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'Acceleration sensors as seismometers'
1996\08\21@125341 by Bernard Seront

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>1. What resolution is typ. for an airbag sensor?

I don't know what your mean by that. The typical resolution of the ADXL05 is
0.005 g. This value, which is the noise floor of the sensor, is actually RMS
and statistically evaluated, and depends on the frequency range you want!
Look at the data sheets for more details.
If you want to know what resolution is needed for a "deceleration" sensor
suitable for car crash detection: I don't know. A quick calculation make me
think you can have 50 g or more in a crash (seems a lot, but my math looks
ok, does this make sense?).

>2. Would any of these be sensitive enough for use in a seismometer?

You read the article in Scientific American, didn't you?

In short: yes and no! It will make (the ADXL05) a very good "strong motion"
sensor (close-by earthquakes), but may be quite limited for recording
teleseisms (i.e. far away; a Magnitude~6 earthquake can be recorded
everywhere on earth with "amateur" equipment).
The article in Scientific American was somehow misleading on that by not
giving enough background information.

A seismic wave is an elastic movement of the ground, and can be recorded as
a displacement, a velocity or an acceleration. The problem lies in the huge
range of displacement and frequencies encountered: <0.01 mm to >10 mm (I'm
not speaking of the permanent movement or deformation on the fault: that can
be tens of meters), and >10Hz to <0.01Hz. The required sensibility for
velocity or acceleration sensors depends on the frequency. Here are some
typical figures:

       1mm     10mm
10 Hz   .04g    .4g
1 Hz   .004g   .04g
.1 Hz   .0004g  .004g

If you want the detailed math on these values read: Bruce Bolt: Earthquakes,
1993. I recommend this book as a very good (and simple) introduction to
earthquakes and seismology.

To understand what these values mean you have to know that 10 mm at 10Hz is
on the order of what you can get at 100 to 200 km of a strong earthquake (M>6).
But high frequency waves are more attenuated than low frequency ones when
they travel in the earth. The same M>6 earthquake recorded 2500-3000 km away
will mainly show waves in the range <<1 to (maybe) 5 mm at 1 to <0.01Hz; the
larger displacements being at the lower frequencies.
This is clearly out of spec. for the ADXL05. Note however that you can
improve the resolution of the ADXL05 by limiting the frequency range (low
pass), averaging a couple of readings ... but that won't give you .0004g
resolution, I think.

In conclusion: if you live in a seismically active area and/or want to
record local activity: the ADXL05 is suitable. Otherwise (teleseisms) you
may be better of with a velocity sensor (type "Lehman", check
http://psn.quake.net/equip.html for details). Here an ADXL05 is a good
complement in case of strong/close earthquake (which will be out of range
for the velocity seismometer). Another advantage of the ADXL05 is that it
can work vertically (not possible for a "pendulum" type seismometer like the
Lehman).

Bernard.


-----
Bernard Seront, spam_OUTserontTakeThisOuTspamseism1.ess.sunysb.edu (MIME ok)
http://rock.ess.sunysb.edu:8080/

1996\08\22@010626 by Steve Childress

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part 0 480 bytes
I didn't see the Scientific American article. I recall one from years ago re
seismometers using a pendulum and voice coils or some such.

I wonder if I can get a donated (junked) seismometer from the state or an oil
company?
Something  I can interface to my PC.

I built a complicated microprocessor data logger box which catches the weather
data on my old Heathkit WX station - and sends in via RS232 to the PC on demand.
Been using it for years.

regards, steve childress





1996\08\25@183603 by Scott Stephens

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For sensing the very low frequency accerlerations of siesmic activity, you
might consider using a laser diode. A laser can be used to monitor
vibrations (velocimeter or 'laser-bug') by knife-edge detection from
reflection off a mirror, or by use as an interferometer. Not an OEM
application for sure. A cheap diode laser, a  mirror, lens, razor blade,
photo-transistor and amplifier are required. Or a junk CD player has great
optics.

I also found that by directly reflecting light back into the laser diode,
the power-regulating photo-diode would provide an interference signal.
Nanometer displacemnts of a piezo ceramic disks could be measured this way.
I think it took around 40db of voltage gain to get a clear audio signal.

I have a feeling that measuring sub-hertz frequencies will require
temperature regulation and insulation from rapid temperature changes, and
compensation.

