| Author |
Message |
Jeroen Belleman
Guest
|
 Posted:
Tue Feb 15, 2005 2:11 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
Gregory L. Hansen wrote:
| Quote: | In article <cuql59$16v$1@sunnews.cern.ch>,
Jeroen Belleman <jeroen@nospam.please> wrote:
Usually the hinges are (very) thin hard metal strips. You want something
that bends easily, but doesn't stretch or creep.
What does it matter if the hinges creep? [...]
|
Well, a folded pendulum gets it long period from balancing
a stable pendulum against an unstable one. For long periods,
this balance gets increasingly delicate.
If the hinges change their properties for whatever reason,
the period would drift strongly and the pendulum might
even 'collapse'. (Meaning it no longer seeks a central
position, like a normal pendulum.)
However, I agree that if you do not push things too close
to the edge, it doesn't matter.
Jeroern Belleman
|
|
| Back to top |
|
 |
Gregory L. Hansen
Guest
|
| Posted:
Tue Feb 15, 2005 2:50 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <1108467601.341493@cswreg.cos.agilent.com>,
Ian <Ian_Buckner_not_@agilent.com> wrote:
| Quote: |
"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:cuqilo$9uu$2@rainier.uits.indiana.edu...
In article <pan.2005.02.14.05.14.44.882742@example.net>,
Rich Grise <richgrise@example.net> wrote:
On Sun, 13 Feb 2005 18:24:45 +0000, Ian 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.
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.
A suspended mass ground motion sensor acts as a 2nd order high pass
filter to the ground motion. Above the resonant frequency the amplitude
of the proof mass approaches zero quite quickly if you get the damping
right, and the frame (obviously) will still have the full ground motion.
The difference is what you are trying to measure.
What resonant frequency do you need, to be able to measure what
you want? Let's say you can get hold of a speaker drive unit with a
resonant frequency of 16Hz. If you tweak the Q to a little more than
0.85 you get essentially the full ground motion above about 20Hz.
Is that low enough for you?
|
I'd like to go to about a Hz.
| Quote: |
A loudspeaker driver gives you a proof mass, a way of controlling
the damping, support spring and structures to constrain the motion
of the proof mass to be axial. It is also cheap and readily available.
It is also easy to rotate the axis of the driver to let you check for any
horizontal motion as well.
|
Loud speakers make me nervous because I don't know the stiffness, voltage
versus velocity, the sensitivity, etc.
| Quote: |
Low resonant frequency mechanical structures are a significant problem,
in terms of keeping them stable and avoiding multiple response modes
and resonances. The pivots are critical, stability of the springs is a
nightmare. A very common mistake is to think that a large proof mass
is needed - it isn't, even for sensing very small ground motion, many
orders of magnitude below what you can perceive.
The next issue is what to use for a sensor, and how to calibrate the
whole system. A simple optical lever, geometry and a ruler will give
you this, and will let you see if you need to go any further (if the
measured amplitude of motion is negligible, you are done, if not then
ask again).
|
I don't understand the role of the mirror. A simple displacement won't
mean anything, it doesn't help unless the mirror rotates. But my
vibrations are too small to make water ripple, whatever amplitude that
might be.
--
"Yes, I revere you much, honored ones, and wish to fart in response." --
Aristophanes, Clouds |
|
| Back to top |
|
|
Ian
Guest
|
| Posted:
Tue Feb 15, 2005 5:39 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:cuqilo$9uu$2@rainier.uits.indiana.edu...
| Quote: | In article <pan.2005.02.14.05.14.44.882742@example.net>,
Rich Grise <richgrise@example.net> wrote:
On Sun, 13 Feb 2005 18:24:45 +0000, Ian 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.
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.
|
A suspended mass ground motion sensor acts as a 2nd order high pass
filter to the ground motion. Above the resonant frequency the amplitude
of the proof mass approaches zero quite quickly if you get the damping
right, and the frame (obviously) will still have the full ground motion.
The difference is what you are trying to measure.
What resonant frequency do you need, to be able to measure what
you want? Let's say you can get hold of a speaker drive unit with a
resonant frequency of 16Hz. If you tweak the Q to a little more than
0.85 you get essentially the full ground motion above about 20Hz.
