PSN-L Email List Message

Subject: Re: Vertical Seismometer with Feedback, Transducer Question
From: Brett Nordgren brett3nt@.............
Date: Thu, 18 Sep 2008 11:05:59 -0400

Hi Barry,

I had been wanting to compare your clipping levels with the STS-2, which is 
spec'd at 26mm/sec p-p velocity up to 20Hz and 0.34g p-p acceleration above 
20 Hz.  I am wondering how that compares with what you are getting?  If you 
are clipping much below that, you might want to look at whether you can 
implement some circuit changes to make it higher.  Having made the clipping 
level as high as possible with full sensitivity, you could then look at an 
alternate reduced-gain output for the more local events.  If you are 
wanting to get much higher clipping levels than the STS-2 numbers, I 
suspect that you will probably have to give up on maintaining good 
teleseism sensitivity.

At 07:33 PM 9/17/2008 -0700, you wrote:
>Hi Brett
>After my email I played around with the various components. I have STM's 
>Mcad program, but I was able to present the data in Excel as you did.

I'm sure you had noticed that all the Mathcad seismo. analyses appear to 
contain the line:

"Td = Displacement Xducer time constant (~= 0)",

which actually means that all frequency dependent elements in the 
forward-directed portion of the loop have been ignored.  Since these are 
elements which strongly affect the tendency of the loop to oscillate, and 
which also include the 'inverse filter', required to obtain adequate loop 
gain near the low frequency corner,  they very much do need to be 
considered.  That could probably be done by adding complexity to the 
Mathcad equations, but I agree that including them in an Excel analysis 
should be easier to do.

>I currently have a max output of around 2000-2500 v/m/s for my sensors. I 
>was able to reduce this down to around 400 by reducing the mass to around 
>0.2 kg and reducing r from ~240000 to around 100000. I then adjusted the 
>other feedback components to get a good response at the ends of the pass 
>band and an appropriate damping value. I then could use this as one 
>output, increase output back up by ~6x  or more and make this my second 
>output. I'm not sure if a gain reduction of 6 is enough to make much of a 
>difference. Right now I'm "clipping" with a magnitude of 7.0 @ about 4000 
>km or 5.0 @ ~200 km. Looking at my log-log graph of distance vs counts a 
>gain reduction of 6 would correspond to an increase in non- clipped 
>magnitude of ~ 0.7 at a given distance. Hardly seems worth the bother.



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