PSN-L Email List Message

Subject: Re: VRDT Experience
From: Charles Patton charles.r.patton@........
Date: Fri, 19 Sep 2008 00:55:31 -0700

I want to mention a couple of phenomenon that may have a bearing on your 

One aspect of a VRDT is that it exerts force on the vane and if the vane 
is not centered the force is not balance, being attracted to the closer 
core.  This can also happen with capacitance sensors and has to be 
accounted for in very sensitive applications (like seismometers?).

The second point.  Among other things, some experimenters trying to make 
a Focault pendulum would drive the pendulum with a coil excited with 60 
Hz in series with a capacitor.  No active components were necessary.  It 
basically makes a motor armature out of the bob.  These drive systems 
have been known for years.  The Amateur Scientist column in the 
Scientific American discussed construction details many years ago.  Here 
you have the same conditions, a low frequency bob (long period) and a 
high frequency drive.  Additional evidence that this is possible, an 
experimenter took an RF coil in an oscillator tank tuned to the AM 
broadcast band, delicately balanced a small core so it could bob in and 
out of the coil.  It basically became a Bouncing Betty that modulated 
the coil frequency.  He could listen to it on the radio and use it as a 
seismic detector by hearing the change in the rhythm.  The point here is 
that given a resonant spring system interacting with an RF source, it 
went into physical oscillation.  Sounds suspiciously like your VRDT 
oscillation problem.  The other point is that it isn’t because the 
feedback loop is unstable, it’s a basic motor driving reaction.

Charles Patton

Barry Lotz wrote:
> Hi Brett
> 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. 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? For others using the VRDT is degaussing the 
> center element very important and does this have to be done more than 
> once? I did make a circuit diagram using easytrax which I can email you. 
> More questions later.
> Regards
> Barry
> --- On *Thu, 9/18/08, Brett Nordgren //* wrote:
>     From: Brett Nordgren 
>     Subject: Re: VRDT Experience
>     To: psn-l@..............
>     Date: Thursday, September 18, 2008, 4:43 PM
>     Chuck,
>     The biggest drawback that I can see is that they don't seem to do too well 
>     with higher drive frequencies.  I usually see them run at 5-6kHz or so, 
>     while some other sensor types are happy at much higher frequencies.  After 
>     you demodulate the signal, it contains a large second harmonic component 
>     along with the higher even harmonics, and you need to filter it to recover 
>     the baseband seismic signal.  But in a feedback seismograph, that filter 
>     can introduce phase shifts which tend to make the loop oscillate.
>     If you move your carrier up to 20kHz, you can design a filter which cuts 
>     off at a higher frequency and which, as a result, doesn't add as much phase
>     shift in the 10-30 Hz region where loop oscillations are an issue.  The 
>     STS-2 uses a 20kHz carrier and a 3rd order Bessel filter designed to cut 
>     off at 1600 Hz.  That filter adds less than two degrees of phase at 
>     30Hz--practically nothing, but it reduces the 40kHz 2nd harmonic to below 
>     0.018% of its starting value.
>     With a 5kHz carrier it becomes harder to fit the filter between the high 
>     frequency corner at 10-30Hz (gain crossover) and the 10khz second 
>     harmonic.  So I am not that fond of magnetic transducers.
>     If you do want to go that route and use 5kHz, a 3rd order filter that cuts 
>     off somewhere around 400Hz should be about optimum.  It would add about 5 
>     degrees at 20Hz, not too bad, and it should nicely filter the harmonics.
>     Does anyone know of a nice 400Hz 3rd order, low noise, Bessel filter design 
>     to use here?
>     Regards,
>     Brett

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