1996\08\25@203730 by Steve Hardy

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> From: Scott Stephens <.....stephnssKILLspamspam@spam@PYROTECHNICS.COM>
>
> For sensing the very low frequency accerlerations of siesmic activity, you
> might consider using a laser diode. A laser can be used to monitor
> vibrations (velocimeter or 'laser-bug') by knife-edge detection from
> reflection off a mirror, or by use as an interferometer. Not an OEM
> application for sure. A cheap diode laser, a  mirror, lens, razor blade,
> photo-transistor and amplifier are required. Or a junk CD player has great
> optics.
>
> I also found that by directly reflecting light back into the laser diode,
> the power-regulating photo-diode would provide an interference signal.
> Nanometer displacemnts of a piezo ceramic disks could be measured this way.
> I think it took around 40db of voltage gain to get a clear audio signal.
>
> I have a feeling that measuring sub-hertz frequencies will require
> temperature regulation and insulation from rapid temperature changes, and
> compensation.
>

Interesting idea.  The technique described above measures absolute value
of relative velocity.  To be useful, the sign of the velocity needs to
be determined.  This can be done with quadrature decoding, which is easier
said than done.  Perhaps the easiest way to get the interference pattern
is to use a cube corner reflector and beam splitter.  Lateral and angular
displacements will have a much reduced effect.

Unfortunately, diode lasers don't have very good coherence length so they
can't be used over long path length differences.  HeNe tubes are quite
good over distances less than the tube length.  Once it is warmed up,
the tube is very stable e.g. my 5mW Hughes tube exhibits shift of less than
10ppm in wavelength over 1 min.  This slow (and steady) variation would
not be important for seismic events.

Regards,
SJH
Canberra, Australia

1996\08\26@084852 by Mark A. Corio
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In a message dated 96-08-25 18:46:53 EDT, you write:

>I also found that by directly reflecting light back into the laser diode,
>the power-regulating photo-diode would provide an interference signal.

Sending the laser light back into the diode sets up an external cavity and
the laser may go unstable.  Also, if the back-facet diode is used for
closed-loop output power regulation you may end up with the diode in a
run-away situation.  Be careful.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel:  (716) 328-5850 --- Fax:  (716) 328-1144
http://www.frontiernet.net/~rmi/

***** Designing Electronics For Research & Industry *****

1996\08\26@192803 by Steve Hardy

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> From: "Mark A. Corio" <McoriospamKILLspamaol.com>
>
> In a message dated 96-08-25 18:46:53 EDT, you write:
>
> >I also found that by directly reflecting light back into the laser diode,
> >the power-regulating photo-diode would provide an interference signal.
>
> Sending the laser light back into the diode sets up an external cavity and
> the laser may go unstable.  Also, if the back-facet diode is used for
> closed-loop output power regulation you may end up with the diode in a
> run-away situation.  Be careful.

Getting a bit off-topic, but... not a problem for the following reasons:
. external cavity has less than unity gain i.e. loss.
. the injection efficiency is so low that the photodiode should be able to
compensate.
. the coherence length of a laser diode is only of the order
of 10x cavity length so any external feedback would not correlate i.e.
it would have no net contribution to stimulated emission.

However I speak from the position of armchair theoretician.  The real world
may, indeed probably will, prove me wrong.

Regards,
SJH
Canberra, Australia

1996\08\27@135336 by Scott Stephens

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>> >I also found that by directly reflecting light back into the laser diode,
>> >the power-regulating photo-diode would provide an interference signal.
>>
>> Sending the laser light back into the diode sets up an external cavity and
>> the laser may go unstable.  Also, if the back-facet diode is used for
>> closed-loop output power regulation you may end up with the diode in a
>> run-away situation.  Be careful.

At the time, I was using a current regulator to supply the laser independant
of the photo-diode. Probably not too smart, yes I've blown out a few fragile
diodes, probably due to ESD or transients.

>. the coherence length of a laser diode is only of the order
>of 10x cavity length so any external feedback would not correlate i.e.
>it would have no net contribution to stimulated emission.

I never noticed the effect of coherence length. I noticed that around the
lasing threshold current (about 40 ma for this $5 NIR (Timeline Inc.) laser)
it was very noisey. A nearby mirror or white paper less that a few cm
provided a strong signal. Mirrors farther away, up to a couple feet, would
also work but much less well. Lots of gain was needed for the photodiode
audio spectrum amp.

Alignment was very critical and difficult. When in alignment, the slightest
pressure or movement near a desk or the tripod a mirror was mounted on,
resulted in a tone which oscillated in frequency. Exactly like 40KHz doppler
ultrasound or a microwave doppler  radar, execpt that the displacement
frequency deviation sensitivity was orders of magnitude greater :-)

Another effect I noticed was when sprayed freeze mist on the laser, it
chirped. I suspect that it might make a nice FM laser radar or lidar with
some good optics and a sawtooth current drive to thermaly chirp the laser
with current, much like I've read about with microwave gunplexor transcievers.

As far as practical vibration measurement, using knife edge detection with
an external photo-diode delivered far more signal to work with. It produces
photo-diode amplitude variation rather than frequency variation in
proportion to the vibration of the system.

A realy cool effect I noticed was when I exhaled in the path of the laser
beam, it was plainly audible as if I had blown into a microphone. I suspect
the laser bugs that are shown in movies and advertised are somewhat
effective, but practicaly require excellent vibration isolation at the
recievers end & alignment. The setup could 'hear' me walking across a
concrete floor to a steel desk or mirror tripod over 10 feet away.

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