| Author |
Message |
Gregory L. Hansen
Guest
|
 Posted:
Sun Feb 13, 2005 3:34 am Post subject:
Seismometers and Pickup Coils |
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I'm starting to think about building a seismometer, partly because nobody
else seems to believe that Gaithersburg vibrates, and I want to get some
data. I'm pretty naive when it comes to actual availability of stuff
and implementation, so I'm hoping to get some general comments. My
numbers aren't very precise, but good enough for a feasibility check, I
think.
The sensitivity of the human butt is proportional to frequency and has a
limit of around 0.5 mm/s, which at 50 Hz is an amplitude of around 2
micrometers. I don't know for sure that I'm feeling something vertical,
but addressing that is a structural problem, not an electronic problem.
What I allege I am feeling is certainly above 1 Hz, so I've made my
mechanical design goal a boom with weight and spring to have a natural
frequency of around 1/10 Hz. My nominal target then is to measure a 50 Hz
vibration with an amplitude of 2 micrometers, with a range of interest
from 1 Hz to about 100 Hz.
My pickup design is to pair up two horseshoe magnets with a small gap
between them, and a square pickup coil of a few hundred turns dimensioned
and positioned so that the upper segment is between the upper poles and
the lower segment is between the lower poles. I'd thought about a second
coil with the connection and the magnets reversed to reduce noise from
power lines, but those sorts of details are still in the future.
I have little idea what kinds of magnets are available or where to get
them. But I assumed square poles 2 cm on a side with a field strength at
the poles of 1/100 tesla, and presumed that pairing up the magnets would
give a field strength of 2/100 tesla. That's probably a bad assumption
when working with ferromagnetic materials, but I thought it must be good
enough at this stage.
So I get from that a signal of around 0.1 mV. And my Horowitz & Hill is
at work, so I can't compare that with various noises until Monday. But
I don't think 0.1 mV is really in the regime of special low-noise
techniques. The vibrations I allege that I'm feeling have a time
structure of about five seconds on (sometimes multiples of five) and two
seconds off, so I thought it wouldn't be too hard to put a low-pass
filter in there and measure the on/off difference on a voltmeter. If I
carry it around and map out some amplitudes maybe I can figure out where
it's coming from.
--
"A nice adaptation of conditions will make almost any hypothesis agree
with the phenomena. This will please the imagination but does not advance
our knowledge." -- J. Black, 1803.
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Gregory L. Hansen
Guest
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| Posted:
Sun Feb 13, 2005 1:54 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <420f181e$1_4@127.0.0.1>,
Dave Garnett <dave.garnett@metapurple.co.uk> wrote:
| Quote: |
My pickup design is to pair up two horseshoe magnets with a small gap
between them, and a square pickup coil of a few hundred turns dimensioned
and positioned so that the upper segment is between the upper poles and
the lower segment is between the lower poles. I'd thought about a second
coil with the connection and the magnets reversed to reduce noise from
power lines, but those sorts of details are still in the future.
There is a lot to be said for optical pickup - you can easily give yourself
a long 'lever' arm, and these days a small laser will provide a very
convenient light source. You also don't have to worry about the damping
effects of a magnetic pickup - (in fact, you will probably want to add some
damping)
|
I don't know much about optical pickups except that I'd decided building
an interferometer would be a project in itself. Is it easier than that?
--
"You're not as dumb as you look. Or sound. Or our best testing
indicates." -- Monty Burns to Homer Simpson |
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Robert Baer
Guest
|
| Posted:
Sun Feb 13, 2005 2:00 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
"Gregory L. Hansen" wrote:
| Quote: |
I'm starting to think about building a seismometer, partly because nobody
else seems to believe that Gaithersburg vibrates, and I want to get some
data. I'm pretty naive when it comes to actual availability of stuff
and implementation, so I'm hoping to get some general comments. My
numbers aren't very precise, but good enough for a feasibility check, I
think.
The sensitivity of the human butt is proportional to frequency and has a
limit of around 0.5 mm/s, which at 50 Hz is an amplitude of around 2
micrometers. I don't know for sure that I'm feeling something vertical,
but addressing that is a structural problem, not an electronic problem.