Is that low enough for you?
A loudspeaker driver gives you a proof mass, a way of controlling
the damping, support spring and structures to constrain the motion
of the proof mass to be axial. It is also cheap and readily available.
It is also easy to rotate the axis of the driver to let you check for any
horizontal motion as well.
Low resonant frequency mechanical structures are a significant problem,
in terms of keeping them stable and avoiding multiple response modes
and resonances. The pivots are critical, stability of the springs is a
nightmare. A very common mistake is to think that a large proof mass
is needed - it isn't, even for sensing very small ground motion, many
orders of magnitude below what you can perceive.
The next issue is what to use for a sensor, and how to calibrate the
whole system. A simple optical lever, geometry and a ruler will give
you this, and will let you see if you need to go any further (if the
measured amplitude of motion is negligible, you are done, if not then
ask again).
Regards
Ian
|
|
| Back to top |
|
|
Mark Jones
Guest
|
| Posted:
Wed Feb 16, 2005 6:11 am Post subject:
OT: was Re: Seismometers and Pickup Coils |
|
|
Just side note, I heard that scientists saw where the north pole had moved an
inch after the Asian Tsunami. |
|
| Back to top |
|
|
Ken Smith
Guest
|
| Posted:
Wed Feb 16, 2005 2:42 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <1108560675.382559@cswreg.cos.agilent.com>,
Ian <Ian_Buckner_not_@agilent.com> wrote:
[..]
| Quote: | 1Hz rules out quite a lot of methods. Your original post said you thought
the vibration was around 50Hz, is that no longer the case?
|
The OP could just break down and buy a geophone
--
--
kensmith@rahul.net forging knowledge |
|
| Back to top |
|
|
Gregory L. Hansen
Guest
|
| Posted:
Wed Feb 16, 2005 3:25 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <1108560675.382559@cswreg.cos.agilent.com>,
Ian <Ian_Buckner_not_@agilent.com> wrote:
| Quote: |
"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:cut28c$4mm$2@rainier.uits.indiana.edu...
The next issue is what to use for a sensor, and how to calibrate the
whole system. A simple optical lever, geometry and a ruler will give
you this, and will let you see if you need to go any further (if the
measured amplitude of motion is negligible, you are done, if not then
ask again).
I don't understand the role of the mirror. A simple displacement won't
mean anything, it doesn't help unless the mirror rotates. But my
vibrations are too small to make water ripple, whatever amplitude that
might be.
1Hz rules out quite a lot of methods. Your original post said you thought
the vibration was around 50Hz, is that no longer the case?
|
Sometimes what I (allege that I) feel is like an unbalanced flywheel
spinning up and down, as low at 1/2 Hz or so.
But I'm not moving very quickly on this for a variety of reasons related
to time, money, logistics, and attention span. I wouldn't actually mind
very much if this phenomon stops before I'm ready to build something.
--
"Is that plutonium on your gums?"
"Shut up and kiss me!"
-- Marge and Homer Simpson |
|
| Back to top |
|
|
Gregory L. Hansen
Guest
|
| Posted:
Wed Feb 16, 2005 3:37 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <cuvm46$i5n$2@blue.rahul.net>,
Ken Smith <kensmith@green.rahul.net> wrote:
| Quote: | In article <1108560675.382559@cswreg.cos.agilent.com>,
Ian <Ian_Buckner_not_@agilent.com> wrote:
[..]
1Hz rules out quite a lot of methods. Your original post said you thought
the vibration was around 50Hz, is that no longer the case?
The OP could just break down and buy a geophone
--
--
kensmith@rahul.net forging knowledge
|
He might look into that.
