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Seismic Sensors
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mark03:

--- Quote from: radioactive on January 11, 2019, 09:44:51 pm ---The reason I brought up the video with the laser interferometer measurement earlier in the thread, is that it seems like it might be another way to pick up the displacement.  I was thinking that a voice-coil (speaker) with retro-reflective tape (like in the video) could be used for the correction.  I have no idea if this could be made to work as well as the capacitive type sensor (probably not I'm guessing), but it seems like it would also be fun to play with.  I also ran across something describing Michelson interferometer involving a couple of mirrors / beam splitter to get higher resolution measurements, but that looks like it would be a bit much for a first attempt.

--- End quote ---

You might have seen the same paper that I recall, where they used the guts of an STS-1 (?) retrofitted with optical position sensing, and DSP to keep track of the fringes using quadrature signals.  My recollection is that they were running this open-loop.  I have been assured by people who know this stuff much better than I do, that a digital control loop (for force feedback) is impractical because any realizable D/A will sacrifice too much dynamic range.  This may be an insoluble issue with the interferometric measurements.  OTOH with sufficiently low measurement noise you may be able to achieve what you want without force feedback... which I think was the point of that paper.

Ok, found the quote I was looking for, from Erhard Wielandt, Design considerations for broadband seismometers (http://ds.iris.edu/stations/seisWorkshop04/PDF/Wielandt-Design3.pdf)


--- Quote ---As an example for the way in which feedback can increase the dynamic range, we may consider a system whose displacement transducer has only two output states indicating whether the mass position is “high” or “low”. A suitably designed force-balance system would nevertheless generate a feedback current that is, in the time average, proportional to ground acceleration. The seismic mass would oscillate around its equilibrium position and the feedback signal would oscillate around the value corresponding to ground acceleration. This does not necessarily disturb the seismic record provided that the oscillations have a frequency above the seismic bandwidth or an amplitude below the desired resolution. A system according to the latter condition was in 1924 used by A. De Quervain and Auguste Piccard to stabilize the mass position of a 20-ton mechanical seismograph at the Swiss Federal Institute of Technology, by adding or subtracting every minute a few drops of water from a ballast tank on top of the mass. This system came amazingly close to being a digital seismograph – had they been able to make their feedback loop some 10 000 times faster and to count the number of water drops, then their system would have been the first (and so far only) seismograph with true digital feedback.

A digital feedback circuit oscillating at high frequency is used in modern Sigma-Delta A/D converters. It is also possible to design the feedback loop of a Sigma-Delta A/D converter around a mechanical pendulum as the summing point, thus creating a digital seismometer. The concept of digital feedback cannot, however, be fully realized because a digitally controlled forcer with an adequate dynamic range is not available. One would have to couple an electronic DA converter to an electrostatic or electromagnetic forcer, so the feedback force would still be generated by an analog signal; the dynamic range would be limited by the DAC to something like 140 dB (7 orders of magnitude). In comparison, the combination of an electromagnetic forcer with a solid-dielectric feedback capacitor, driven by an analog circuit, produces feedback currents over a range of 240 dB or 12 orders of magnitude, say from a noise level of 10^-14 A at 0.3 mHz to a clip level of 10^-2 A at 30 Hz. So it appears that the analog feedback loop of VBB seismometers cannot easily be replaced by a digital circuit.

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rhb:
FWIW An interferometer based seismograph produces an FM signal.  The larger the amplitude, the faster the fringes change.  Much better than trying to count fringes to just use regular FM demodulation.
radioactive:

--- Quote ---better than trying to count fringes to just use regular FM demodulation.
--- End quote ---

I noticed that right away (FM signal) from watching the applied science video too.. for the stable laser power supply.  Still *very* cool response for pretty small measurements.  Later in the video where it was demonstrated that wavelength could be manipulated via power supply, I started thinking about DSP and what might be done with that.  (even though he wasn't very optimistic about it being useful).  As mark03 pointed out, this is probably not something to be optimistic about for a closed loop system.

Thank you both for your inputs.  Much appreciated.  I'm still working on another unrelated project, but I hope to transition to playing around with a seismic sensor soon.  That capacitive inclinometer from the other thread is going to be the first thing I want to experiment with I think.


--- Quote ---Sorry, the more correct term was Optical Tweezers, Its how they measure extremly small forces


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@Rerouter,  sorry I thought you were being sarcastic or something.  That is a *very* interesting video.  Also wayyy out of my capabilities..
rhb:
Also look into the Zeeman effect with lasers.

http://wanda.fiu.edu/teaching/courses/Modern_lab_manual/zeeman_effect.html

HP uses this in their optical measuring system to derive absolute position from the interferometer fringes.  There's an HP application note describing it in detail.
max_torque:

--- Quote from: EEVblog on January 11, 2019, 02:38:56 am ---
Maybe I should try and ask if it's still there and get a tour?

--- End quote ---


It would be interesting to see that!  It's a field (should that be a "lake"?) i know nothing about and looks to have quite a few interesting challenges and solutions
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