What I allege I am feeling is certainly above 1 Hz, so I've made my
mechanical design goal a boom with weight and spring to have a natural
frequency of around 1/10 Hz. My nominal target then is to measure a 50 Hz
vibration with an amplitude of 2 micrometers, with a range of interest
from 1 Hz to about 100 Hz.
My pickup design is to pair up two horseshoe magnets with a small gap
between them, and a square pickup coil of a few hundred turns dimensioned
and positioned so that the upper segment is between the upper poles and
the lower segment is between the lower poles. I'd thought about a second
coil with the connection and the magnets reversed to reduce noise from
power lines, but those sorts of details are still in the future.
I have little idea what kinds of magnets are available or where to get
them. But I assumed square poles 2 cm on a side with a field strength at
the poles of 1/100 tesla, and presumed that pairing up the magnets would
give a field strength of 2/100 tesla. That's probably a bad assumption
when working with ferromagnetic materials, but I thought it must be good
enough at this stage.
So I get from that a signal of around 0.1 mV. And my Horowitz & Hill is
at work, so I can't compare that with various noises until Monday. But
I don't think 0.1 mV is really in the regime of special low-noise
techniques. The vibrations I allege that I'm feeling have a time
structure of about five seconds on (sometimes multiples of five) and two
seconds off, so I thought it wouldn't be too hard to put a low-pass
filter in there and measure the on/off difference on a voltmeter. If I
carry it around and map out some amplitudes maybe I can figure out where
it's coming from.
--
"A nice adaptation of conditions will make almost any hypothesis agree
with the phenomena. This will please the imagination but does not advance
our knowledge." -- J. Black, 1803.
|
It would seem that a speaker would work; connect the moving load to
the cone near the voice coil, or at/near the center of the dome in the
middle (center).
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Dave Garnett
Guest
|
| Posted:
Sun Feb 13, 2005 3:12 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
| Quote: |
My pickup design is to pair up two horseshoe magnets with a small gap
between them, and a square pickup coil of a few hundred turns dimensioned
and positioned so that the upper segment is between the upper poles and
the lower segment is between the lower poles. I'd thought about a second
coil with the connection and the magnets reversed to reduce noise from
power lines, but those sorts of details are still in the future.
|
There is a lot to be said for optical pickup - you can easily give yourself
a long 'lever' arm, and these days a small laser will provide a very
convenient light source. You also don't have to worry about the damping
effects of a magnetic pickup - (in fact, you will probably want to add some
damping)
Dave
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Dave Garnett
Guest
|
| Posted:
Sun Feb 13, 2005 3:12 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
| Quote: |
My pickup design is to pair up two horseshoe magnets with a small gap
between them, and a square pickup coil of a few hundred turns dimensioned
and positioned so that the upper segment is between the upper poles and
the lower segment is between the lower poles. I'd thought about a second
coil with the connection and the magnets reversed to reduce noise from
power lines, but those sorts of details are still in the future.
|
There is a lot to be said for optical pickup - you can easily give yourself
a long 'lever' arm, and these days a small laser will provide a very
convenient light source. You also don't have to worry about the damping
effects of a magnetic pickup - (in fact, you will probably want to add some
damping)
Dave
Posted Via Nuthinbutnews.Com Premium Usenet Newsgroup Services
----------------------------------------------------------
** SPEED ** RETENTION ** COMPLETION ** ANONYMITY **
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http://www.nuthinbutnews.com |
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Ian
Guest
|
| Posted:
Sun Feb 13, 2005 6:24 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:cunm7d$ajf$2@rainier.uits.indiana.edu...
| Quote: | In article <420f181e$1_4@127.0.0.1>,
Dave Garnett <dave.garnett@metapurple.co.uk> wrote:
My pickup design is to pair up two horseshoe magnets with a small gap
between them, and a square pickup coil of a few hundred turns
dimensioned
and positioned so that the upper segment is between the upper poles and
the lower segment is between the lower poles. I'd thought about a
second
coil with the connection and the magnets reversed to reduce noise from
power lines, but those sorts of details are still in the future.