--
"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. |
|
| Back to top |
|
|
Gregory L. Hansen
Guest
|
| Posted:
Wed Feb 16, 2005 6:33 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <11172qmr3egdf7d@corp.supernews.com>,
Phil Hobbs <pcdh@SpamMeSenseless.us.ibm.com> wrote:
| Quote: | Gregory L. Hansen wrote:
Another compact method, probably quite a bit easier to build than the
folded pendulum, is a superspring. You hang a real mass/spring
combination from the centre of a small loudspeaker, sense the mass's
position using e.g. a laser and split photodiode, and apply positive
feedback to the speaker using a dc amplifier. Overall loop gain should
be 0.99 or thereabouts. This reduces the effective spring constant by a
factor of 100, leading to a 10x lower resonant frequency for the same
physical size.
|
Supersprings, folded pendulums, geophones... all sorts of things I didn't
know existed. That's what's nice about a group like this.
I'm having a little trouble picturing the superspring. Are you sure it's
positive feedback? Feedback is a straight proportionality? If the mass
is supposed to stay still and the ground is moving, that would be sense
the position of the mass with respect to the ground?
--
"The average person, during a single day, deposits in his or her underwear
an amount of fecal bacteria equal to the weight of a quarter of a peanut."
-- Dr. Robert Buckman, Human Wildlife, p119. |
|
| Back to top |
|
|
Ian
Guest
|
| Posted:
Wed Feb 16, 2005 7:31 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:cut28c$4mm$2@rainier.uits.indiana.edu...
| Quote: |
What resonant frequency do you need, to be able to measure what
you want? Let's say you can get hold of a speaker drive unit with a
resonant frequency of 16Hz. If you tweak the Q to a little more than
0.85 you get essentially the full ground motion above about 20Hz.
Is that low enough for you?
I'd like to go to about a Hz.
A loudspeaker driver gives you a proof mass, a way of controlling
the damping, support spring and structures to constrain the motion
of the proof mass to be axial. It is also cheap and readily available.
It is also easy to rotate the axis of the driver to let you check for any
horizontal motion as well.
Loud speakers make me nervous because I don't know the stiffness, voltage
versus velocity, the sensitivity, etc.
The next issue is what to use for a sensor, and how to calibrate the
whole system. A simple optical lever, geometry and a ruler will give
you this, and will let you see if you need to go any further (if the
measured amplitude of motion is negligible, you are done, if not then
ask again).
I don't understand the role of the mirror. A simple displacement won't
mean anything, it doesn't help unless the mirror rotates. But my
vibrations are too small to make water ripple, whatever amplitude that
might be.
|
1Hz rules out quite a lot of methods. Your original post said you thought
the vibration was around 50Hz, is that no longer the case?
I was not suggesting using the speaker unit as a speaker, or using the
voice coil to sense the motion, just to control the damping with an external
resistor. Using it just as a suspended mass on a spring above the
resonant frequency means the stiffness, mass, Bl product have absolutely
no effect on the output, other than on the damping. The worst aspect is
sensitivity to air movements, but you are going to need a pretty
airtight box whatever you do.
The role of the mirror and laser pointer are to provide an optical lever
to sense the relative displacement of the cone and frame. You're
right about needing rotation.
Regards
Ian |
|
| Back to top |
|
|
Gregory L. Hansen
Guest
|
| Posted:
Wed Feb 16, 2005 8:02 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <11177u46ar7so96@corp.supernews.com>,
Phil Hobbs <pcdh@SpamMeSenseless.us.ibm.com> wrote:
| Quote: | Gregory L. Hansen wrote:
In article <11172qmr3egdf7d@corp.supernews.com>,
Phil Hobbs <pcdh@SpamMeSenseless.us.ibm.com> wrote:
Gregory L. Hansen wrote:
Another compact method, probably quite a bit easier to build than the
folded pendulum, is a superspring. You hang a real mass/spring
combination from the centre of a small loudspeaker, sense the mass's
position using e.g. a laser and split photodiode, and apply positive
feedback to the speaker using a dc amplifier. Overall loop gain should
be 0.99 or thereabouts. This reduces the effective spring constant by a
factor of 100, leading to a 10x lower resonant frequency for the same
physical size.
Supersprings, folded pendulums, geophones... all sorts of things I didn't
know existed. That's what's nice about a group like this.
I'm having a little trouble picturing the superspring. Are you sure it's
positive feedback? Feedback is a straight proportionality? If the mass
is supposed to stay still and the ground is moving, that would be sense
the position of the mass with respect to the ground?
