## PSN-L Email List Message

Subject: Re: Optical siesmometer
From: chrisatupw@.......
Date: Fri, 18 Jan 2013 11:10:32 -0500 (EST)

```

From: Charles R Patton charles.r.patton@........
Sent: Wed, 16 Jan 2013 17:55
Subject: Re: Optical siesmometer

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On 1/16/2013 3:14 AM,      chrisatupw@....... wrote:
=20
=20
Subject: Optical seismometer
Date: Tue, 15 Jan 2013 15:33:51 -0600

Larry,
I thought you might find this interesting:
0J_Berger%20Optical%20Seismometer.pdf
-Charlie

Hi Charlie,=20
It looks as if Mark & Co have done quite a bit more development work.=
=20

=20
But amateurs would likely to have difficulty measuring optical fringes to=
=20
=20
1/2 ppm / Root Hz and Michaelson Interferometers are not cheap.=20
=20
Amateurs can get about 10 nano metres resolution over 10 Hz using large are=
a=20
=20
photocells and a stabilised light source, but this is likely to be adequate=
-=20
=20
unless you can 'lay your hands' on a couple of redundant Streckheisens !
=20
Regards,
=20
Chris Chapman
=20
=20
Take        a standard \$10 or so USB  web        cam --        definite=
ly a cheaper one, no autofocus, but rather one where you        can unscrew=
the lens easily.  It        will have a 640 x        480 or better =
resolution at a 60 Hz sample rate.   Use it in an optical lever arrangement=
on the        seismometer and        project a laser beam spot on the face=
of the sensor.  So how sensitive could it be?
=20
Let's        assume a 20" pendulum and a 20" optical lever length.   We're =
interested in        duplicating the        interferometer capability in th=

3e-7        * 1e-6 =3D 3e-13m=3D1.18e-12"   (see        pg       =
8). =20
=20
Although        I have a problem with this number.  They        describe a =
16 bit conversion so the number can't be much better        than:
=20
3e-7        / 65536 =3D 4.6e-11m=3D1. 8e-9" =20

****I'm a bit confused 3x10^-13 m =3D 1.18x10^-11 inches=20

I think that there may be 100cm in every metre and 2.54 cm in every inch...=
.....
=20
And I'm not sure why are we talking about inches when the seismometer is ca=
librated in metres ?
=20
So,        assuming the typical 1/3" sensor in the webcam.  Therefore
=20
0.33"        / 640 =3D 5.21e-4"     640        pixel        spacing=20
=20
5.21e-4"        / 256  =3D 2.03e-6"  due to interpolation from        the 8=
-bit analog        digitization

**** (0.33x25.4/640)x1000 =3D 13.1 microns=20

I quite don't understand where the 256 bit interpolation fits in ?=20
If the movement covered several pixels, you could reduce the pixel 'noise'.=
=20
If there is only one pixel concerned, you couldn't interpolate. =20
=20
As        the optical lever length is assumed equal to the pendulum        =
length, then for        small movements, the projected laser dot displaceme=
nt will equal        the pendulum        movement.=20
=20
**** One problem is that semiconductor lasers are VERY VERY noisy. Another=
is that the light output is highly temperature sensitive - a factor of ~x7=
between 0c and 100C.=20

So        the optical sensor is still a factor of 1000 away from the       =
interferometer.  Averaging        the sample rate from 60 Hz/16 down      =
to approx 4Hz, could add another 4x resolution improvement or        abou=
t:
=20
2.03e-6"        / 4 =3D 0.5e-6" =20
=20
Not        really close enough.  No        joy there. =20

****???? I don't think that it will make much difference. The pixel size / =
count stay the same - assuming a 'quiet' signal. If the noise was several p=
ixels (unlikely) you could reduce the error - but it would take more than a=
factor of x2 -> 4 counts !
=20
I        can't think of another major improvement to the resolution    =
except:
=20
1)        Maybe project the beam through a cylindrical lens that would     =
increase the deviation,        but also spread the beam so probably a wa=
sh.        =20
=20
2)        An optical lever distance of 1000 x 20" =3D 2e4"=3D 1667'.  I don=
't think so, unless        we did it with a        set of parallel mirrors =
spaced perhaps 2' apart and where we        allow the laser        beam to =
enter at an almost perpendicular angle to bounce back        and forth 800 =
times before exiting.  Mirror        loss per        bounce of 1% wo=
uld attenuate the beam by 8.         That shoudldn't be a problem -- just t=
he quality of the        mirrors would be        tough. =20
=20
****Umm ? Just 2^100 =3D 1.27x10^30 angular gain..... Do we want that much=
?

Any        other ideas? =20
=20
**** Larger photocells and more light ? The noise was about 13 nano metres =
using cheap BPW34 7 sq mm photocells, so we should be able to reduce it qui=
te a bit.=20
Alternatively, use either an LVDT or a capacitative sensor ?=20

Regards,
=20
ChrlsChapman                       =20
=20
=20
=20
=20
=20

From: Charles R Patton ch=
arles.r.patton@........