There is a lot to be said for optical pickup - you can easily give
yourself
a long 'lever' arm, and these days a small laser will provide a very
convenient light source. You also don't have to worry about the damping
effects of a magnetic pickup - (in fact, you will probably want to add
some
damping)
I don't know much about optical pickups except that I'd decided building
an interferometer would be a project in itself. Is it easier than that?
If you are sure the vibration is 50Hz or thereabouts, you only need to |
get the resonant frequency of the proof mass suspension a way below
that frequency. 1/10Hz is a major engineering exercise.
The loudspeaker suggestion could be used directly. Pick a drive unit
with resonant frequency well below the vibration frequency you expect,
and fix a small mirror to the cone near the middle. Fix a laser pointer to
the frame, reflect the beam off the mirror onto a convenient wall, and
use a ruler to measure the amplitude of the beam movement. Simple
geometry let's you work back to the amplitude at the loudspeaker cone, and
that is the amplitude of ground motion you are getting.
There are plenty of woofers out there with resonances below 20Hz, you
can tweak the damping using the drive coil and a resistor. Just putting
the driver resting vertically on a bench is good enough.
DON'T look into the laser. DO make sure there are no highly reflective
surfaces anywhere near the beam path.
Regards
Ian |
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Gregory L. Hansen
Guest
|
| Posted:
Sun Feb 13, 2005 8:25 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <cuo61d$g6g$1@sparta.btinternet.com>,
Ian <ian.buckner@btinternet.com> wrote:
| Quote: |
"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:cunm7d$ajf$2@rainier.uits.indiana.edu...
In article <420f181e$1_4@127.0.0.1>,
Dave Garnett <dave.garnett@metapurple.co.uk> wrote:
My pickup design is to pair up two horseshoe magnets with a small gap
between them, and a square pickup coil of a few hundred turns
dimensioned
and positioned so that the upper segment is between the upper poles and
the lower segment is between the lower poles. I'd thought about a
second
coil with the connection and the magnets reversed to reduce noise from
power lines, but those sorts of details are still in the future.
There is a lot to be said for optical pickup - you can easily give
yourself
a long 'lever' arm, and these days a small laser will provide a very
convenient light source. You also don't have to worry about the damping
effects of a magnetic pickup - (in fact, you will probably want to add
some
damping)
I don't know much about optical pickups except that I'd decided building
an interferometer would be a project in itself. Is it easier than that?
If you are sure the vibration is 50Hz or thereabouts, you only need to
get the resonant frequency of the proof mass suspension a way below
that frequency. 1/10Hz is a major engineering exercise.
|
I've been discovering the joys of 1/10 Hz. It seems so easy when you plug
in w=sqrt(k/m), but then you discover the spring reaches its maximum
extension before it can support the required m. I'm going to brush up on
my differential equations and try revising my design goal to the measured
amplitude being 90% of the actual amplitude.
| Quote: |
The loudspeaker suggestion could be used directly. Pick a drive unit
with resonant frequency well below the vibration frequency you expect,
and fix a small mirror to the cone near the middle. Fix a laser pointer to
the frame, reflect the beam off the mirror onto a convenient wall, and
use a ruler to measure the amplitude of the beam movement. Simple
geometry let's you work back to the amplitude at the loudspeaker cone, and
that is the amplitude of ground motion you are getting.
|
I'm not sure what role the loudspeaker plays in that case.
--
"In any case, don't stress too much--cortisol inhibits muscular
hypertrophy. " -- Eric Dodd |
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Fred Bartoli
Guest
|
| Posted:
Mon Feb 14, 2005 3:05 am Post subject:
Re: Seismometers and Pickup Coils |
|
|
"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> a écrit dans le message
de news:cuod3p$ic4$2@rainier.uits.indiana.edu...
| Quote: | In article <cuo61d$g6g$1@sparta.btinternet.com>,
Ian <ian.buckner@btinternet.com> wrote:
If you are sure the vibration is 50Hz or thereabouts, you only need to
get the resonant frequency of the proof mass suspension a way below
that frequency. 1/10Hz is a major engineering exercise.