It's definitely positive feedback. If you put a dot of paint 99% of the
way down a spring, that point moves 0.99 times as much as the end of the
spring does. The superspring replaces the upper 99% of the spring by
moving the mounting point of the bottom 1% in just the way the paint dot
would move. Negative feedback will effectively shorten the spring.
|
Oh, right. When the mass starts to move down, you want to keep moving it
down-- longer displacement, longer period. If it were negative, when the
mass moves down the spring would move up and pull the mass up with it.
Unfortunately, supersprings are also vehicle suspension, connectors, toys,
a theory of black hole thermodynamics... a lot of unrelated hits on
Google, and a reference to Rinkler's PhD thesis which I'm sure would cost
some money to get. But it doesn't seem so complicated once the idea has
already been explained.
| Quote: |
The limits on the spring length are basically stability and SNR--if your
loudspeaker isn't linear enough, it'll oscillate in high-gain regions
and have degraded response in low-gain regions. 0.99 is about as high
as you want to go, although you could probably do 0.999 if you
linearized the speaker really carefully, maybe with its own sensor and
feedback loop.
|
And could be made as complicated as you like.
| Quote: |
Really low frequency negative feedback (say another factor of 10 lower)
will stabilize the operating position of the mass without doing much
violence to the system response. Supersprings can have nasty turn-on
transients, so you have to be a bit careful designing one, but it's a
fun exercise. People have done it using a servomotor and a reel of
cable instead of the speaker. You can get a much larger deflection
range that way at the price of much more vibration.
|
I don't know about servomotors for seismology, but maybe the controlled
portion on the short end of a lever arm and the bouncing mass on the long
end. A factor of 10 shouldn't be so hard. But I gather supersprings are
used for a lot more than seismology. (And I'm not including leaf springs,
etc.)
--
"In any case, don't stress too much--cortisol inhibits muscular
hypertrophy. " -- Eric Dodd |
|
| Back to top |
|
|
Phil Hobbs
Guest
|
| Posted:
Thu Feb 17, 2005 12:05 am Post subject:
Re: Seismometers and Pickup Coils |
|
|
Gregory L. Hansen wrote:
| Quote: | In article <1108560675.382559@cswreg.cos.agilent.com>,
Ian <Ian_Buckner_not_@agilent.com> wrote:
"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:cut28c$4mm$2@rainier.uits.indiana.edu...
The next issue is what to use for a sensor, and how to calibrate the
whole system. A simple optical lever, geometry and a ruler will give
you this, and will let you see if you need to go any further (if the
measured amplitude of motion is negligible, you are done, if not then
ask again).
I don't understand the role of the mirror. A simple displacement won't
mean anything, it doesn't help unless the mirror rotates. But my
vibrations are too small to make water ripple, whatever amplitude that
might be.
1Hz rules out quite a lot of methods. Your original post said you thought
the vibration was around 50Hz, is that no longer the case?
Sometimes what I (allege that I) feel is like an unbalanced flywheel
spinning up and down, as low at 1/2 Hz or so.
But I'm not moving very quickly on this for a variety of reasons related
to time, money, logistics, and attention span. I wouldn't actually mind
very much if this phenomon stops before I'm ready to build something.
|
Another compact method, probably quite a bit easier to build than the
folded pendulum, is a superspring. You hang a real mass/spring
combination from the centre of a small loudspeaker, sense the mass's
position using e.g. a laser and split photodiode, and apply positive
feedback to the speaker using a dc amplifier. Overall loop gain should
be 0.99 or thereabouts. This reduces the effective spring constant by a
factor of 100, leading to a 10x lower resonant frequency for the same
physical size.
The SNR isn't improved by this idea, but the frequency response is--it's
a mechanical version of the old idea of driving a coaxial shield from an
emitter follower on the sensor.