Sent: Wed, 16 Jan 2013 17:55

Subject: Re: Optical siesmometer

=20
=20
=20
=20
=20

On 1/16/2013 3:14 AM,
chrisatupw@....... wrote:

Subject: Optical seismometer

Date: Tue, 15 Jan 2013 15:33:51 -0600

Larry,

I thought you might find this interesting:

ations/T3-O5%20J_Berger%20Optical%20Seismometer.pdf

-Charlie

Hi Charlie,

It looks as if Mark & Co have d=
one quite a bit more development work.

But a=
mateurs would likely to have difficulty measuring optical fringes to
1/2 p=
pm / Root Hz and Michaelson Interferometers are not cheap.
Amate=
urs can get about 10 nano metres resolution over 10 Hz using large area
photo=
cells and a stabilised light source, but this is likely to be adequate -
unles=
s you can 'lay your hands' on a couple of redundant Streckheisens !<=
/tt>
Regar=
ds,
Chris=
Chapman

OK,

=20

Take
a standard \$10 or so USB  <=
/span>web
cam --
definitely a cheaper one, no autofocus, but rather one where you
can unscrew
the lens easily.  It
will have a 640 x
480 or better resolution at a 60 Hz sample rate.   Use it in an optical lever arrangement =
on the
seismometer and
project a laser beam spot on the face of the sensor.  So how sensitive could it be?

=20

Let's
assume a 20" pendulum and a 20" optical lever length.   We're interested in
duplicating the
interferometer capability in the Zumberge/Berger paper -- about:

=20

3e-7
* 1e-6 =3D 3e-13m=3D1.18e-12"&nb=
sp;  (see
pg
8).

=20

Although
I have a problem with this number.3e-7
/ 65536 =3D 4.6e-11m=3D1. 8e-9"&=
nbsp;

****I'm a bit confused 3x10^-13 m =3D 1.18x10^=
-11 inches

I think that there may be 100cm in every metre and =
2.54 cm in every inch.......

=20

And I'm not s=
ure why are we talking about inches when the seismometer is calibrated in m=
etres ?

=20

So,
assuming the typical 1/3" sensor in the webcam.  Therefore

=20

0.33"
/ 640 =3D 5.21e-4"  &n=
bsp;  640
pixel
spacing

=20

5.21e-4"
/ 256  =3D 2.03e-6"<=
span style=3D"mso-spacerun: yes;">  due to interpolation from
the 8-bit analog
digitization

**** (0.=
33x25.4/640)x1000 =3D 13.1 microns

I quite don't=
understand where the 256 bit interpolation fits in ?

If the moveme=
nt covered several pixels, you could reduce the pixel 'noise'.

If there is o=
nly one pixel concerned, you couldn't interpolate.

=20

As
the optical lever length is assumed equal to the pendulum
length, then for
small movements, the projected laser dot displacement will equal
the pendulum
movement.

=20

**** On=
e problem is that semiconductor lasers are VERY VERY noisy. Another is that=
the light output is highly temperature sensitive - a factor of ~x7 between=
0c and 100C.

So
the optical sensor is still a factor of 1000 away from the
interferometer.  Ave=
raging
the sample rate from 60 Hz/16 down
to approx 4Hz, could add another 4x resolution improvement or

=20

2.03e-6"
/ 4 =3D 0.5e-6"

=20

Not
really close enough.  No
joy there.  <=
/div>

****???? I don't think that it will make much diffe=
rence. The pixel size / count stay the same - assuming a 'quiet' signal. If=
the noise was several pixels (unlikely) you could reduce the error - but i=
t would take more than a factor of x2 -> 4 counts !

=20

=20

I
can't think of another major improvement to the resolution
except:

=20

1)
Maybe project the beam through a cylindrical lens that would
increase the deviation,
but also spread the beam so probably a wash.

=20

2)
An optical lever distance of 1000 x 20" =3D 2e4"=3D 1667'.  I don't think so, unless
we did it with a
set of parallel mirrors spaced perhaps 2' apart and where we
allow the laser
beam to enter at an almost perpendicular angle to bounce back
and forth 800
times before exiting.  Mirror
loss per
bounce of 1% would attenuate the beam by 8.
That shoudldn't be a problem -- just the quality of the
mirrors would be
tough.

=20

****Umm=
? Just 2^100 =3D 1.27x10^30 angular gain..... Do we want that much ?<=
/span>

=
Any
other ideas?

=20

**** Lar=
ger photocells and more light ? The noise was about 13 nano metres using ch=
eap BPW34 7 sq mm photocells, so we should be able to reduce it quite a bit=
..

Alterna=
tively, use either an LVDT or a capacitative sensor ?

=

=20

Regards,

Chrls
Chapman
=20
=20
=20
=20
=20
#AOLMsgPart_1_36365860-fc1b-4226-ae48-c04f2d3e4c02 td{color: bla=
ck;}

=20

```