I've been discovering the joys of 1/10 Hz. It seems so easy when you plug
in w=sqrt(k/m), but then you discover the spring reaches its maximum
extension before it can support the required m. I'm going to brush up on
my differential equations and try revising my design goal to the measured
amplitude being 90% of the actual amplitude.
|
Sure.
w=sqrt(k/m)
but also for a spring, L-L0 = g m / k
so you have L-L0 = g /(w^2)
If you want 0.1Hz resonance frequency that gives you about 25 meters spring
displacement. A pretty nice spring :-)
If you really want 0.1Hz, you'll have to go for the right k/m ratio, then
compensate for the mass weight by some *constant force* mean (either a short
solenoid with a long core, or a short core in a long solenoid).
You'll probably have to add some rest position servoing with a low corner
frequency.
Another possibility is, with the same setup, to servo your sensing mass
position up to the highest frequency of interest. Then the solenoid current
is proportionnal to the mass acceleration.
--
Thanks,
Fred. |
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Rich Grise
Guest
|
| Posted:
Mon Feb 14, 2005 6:10 am Post subject:
Re: Seismometers and Pickup Coils |
|
|
On Sun, 13 Feb 2005 18:24:45 +0000, Ian wrote:
| Quote: |
"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:cunm7d$ajf$2@rainier.uits.indiana.edu...
In article <420f181e$1_4@127.0.0.1>,
Dave Garnett <dave.garnett@metapurple.co.uk> wrote:
My pickup design is to pair up two horseshoe magnets with a small gap
between them, and a square pickup coil of a few hundred turns
dimensioned
and positioned so that the upper segment is between the upper poles and
the lower segment is between the lower poles. I'd thought about a
second
coil with the connection and the magnets reversed to reduce noise from
power lines, but those sorts of details are still in the future.
There is a lot to be said for optical pickup - you can easily give
yourself
a long 'lever' arm, and these days a small laser will provide a very
convenient light source. You also don't have to worry about the damping
effects of a magnetic pickup - (in fact, you will probably want to add
some
damping)
I don't know much about optical pickups except that I'd decided building
an interferometer would be a project in itself. Is it easier than that?
If you are sure the vibration is 50Hz or thereabouts, you only need to
get the resonant frequency of the proof mass suspension a way below
that frequency. 1/10Hz is a major engineering exercise.
|
It doesn't look too hard to me:
http://mariottim.interfree.it/doc02hl_e.htm
Cheers!
Rich |
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Gregory L. Hansen
Guest
|
| Posted:
Mon Feb 14, 2005 4:08 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <cupubm$7n1$1@sunnews.cern.ch>,
Jeroen Belleman <jeroen@nospam.please> wrote:
| Quote: | Gregory L. Hansen wrote:
I've been discovering the joys of 1/10 Hz. It seems so easy when you plug
in w=sqrt(k/m), but then you discover the spring reaches its maximum
extension before it can support the required m. [...]
Google on 'folded pendulum'. That should bring up a wealth
of ways to suspend a weight with low resonant frequencies.
Jeroen Belleman
|
Nice. Looks like that could be a compact design.
On a related note, I was thinking of using knife edges for pivot points.
That's nice and scientific, right? But as I thought about it, I couldn't
think of any reason not to use strips of rubber sheet, fabric, or even
string for hinges. String hinges would be strong enough, easy to
construct, little friction, no stiction, cheap... what's not to like?
--
"Coincidences, in general, are great stumbling blocks in the way of that
class of thinkers who have been educated to know nothing of the theory of
probabilities." -- Edgar Allen Poe |
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Gregory L. Hansen
Guest
|
| Posted:
Mon Feb 14, 2005 4:12 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <pan.2005.02.14.05.14.44.882742@example.net>,
Rich Grise <richgrise@example.net> wrote:
| Quote: | On Sun, 13 Feb 2005 18:24:45 +0000, Ian wrote:
"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:cunm7d$ajf$2@rainier.uits.indiana.edu...
In article <420f181e$1_4@127.0.0.1>,
Dave Garnett <dave.garnett@metapurple.co.uk> wrote:
My pickup design is to pair up two horseshoe magnets with a small gap
between them, and a square pickup coil of a few hundred turns
dimensioned
and positioned so that the upper segment is between the upper poles and
the lower segment is between the lower poles. I'd thought about a
second
coil with the connection and the magnets reversed to reduce noise from
power lines, but those sorts of details are still in the future.
