PSN-L Email List Message

Subject: Re: VRDT Experience
From: ChrisAtUpw@.......
Date: Fri, 19 Sep 2008 19:42:23 EDT


In a message dated 2008/09/19, barry_lotz@............. writes:

> I have been running three sensors with the smt8 type triple feedback. The 
> ~30" vertical with woofer coil and LVDT seems to work very well. I oscillate 
> the LVDT with a 6500hz phase shift oscillator. I have a 10 Hz single pole 
> passive low pass filter after the maxim dg419 demodulator. 

Hi Barry,

       Am I correct in assuming that your feedback is derived from the 10 Hz 
limited signal? The output of the demodulator needs to be limited to about 
1/10 the drive oscillator frequency. This smooths the response and it needs to be 
about 10x the final maximum response frequency.

       What low pass filter are you using on the final output?
 
I then run it thru a gain of 5 and into the triple feedback. The ~12" leaf 
spring 
> vertical uses the same 6500 hz oscillator with a VRDT and STM home built  
> feedback coil. I have been getting a large very low frequency oscillation 
> around 0.005 Hz. Looking at the winquake helicorder it doesn't last all day 
> (maybe 2-10 hours at a time) . I have been trying to see if it's environmental but 
> it doesn't appear to occur at the same time periods each day.Could this 
> oscillation have anything to do with the feedback? 

    The only circuit element which has a Tc about this is likely to be the 
integrator.

For others using the VRDT is degaussing the center element very important and 

> does this have to be done more than once? 

       There may be a small remanent magnetism in the Silicon Iron often used 
in transformer laminations. To get a really stable output level, the 
lamination should preferably be demagnetised. This only normally needs to be done once 
- unless the lamination comes in contact with a fairly strong magnetic field 
/ a permanent magnet / a DC voltage applied across the sensor coil windings.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/09/19, barry_lotz@............. writes:

I have been running three senso= rs with the smt8 type triple feedback. The ~30" vertical with woofer coil an= d LVDT seems to work very well. I oscillate the LVDT with a 6500hz phase shi= ft oscillator. I have a 10 Hz single pole passive low pass filter after the=20= maxim dg419 demodulator.


Hi Barry,

       Am I correct in assuming that your feed= back is derived from the 10 Hz limited signal? The output of the demodulator= needs to be limited to about 1/10 the drive oscillator frequency. This smoo= ths the response and it needs to be about 10x the final maximum response fre= quency.

       What low pass filter are you using on t= he final output?

I then run it thru a gain of 5 and into the triple feedback. The ~12" leaf s= pring

vertical uses the same 6500 hz= oscillator with a VRDT and STM home built  feedback coil. I have been=20= getting a large very low frequency oscillation around 0.005 Hz. Looking at t= he winquake helicorder it doesn't last all day (maybe 2-10 hours at a time)=20= .. I have been trying to see if it's environmental but it doesn't appear to o= ccur at the same time periods each day.Could this oscillation have anything=20= to do with the feedback?


    The only circuit element which has a Tc about this is lik= ely to be the integrator.

For others using the VRDT is degaussing the center element very important an= d

does this have to be done more= than once?


       There may be a small remanent magnetism= in the Silicon Iron often used in transformer laminations. To get a really=20= stable output level, the lamination should preferably be demagnetised. This=20= only normally needs to be done once - unless the lamination comes in contact= with a fairly strong magnetic field / a permanent magnet / a DC voltage app= lied across the sensor coil windings.

       Regards,

       Chris Chapman

[ Top ] [ Back ] [ Home Page ]