PSN-L Email List Message

Subject: Re: "EPICS" seismo suggestion
From: ChrisAtUpw@.......
Date: Wed, 18 Sep 2002 22:20:06 EDT


In a message dated 18/09/02, twleiper@........ writes:

> But it occured to me that if one were to use the plates as capacitors in a 
> pair of tuned circuits rather than as antennas, it would be a simple matter 
> to "lock" these
> two tank circuits in phase to a very high degree of precision using a 
> couple of counters (just like a freq counter) and apply any offset into a 
> force feedback
> 

       The capacitor plates have an inverse separation capacity relationship. 
Depends whether you want to compare frequency or period. Using moving cores 
in inductors might give a more linear relationship and allow larger movements 
of the pendulum.

> That original experiment breadboard is currently hacked up in preparation 
> for phase two, which is to use two coils and an iron bar with two 
> oscillators, thus avoiding the fixed reference and allowing all temperature 
> effects to be nulled out, as well as the precision tuning required to 
> synchronize with the crystal standard. Depending upon how far you want to 
> divide your oscillators down, you can also adjust the freqency response and 
> "sampling" rate to an incredible degree while achieving displacement 
> sensitivity that is difficult to calculate.

       You could cut a ferrite rod in half and use the halves to tune two 
coils. You might have to provide screening to prevent the oscillators from 
locking.

       You might put a flat ferrite plate on the pendulum in between two C 
ferrite transformer cores. You can buy coil formers with two equal sections 
for some core sizes.

       A CD4046 phase lock loop IC comes complete with two types of precision 
phase comparitors. This would allow continuous sampling. 
 
       Another alternative which avoids oscillator lock problems is to use 
the two coils in a variable reluctance switching oscillator and look for tiny 
changes in the mark / space ratio. 

       Regards,

       Chris Chapman   
In a message dated 18/09/02, twleiper@........ writes:


But it occured to me that if one were to use the plates as capacitors in a pair of tuned circuits rather than as antennas, it would be a simple matter to "lock" these
two tank circuits in phase to a very high degree of precision using a couple of counters (just like a freq counter) and apply any offset into a force feedback
loop. One could, of course use moving cores in inductors as well....


      The capacitor plates have an inverse separation capacity relationship. Depends whether you want to compare frequency or period. Using moving cores in inductors might give a more linear relationship and allow larger movements of the pendulum.

That original experiment breadboard is currently hacked up in preparation for phase two, which is to use two coils and an iron bar with two oscillators, thus avoiding the fixed reference and allowing all temperature effects to be nulled out, as well as the precision tuning required to synchronize with the crystal standard. Depending upon how far you want to divide your oscillators down, you can also adjust the freqency response and "sampling" rate to an incredible degree while achieving displacement sensitivity that is difficult to calculate.


      You could cut a ferrite rod in half and use the halves to tune two coils. You might have to provide screening to prevent the oscillators from locking.

      You might put a flat ferrite plate on the pendulum in between two C ferrite transformer cores. You can buy coil formers with two equal sections for some core sizes.

      A CD4046 phase lock loop IC comes complete with two types of precision phase comparitors. This would allow continuous sampling.

      Another alternative which avoids oscillator lock problems is to use the two coils in a variable reluctance switching oscillator and look for tiny changes in the mark / space ratio.

      Regards,

      Chris Chapman   

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