PSN-L Email List Message

Subject: Re: Seismic Sensors
From: Geoff gmvoeth@.........
Date: Sat, 30 Mar 2013 17:22:52 +0000


To go a bit further, you might make such a thing
be linear by the geometry of the plates meshing together.
Also, increasing area you can increase the space.
Also, this is an analog and not a digital solution.
To get rid of noise like they did with Video I think
some kind of digital sensor is in order.
Direct reading of the X/Y axis of a single illuminated pixel
might possibly provide this digital solution.
You are looking at jumps of discrete distances
and nothing in between on a basic algebra graph
which is in fact the sensor itself.
Such a capacitive sensor would work like a capacitive
keyboard with a moving mass pressing only one key at a time
according to relative motion.
We got to think micro switches here and not
those purely analog amplitude oriented things.

If you get my drift (chuckle).

I feel really stupid saying these things even tho I know what I'm 
looking for.


On 3/28/2013 5:38 PM, Brett Nordgren wrote:
> Geoff,
> The guys who build most of the seismographs have found that 
> capacitance sensors generally worked the best.  However, in order to 
> make them sufficiently sensitive, the spacing between the plates needs 
> to be very small.  In his latest FBV design Dave is using 0.015" plate 
> spacing.  But, doing that requires you to have some sort of active 
> feedback mechanism to keep the plates from moving.
> What you do is take the output of the capacitance sensor circuit, 
> amplify it a whole lot and feed it back to a coil/magnet which pushes 
> in the right direction to force the boom to stay centered. Doing that 
> means that the boom will barely move when a quake comes along, so the 
> plate gap hardly changes at all.
> Then, you take the electical signal that the loop generated to resist 
> the boom motion, modify it a bit to get it to represent ground 
> velocity, maybe amplify some more and presto, you have a very 
> sensitive velocity-responding instrument.
> In practice, amplifying the signals is easy, the hard part is to 
> design the electronics and mechanism to have extremely low 
> internally-generated noise which is really the limiting factor when 
> considering sensitivity.
> Regarding the optical lever.  To be equivalent to what the FBV's see, 
> which generally is < M 5.0, if the quakes are pointed in the right 
> direction, you need 10 pixels per 8nm of ground displacement.  Using a 
> 3Mp 1/3" camera chip (~4.9E-6 m/row), you'd need a ratio of about 
> 6220.  But by *far*!! the biggest problem will be keeping the light 
> beam from wandering off the chip.  Also the instrument response would 
> probably be flat to displacement, which tends to accentuate the high 
> frequencies (mostly noise) at the expense of the slower waves you are 
> looking for.
> Regards,
> Brett
> At 12:08 PM 3/28/2013, you wrote:
>> Let us say we are looking for EQ at M5 or above from anywhere in the 
>> world
>> and not the 4.5 of a Richter device.
>> How much mechanical advantage must we have to detect this
>> motion using a simple lever arm and lets say bouncing a laser light 
>> between two
>> surface silvered mirrors ?
>> I think you might get a mile of mechanical advantage this way.
>> Talking about increase of motion and not the force of a lever.
>> The ratio between the two ends of the fulcrum is of
>> what I speak for mechanical advantage of motion.
>> All the amplification happens, with this ratio.
>> Two opposed mirrors are a decent way to keep things relatively small.
>> Like sitting at the Barber shop looking at your reflection
>> fading into the distance many times over.
>> I just did this very same thing with my web camera
>> pointing into the monitor.
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It is by the Entropy, That I set my mind in Motion.
Entropy is the seeking of equilibrium.
It is by the Entropy that, I set my mind in motion.


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