PSN-L Email List Message
Subject: Re: VRDT Experience
From: Brett Nordgren brett3nt@.............
Date: Thu, 18 Sep 2008 19:43:02 -0400
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?
At 12:52 AM 9/19/2008 -0600, you wrote:
>Does anyone have experience with Variable Reluctance Displacement Transducers
>(VRDTs)? Morrissey appears to have used them extensively and they seem to
>a number of advantages. They are inexpensive and easy to fabricate (so
>says). They are sensitive and have a low impedance output so parasitic
>wouldn't be a problem.
>It's true that they do produce heat, of course. Any other drawbacks?
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