PSN-L Email List Message

Subject: Re: Seismic Sensors
From: Geoff gmvoeth@.........
Date: Thu, 28 Mar 2013 16:22:47 +0000

If you could place a point of light on a single pixel of a CCD
you might get such motion as I indicated in X,Y form directly
from the CCD simply by orientation of the chip.
It is not far from one pixel to the next on the surface of a CCD
or other such chip.

A CCD can be 1024X1024 or whatever the laws of physics
and the State of the ART will allow.

It might be difficult or impossible for the layman
to do but easy for the high tech industry.

Would be even better than I think what is called TEMS ???
that accelerometer thing.

Maybe it could be an electron beam and a magnet deflector ?

It is the lever arm doing all the amplification.

Direct reading of the X,Y position of whatever at some
useful sample rate which is the digital solution
which I seek here. A graph with a projection
of some kind upon a single coordinate of four quadrants.
It all seems so simple the final results, but getting there
is another problem which only industry may have the
resources to solve.
If you could put it all on one sensor chip ?????
The size of a CPU. It might be very cheap to create
and to buy.
I am no inventor but I see possibilities here
for a jump in favor of amateur seismology.

On 3/27/2013 10:50 PM, Charles R Patton wrote:
> OK, since I threw out the webcam idea --  a few notes.
> 1) Webcams only send a bitmap picture to the PC.  It's up to the 
> software on the PC what to do next.  And typically most of these pgms 
> do a compression such as JPG or other for transmission over the web.  
> It is not compressed coming from the camera other than realizing there 
> may be amplitude compression of highlights clipped due to the typical 
> 8 bit (256 levels) of the A/D process in the camera chip or the 
> submersion of the picture information in the noise floor due to 
> inadequate light.  Some chips have variable aperture (akin to an old 
> time camera shutter time) times to help with this problem.  The video 
> rate is not due to the typical TV standard of 30 frames a second of 60 
> interlaced fields a second (US standard. NTSC that is, 50 and 100 
> respectively in Britain and many other places.)  Earlier webcams 
> couldn't even hit 30 frames a second and they didn't have a field rate 
> as they don't interlace because they're just sending bitmap pictures 
> anyway.  (Interlace is a solution to a very old problem that started 
> with analog TV picture tubes combined with the retrace (flyback) times 
> associated with the magnetic scanning of the picture tube.) That being 
> said, optical mice hit higher update rates mainly because they're 
> working with smaller images and not sending that data to the PC. (See 
> the next item: 2).
> 2) The optical mouse idea has shown up before in conjunction with a 
> colleague of Randall Peters.  I have discussed it a bit.  I feel it 
> has a couple of problems and I have not heard back on those objections.
> A) The sensor in a mouse is typically only from about 16x16 to 30x30 
> pixels.  A webcam is around 640x480 -- a big difference that plays 
> into your eventual sensitivity and range.  I argue that if you're 
> using a webcam that digitizes to 8 bits you will also be able to 
> improve the position resolution beyond the 640x480 in the same way the 
> optical mouse does -- do a centroid calculation on the dot in the image.
> B) I'm only aware of a few early HP sensors that also allowed a jumper 
> configuration that would allow you to view through the USB interface 
> what the sensor was seeing.  A very necessary condition in light of 
> the next comment.
> C) The experiments I did on optical mice showed that the centroid 
> calculation had "slippage".  I.e., you place the mouse on a surface 
> and move it back and forth across the surface to a hard stop point.  
> The zero will drift indicating that the centroid calculation is 
> apparently rounding off some data and gradually moving the starting 
> data point.  This centroid calculation of the hills and valleys of the 
> roughness of the surface is converted to a distance moved number that 
> is added or subtracted to the running total.  That movement number is 
> then sent to the PC, not the actual picture data.  Since you have a 
> larger sensor in the webcam, you can constrain the centroid 
> calculation to an absolute position and this drift is not a factor.  
> Actual physical drift of the seismic sensor will still exist, but the 
> integration of the physical sensor zero can be set to hours in the PC 
> calculation with no problem until the laser dot drifts off the optical 
> sensor.
> 3) The original post of this question had a link to a video that 
> showed significant movement (noisy ground floor in the signal 
> processing sense?).  That being true, the movement was more than 
> sufficient to move across the laser dot across the surface of an 
> optical sensor in a webcam (some 8x6 mm).  That then being said, the 
> webcam sensor is sufficient to work with the source of the dot.  
> Whether the dot is of any use in a seismic sense is an entirely 
> different question and was not the thrust of this answer.  (I'm not 
> trying to start a flame war here, I'm just trying to contain the scope 
> of the discussion to the original question of, " to digitized the 
> dot's position?").  If it's of interest we can start another 
> discussion about the ultimate possible sensitivity of such a scheme by 
> noting the conversion factor of the physical sensor in converting 
> physical ground motion to light beam displacement and multiplying it 
> by the number of pixels per mm (approximately 80/mm.)
> 4) And as to the starting from ground zero I might suggest this link 
> for a possible starting point:
> It gets you past the USB and menus and allows you direct access to the 
> returned image for follow-on processing of the image.
> Regards,
> Charles R. Patton
> On 3/27/2013 5:28 AM, Brett Nordgren wrote:
>> The only problem I can see is that the sensitivity you typically find 
>> in a good instrument may be somewhat higher than what you are 
>> considering.  For example, with the FBV instruments being operated by 
>> Dave Nelson, a 2 Hz sinusoidal ground motion which registers 10 
>> counts peak-peak on Larry's A/D would represent a ground motion of 
>> ~8nm p-p or about 18x the 0.44nm diameter of a large atom 
>> (Potassium).  So one A/D count at 2Hz represents a motion of about 
>> 1.8x an atomic diameter.  And most seismic observatories using 24-bit 
>> A/Ds are about 5x more sensitive.
>> For a really good instrument, one needs to be thinking in terms of 
>> devices which can detect very small motions, though, in practice, an 
>> instrument's true sensitivity will be determined by its internal 
>> noise, not simply its bit-resolution.
>> Regards,
>> Brett
>> At 06:47 PM 3/26/2013, you wrote:
>>> On 26/03/13 23:56, Geoff wrote:
>>>> I like the idea of using a WEBcam or something to look at the red dot
>>>> then use a custom program to read the video and turn the graphic data
>>>> into rectangular coordinates of a standard four quadrant graph
>>>> with the point resting in the center of the graph
>>>> But to do this in near real time may not be possible for myself to do.
>>>> The max rate will be like 30Hz to 60Hz which is a video frame rate.
>>>> Video is usually encoded and needs to be uncompressed.
>>>> Uncompressing video takes lots of time to do so like a
>>>> series of BMP captures at whatever rate would be the thing to do.
>>>> You would take screenies at like 30 per second in a modulo fashion
>>>> then save all after an alarm and the series of like
>>>> 108000 BMP images would be decoded into a series of (X,Y) 
>>>> coordinates ??
>>> This sounds a bit like a modified optical mouse.
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