There is a lot to be said for optical pickup - you can easily give
yourself
a long 'lever' arm, and these days a small laser will provide a very
convenient light source. You also don't have to worry about the damping
effects of a magnetic pickup - (in fact, you will probably want to add
some
damping)
I don't know much about optical pickups except that I'd decided building
an interferometer would be a project in itself. Is it easier than that?
If you are sure the vibration is 50Hz or thereabouts, you only need to
get the resonant frequency of the proof mass suspension a way below
that frequency. 1/10Hz is a major engineering exercise.
It doesn't look too hard to me:
http://mariottim.interfree.it/doc02hl_e.htm
|
I was thinking of this design, but I think I like the folded pendulum
better, looks like it might be lighter and more compact, easier to carry.
But if you're measuring vertical vibrations I think the engineering would
be a little more challenging.
--
"Outside the camp you shall have a place set aside to be used as a
latrine. You shall keep a trowel in your equipment and with it, when you
go outside to ease nature, you shall first dig a hole and afterward cover
up your excrement." -- Deuteronomy 23:13-14 |
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Jeroen Belleman
Guest
|
| Posted:
Mon Feb 14, 2005 4:25 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
Gregory L. Hansen wrote:
| Quote: | I've been discovering the joys of 1/10 Hz. It seems so easy when you plug
in w=sqrt(k/m), but then you discover the spring reaches its maximum
extension before it can support the required m. [...]
|
Google on 'folded pendulum'. That should bring up a wealth
of ways to suspend a weight with low resonant frequencies.
Jeroen Belleman |
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| Back to top |
|
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Gregory L. Hansen
Guest
|
| Posted:
Mon Feb 14, 2005 6:09 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <cuql59$16v$1@sunnews.cern.ch>,
Jeroen Belleman <jeroen@nospam.please> wrote:
| Quote: | Gregory L. Hansen wrote:
In article <cupubm$7n1$1@sunnews.cern.ch>,
Jeroen Belleman <jeroen@nospam.please> wrote:
Google on 'folded pendulum'. [...]
Nice. Looks like that could be a compact design.
On a related note, I was thinking of using knife edges for pivot points.
[...]
Usually the hinges are (very) thin hard metal strips. You want something
that bends easily, but doesn't stretch or creep. Strings or rubber sheets
don't qualify. Naturally, the resulting pedulum has only one degree of
freedom.
Jeroen Belleman
|
What does it matter if the hinges creep? If their length changes by 1%,
the overall structure might change by 0.1% or 0.01% if the booms,
platforms, etc., aren't creeping with it. Probably nothing else in the
apparatus or the measurement matches that precision anyway.
I didn't think the knife edges were meant to bend, just to have a very
small contact surface. Torque is force times length, so making the length
as small as possible reduces the frictional effect on the pivot.
--
"In any case, don't stress too much--cortisol inhibits muscular
hypertrophy. " -- Eric Dodd |
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Jeroen Belleman
Guest
|
| Posted:
Mon Feb 14, 2005 10:55 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
Gregory L. Hansen wrote:
| Quote: | In article <cupubm$7n1$1@sunnews.cern.ch>,
Jeroen Belleman <jeroen@nospam.please> wrote:
Google on 'folded pendulum'. [...]
Nice. Looks like that could be a compact design.
On a related note, I was thinking of using knife edges for pivot points.
[...]
|
Usually the hinges are (very) thin hard metal strips. You want something
that bends easily, but doesn't stretch or creep. Strings or rubber sheets
don't qualify. Naturally, the resulting pedulum has only one degree of
freedom.
Jeroen Belleman |
|
| Back to top |
|
|
Guest
|
| Posted:
Mon Feb 14, 2005 11:56 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
glhansen@steel.ucs.indiana.edu (Gregory L. Hansen) wrote:
| Quote: |
I'm starting to think about building a seismometer, partly because nobody
else seems to believe that Gaithersburg vibrates, and I want to get some
data.
|
You aren't also known as "NEWS 2020", are you? He could feel things that
nobody else could feel too.
Jim
----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! >100,000 Newsgroups
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