PSN-L Email List Message

Subject: Re: Volksmeter sensor and the AD774x CDC chip
From: Larry Cochrane lcochrane@..............
Date: Wed, 19 Apr 2006 23:40:41 -0700

Hi Chris,

ChrisAtUpw@....... wrote:
> In a message dated 18/04/2006, lcochrane@.............. writes:
>     The discussion last week about the AD7745/6 chip was rather timely.
>     The event files that I have been posting on the event file archive
>     system with the file extension of *.LCTST.PSN are from the new
>     Volksmeter sensor. If you compare the LC8 channel of my S-G sensor
>     and the new Volksmeter sensor you will see they are similar since
>     they both use a short period pendulum and a displacement pickup.
> Hi Larry,
>     What period compensation are you claiming for this sensor?

>     Do the plots show a compensated velocity / compensated displacement 
> / uncompensated signal, or what?
>     How are you performing the velocity feedback / damping necessary to 
> stabilise the response?

There is no period compensation in either the SG sensor or the Volksmeter (VM). Both 
use simple damping. The SG sensor has a feedback loop but it's just there to damp the 
pendulum. It might make the sensor more stable do to the feedback loop, but I'm not 
sure of that. The VM sensor uses eddy-current damping and has no feedback system. 
Randall will be sending a response to this shorty.

The plots are from the raw data right out of the sensor, but with a 60 second 2 pole 
high-pass and 2 Hz 2 pole low-pass filter applied to the data before making the GIF 
image. The event files are the raw data from the AD7746 chip.

>     I note that in your SG EMail reference, you claim a flat response 
> from 50 sec to 3 Hz, but remarked that you had to substantially increase 
> the damping to get this. I note that in your latest circuit diagram, you 
> seem to have greatly reduced the itegration time and wondered if the 
> circuit values were correct?

Like the VM channel (LCTST) my LC8 channel is pretty much the raw data from the 
pickup except there are two 60 second high-pass filters in the signal path. My LC3 
channel, the integrated output, should have a velocity response from about 1 second, 
the period of the pendulum, to about 50 seconds. I have not made any changes to my SG 
sensor for many years so the value of the integrator parts should be the same.



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