Seismometers and Pickup Coils

[Gregory L. Hansen]

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.

 

[Robert Baer]

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.

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).

 

[Dave Garnett]

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

 

[Dave Garnett]

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

 

[Gregory L. Hansen]

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?

 

[Ian]

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

 

[Gregory L. Hansen]

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.

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.

 

[Fred Bartoli]

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.

 

[Rich Grise]

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

 

[Jeroen Belleman]

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

 

[Gregory L. Hansen]

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?

 

[Gregory L. Hansen]

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.

 

[Jeroen Belleman]

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

 

[Guest]

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

 

[Gregory L. Hansen]

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.

 

[Jeroen Belleman]

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

 

[Ian]

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

 

[Gregory L. Hansen]

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.

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.

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.

 

[Mark Jones]

Just side note, I heard that scientists saw where the north pole had moved an
inch after the Asian Tsunami.

 

[Ian]

I'd like to go to about a Hz.


Loud speakers make me nervous because I don't know the stiffness, voltage
versus velocity, the sensitivity, etc.


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