vol 79, no 4, pp 1607-1617 Oct. 1989 See als=
o
http://jclahr.com/=
science/psn/roberts/index.html This=20
circuit selectively amplifies the f^2 'tail' of the normal geophone=20
response below resonance and can provide a period extension of ~x10. Both ty=
pes=20
require the use of very low noise amplifiers, preferably with discreet match=
ed=20
pair low noise input transistors.
Regards,
Chris Chapman
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