PSN-L Email List Message

Subject: Re: Equipment: SG vs. Lehman...Hybrid
From: "David A. Latsch" blottobear@..........
Date: Fri, 25 May 2001 18:59:37 -0600


Hi Chris,

You are almost there-forget the photocells and use a CCD
linear array
and a laser beam reflected off the vertical pendulum. The
CCD array
gives you a clean, robust output that is a direct analog of
the POSITION
of the seismic mass. Hence it is not frequency-response
limited. It is
truly an absolute position sensor, rather than the classic
velocity
sensor with all its' attendant problems. I am cobbling up a
system
using the guts from an old HP Laser-Jet printer.  Another
similar
method is to monitor the relected laser beam with a simple
optical
interferometer and count the resulting fringe patterns. This
is done in the
USGS gravity observatory instrument here in Boulder. Now you
can
measure displacements down to to wavelength of whatever
color of
light your laser is!!

Best wishes-
Dave Latsch



ChrisAtUpw@....... wrote:

> In a message dated 25/05/01, travis5765@...........
> writes:
>
>
>> Although I have not tried it, it seems like it should
>> work. You would have
>> to make sure NO excess light fell onto the sensors
>> though.
>> By the way, I missed the page in scanning that told that
>> the output needs
>> only minimal amplification though it varies with the
>> rated output of the
>> solar panels and the consistency of the light output.
>> My idea for a modification would be to use a laser
>> pointer as a light
>> source. make sure to have a well regulated power supply
>> for it.
>
>       The larger area silicon photocells as opposed to
> 'solar cells' tend to
> be better matched as to sensitivity, change with
> temperature and reverse
> leakage. Solid state Lasers are very noisy indeed. LED's
> are also noisy and
> their output is quite temperature dependant. A tungsten
> bulb driven from a
> voltage stabilised source at about 3/4 it's rated voltage
> will give good
> performance. You also benefit from the Infra Red
> sensitivity of the Si cell.
> You can get good performance by butting two cells end to
> end and using a slit
> which exposes about half each cell.
>
>       Regards,
>
>       Chris Chapman


Hi Chris,

You are almost there-forget the photocells and use a CCD linear array
and a laser beam reflected off the vertical pendulum. The CCD array
gives you a clean, robust output that is a direct analog of the POSITION
of the seismic mass. Hence it is not frequency-response limited. It is
truly an absolute position sensor, rather than the classic velocity
sensor with all its' attendant problems. I am cobbling up a system
using the guts from an old HP Laser-Jet printer.  Another similar
method is to monitor the relected laser beam with a simple optical
interferometer and count the resulting fringe patterns. This is done in the
USGS gravity observatory instrument here in Boulder. Now you can
measure displacements down to to wavelength of whatever color of
light your laser is!!

Best wishes-
Dave Latsch
 
 

ChrisAtUpw@....... wrote:

In a message dated 25/05/01, travis5765@........... writes:
 
Although I have not tried it, it seems like it should work. You would have
to make sure NO excess light fell onto the sensors though.
By the way, I missed the page in scanning that told that the output needs
only minimal amplification though it varies with the rated output of the
solar panels and the consistency of the light output.
My idea for a modification would be to use a laser pointer as a light
source. make sure to have a well regulated power supply for it.

      The larger area silicon photocells as opposed to 'solar cells' tend to
be better matched as to sensitivity, change with temperature and reverse
leakage. Solid state Lasers are very noisy indeed. LED's are also noisy and
their output is quite temperature dependant. A tungsten bulb driven from a
voltage stabilised source at about 3/4 it's rated voltage will give good
performance. You also benefit from the Infra Red sensitivity of the Si cell.
You can get good performance by butting two cells end to end and using a slit
which exposes about half each cell.

      Regards,

      Chris Chapman


[ Top ] [ Back ] [ Home Page ]

Larry Cochrane <cochrane@..............>