PSN-L Email List Message

Subject: RE: New subscriber
From: John Popelish jpopelish@........
Date: Mon, 28 Feb 2005 10:35:58 -0500

ChrisAtUpw@....... wrote:
> John Popelish wrote:
> > One thing that struck me about many of the sensor designs is their
> > lack of optimisation and sophistication.  
> > Either this means that the  sensor is not the limiting part 
> > of most designs or else it means that  considerable improvement 
> > is possible.
>     I don't know where you are finding the 'many'  amplifier designs? There 
> is one on Larry's website which uses LT1007s and is  optimised. You need to 
> optimise both current and voltage noise sources. LT1007s,  OP07s and OP27s can 
> all give satisfactory performance. If you wish to use  very long periods where 
> 1/f noise is a limitation, MAX432 and chopper amp  circuits are available.

I have my eye on the LT1007 for the front end amplifier.
> Don't confuse apparently simple with unsophisticated!  
> You are trying to get the amplifier noise a factor of 10 lower 
> than the seismic  noise. The ready 
> availability of inexpensive but powerful NdFeB magnets has  allowed the use of 
> smaller sensor coils and magnets with increased  sensitivity. 

I haven't addressed the amplifier design, yet.  I am focused on
getting the largest, cleanest possible signal out of the sensor before
thinking about amplification.
> > I think I have  come up with a more sensitive design
> > that also has noise cancelling capability 

> I suggest that you consider 1" square NdFeB  magnets in a quad formation, 
> NS opposing SN, in between two 1/4" thick  rectangular mild steel plates, say 
> 3.5" long by 2" wide. You wind a flat  rectangular coil to half cover the 
> magnet poles, say ~1" square.

I played around with this sort of approach, including using two guitar
pickup coils, but even those long rectangular coils do not use all the
wire to generate signal.  I think I can get more output with a dual
voice coil design, with one having the north pole in the center and
one having the south pole in the center.  I think I may be able to
approach 10,000 gauss field over the entire coil, with every bit of
the wire generating voltage.  This will also surround the coils with
the outer iron pole pieces as part of the shielding.  If I figure out
the file system, here, I may be able to upload a sketch of the magnet
and coil structure.             
>  AC hum is fairly low and is strongly filtered by  your 3 to 10 Hz 
> amplifier filters. 

Agreed, but I hope to eliminate most of the usual noise pickup (had
with the single coil sensor) with the dual coil approach.

> It is preferable to make the  seismometer arm and weight using 
> non magnetic materials. Stainless steel water pipe is quite useful
> for the arm and you can buy brass screw clamp fittings quite easily,
> to fit.  Don't use a knife blade or a point suspension.  
> Ball on a flat, crossed cylinder, crossed wire and crossed foil 
> suspensions are all OK.  Single wire and single foil (Cardan hinge) 
> may also be OK. See 

Thank you for these links.  
I haven't yet gotten very far into the design of the pendulum system.
What do you dislike about the knife edge hinges?  

(re: optical beam sensors)
> You might find some information to interest you at  
> You  can make OK optical sensors using large area photodiode pairs
> (7sq mm) and a tungsten filament lamp with either a resistance 
> or a voltage stabilisation  circuit. 
> Infra red LEDS change their output by about a factor of 5 
> at constant  current between 0 and 100 C, so you would need 
> to use additional photodiode stabilisation if you used one of them.
> I can get down to about +/- 15 nano  metres of 
> noise, or less if I reduce the bandwidth below 10 Hz.

Thanks.  I will get back to this problem, later.
> You can also use NdFeB magnet quads and an A3515 Hall Effect sensor.
> See
> Two  pairs of rectangular magnets, one SN
> and the other NS, are mounted on  parallel soft iron backing plates
> connected by mild steel bolts. The sensor is suspended in the 
> central field join.

I haven't considered Hall effect devices.  Do you have any idea how
stable and clean these are, compared to the optical approach, you
describe, above?
> There are also differential capacitor designs available 
> - if you need sub nanometre resolution. These are a subject 
> in themselves.
> Anyway, here is some "food for thought". 
> Can I suggest that you visit 
> and  download the last few years' letters? 
> There is a great deal of good information and experience 
> detailed therein.

I have been wading in, hip deep the last few days, but with many
people posting in multi part mime and html, the signal to noise ration
is pretty low.

Thank you for your advice.

John Popelish

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