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Old 1st July 2013, 06:45 PM   #11
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by wintermute View Post
Yes and in fact I saw a thread just a short while ago --> Low distortion, DSP based high gain servo controlled woofer controller.

XRK971 would you like it moved to subwoofers instead?

Tony.
I want to use this for full range drivers really but wanted people who know about amps and electronics to give input, which was why I posted it in amps forum at first. Lets keep it here in Exotics for now.
Thanks
X
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Old 1st July 2013, 06:50 PM   #12
xrk971 is offline xrk971  United States
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Interferometric methods are way too complicated (expensive) and do not handle vibration or large motions well. A fiber optic proximity sensor with LED and photodiode has DC to hundred MHz bandwidth. The linearity can be handled optically with a mask on on scattering surface.
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Old 1st July 2013, 07:00 PM   #13
SY is offline SY  United States
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Quote:
Originally Posted by DUG View Post
Accurate but difficult to get a linear signal for full cone excursion.
You're just counting fringes, so the linearity is immaterial. My first "real" project when I got out of school was designing a system like this for an FTIR system. The excursion of the voice coil coupled to the interferometer was 2cm or so. The difficulty with doing it cheaply back then was keeping the beam coherent over the travel length, but these days I would expect that you could pull it off with a laser diode and a few bits of optics. As mentioned by others, this won't account for cone breakup, but at least you can get the voice coil motion linearized.
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Old 1st July 2013, 07:59 PM   #14
DUG is offline DUG  Canada
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Quote:
Originally Posted by SY View Post
You're just counting fringes, so the linearity is immaterial. My first "real" project when I got out of school was designing a system like this for an FTIR system. The excursion of the voice coil coupled to the interferometer was 2cm or so. The difficulty with doing it cheaply back then was keeping the beam coherent over the travel length, but these days I would expect that you could pull it off with a laser diode and a few bits of optics. As mentioned by others, this won't account for cone breakup, but at least you can get the voice coil motion linearized.
Counting fringes...yes...I would like to see the circuit to convert that to a linear signal.

No, not sarcasm...I would really like to see the circuit.

Up/down counter and parallel input d-a? It would have to be fast and keep track of zero position.

I have some parts that could be used for this type of setup.

First surface mirrors and 50% transmission mirrors from cd/dvd drives.

Cone breakup...I would control close to the voice coil...cone breakup will happen with or without feedback.
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Old 1st July 2013, 08:16 PM   #15
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laser doppler velocimetry is well developed - direction ambiguity, zero sensitivity is helped with frequency offset - Bragg Acousto-optic modulators are often used

then you have a (many) MHz FM signal to demodulate linearly
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Old 1st July 2013, 08:28 PM   #16
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laser doppler velocimetry is well developed - direction ambiguity, zero sensitivity is helped with frequency offset - Bragg Acousto-optic modulators are often used

then you have a (many) MHz FM signal to demodulate linearly
This is sounding more complicated than an optical mouse.

I may just stick to 1GHz.

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Old 1st July 2013, 08:57 PM   #17
xrk971 is offline xrk971  United States
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Dug,
Good to hear from you in this forum. I used to do all this laser interferometry stuff in a past life. It's not the electronics that is hard as MHz counters are routine parts of many digital I/O cards, but what is tricky with interferometry is having a phase sensitive counter (two counters offset by known angle between 0 and 90 deg so you know direction of travel. Also the optics need to be really rugged and as someone else said, you can use diode lasers beam splitters etc. They will quickly grow in complexity and the speaker will be an optics experiment rather than a closed loop feedback experiment.

Regarding the cone breakup issue, I would use a very stiff cone and a motor with extra high modulation authority. Rely on the feedback loop to sledgehammer the cone movement to match the input signal.

I have built interferometers, spectrometers and fiber optic scattering proximity probes and I still think the best and most cost effective way is fiber optic coupled light scattering. I would put the dual fiber probe tip on the backside of the dust cap through the hole in the vented pole piece. It is nice and dark and you get direct movement of cone.
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Old 1st July 2013, 09:21 PM   #18
DUG is offline DUG  Canada
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Dug,

...is fiber optic coupled light scattering. ....
Greetings xrk

That is basically what I did with the failed positioning experiment...did not use optic cable on that one but I have some parts at work that I might be able to adapt to that.

One more thing on the list to try.

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Old 1st July 2013, 10:17 PM   #19
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I have built several systems with a piezoelectric tweeter crystal as an accellerometer.
Works really good!
use two wery thin twisted wires to the basket where you place an OPamp buffer. from there you can have a pretty long feedback cable to the servo cirquit/power amp.
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Old 1st July 2013, 11:16 PM   #20
xrk971 is offline xrk971  United States
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Esl63,
Neat idea to use accelerometer as sensor based on piezo crystal. Acceleration is two steps of integration away from position, which I think is the variable we want to use for feedback. Do you have any closed loop feedback applications you can share ?
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