Cheers,
Phil Hobbs |
|
| Back to top |
|
|
Phil Hobbs
Guest
|
| Posted:
Thu Feb 17, 2005 1:32 am Post subject:
Re: Seismometers and Pickup Coils |
|
|
Gregory L. Hansen wrote:
| Quote: | In article <11172qmr3egdf7d@corp.supernews.com>,
Phil Hobbs <pcdh@SpamMeSenseless.us.ibm.com> wrote:
Gregory L. Hansen wrote:
Another compact method, probably quite a bit easier to build than the
folded pendulum, is a superspring. You hang a real mass/spring
combination from the centre of a small loudspeaker, sense the mass's
position using e.g. a laser and split photodiode, and apply positive
feedback to the speaker using a dc amplifier. Overall loop gain should
be 0.99 or thereabouts. This reduces the effective spring constant by a
factor of 100, leading to a 10x lower resonant frequency for the same
physical size.
Supersprings, folded pendulums, geophones... all sorts of things I didn't
know existed. That's what's nice about a group like this.
I'm having a little trouble picturing the superspring. Are you sure it's
positive feedback? Feedback is a straight proportionality? If the mass
is supposed to stay still and the ground is moving, that would be sense
the position of the mass with respect to the ground?
It's definitely positive feedback. If you put a dot of paint 99% of the |
way down a spring, that point moves 0.99 times as much as the end of the
spring does. The superspring replaces the upper 99% of the spring by
moving the mounting point of the bottom 1% in just the way the paint dot
would move. Negative feedback will effectively shorten the spring.
The limits on the spring length are basically stability and SNR--if your
loudspeaker isn't linear enough, it'll oscillate in high-gain regions
and have degraded response in low-gain regions. 0.99 is about as high
as you want to go, although you could probably do 0.999 if you
linearized the speaker really carefully, maybe with its own sensor and
feedback loop.
Really low frequency negative feedback (say another factor of 10 lower)
will stabilize the operating position of the mass without doing much
violence to the system response. Supersprings can have nasty turn-on
transients, so you have to be a bit careful designing one, but it's a
fun exercise. People have done it using a servomotor and a reel of
cable instead of the speaker. You can get a much larger deflection
range that way at the price of much more vibration.
Cheers,
Phil Hobbs |
|
| Back to top |
|
|
Active8
Guest
|
| Posted:
Thu Feb 17, 2005 6:11 am Post subject:
Re: OT: was Re: Seismometers and Pickup Coils |
|
|
On Tue, 15 Feb 2005 23:24:59 -0500, Mark Jones wrote:
| Quote: | Just side note, I heard that scientists saw where the north pole had moved an
inch after the Asian Tsunami.
|
Which pole and in relation to which of <GEO MAG ZENITH>?
--
Best Regards,
Mike |
|
| Back to top |
|
|
Gregory L. Hansen
Guest
|
| Posted:
Thu Feb 17, 2005 3:33 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
In article <b2v811lr032icns6nad24ao6bpsmcb2pd3@4ax.com>,
Boris Mohar <borism_-void-_@sympatico.ca> wrote:
| Quote: | On Sun, 13 Feb 2005 03:34:47 +0000 (UTC), glhansen@steel.ucs.indiana.edu
(Gregory L. Hansen) wrote:
I'm starting to think about building a seismometer
Have you considered a head positioning mechanism from a hard drive?
|
Briefly. But I don't have a dead hard drive laying around, so I didn't
try very hard to learn the mechanical characteristics and decide whether
they're suitable.
--
"The main, if not the only, function of the word aether has been to
furnish a nominative case to the verb 'to undulate'."
-- the Earl of Salisbury, 1894 |
|
| Back to top |
|
|
Boris Mohar
Guest
|
| Posted:
Thu Feb 17, 2005 5:14 pm Post subject:
Re: Seismometers and Pickup Coils |
|
|
On Sun, 13 Feb 2005 03:34:47 +0000 (UTC), glhansen@steel.ucs.indiana.edu
(Gregory L. Hansen) wrote:
| Quote: |
I'm starting to think about building a seismometer
|
Have you considered a head positioning mechanism from a hard drive?
Regards,
Boris Mohar
Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca |
|
| Back to top |
|
|
|
|
|
|
FAQ
Memberlist
Register
Profile
Log in to check your private messages
Log in