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Old 10th December 2001, 05:56 PM   #21
jduncan is offline jduncan  United States
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pkgum:

Designing a servo feedback system for a subwoofer, while being relatively simple, still requires some fundamental understandings that are a little beyond a quick list of steps you need to make it work.

Also, as noted in the thread specifically relating to this topic, no company could realistically produce a servo feedback system that matched people's systems. They definitely need to be custom designed (at least if you want any decent performance from them).
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Old 10th December 2001, 06:32 PM   #22
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what about measuring current & voltage used by the loudspeaker, and using a programmable chip to provide the correct feedback?

theoretically, the excursion should be able to be calculated if you know the current and voltage going through the speaker.

I may be wrong, this is wayyy out of my depth.
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Old 10th December 2001, 06:36 PM   #23
jduncan is offline jduncan  United States
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I'm interested in knowing people's reasons against avoiding accelerometers (except cost) for this task. They seem to me perfectly suited.

Any suggestions?
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Old 10th December 2001, 07:14 PM   #24
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downhere,

While it is true that you can piece together information about a system from current and voltage or some other set of information, it is difficult. The key is an accurate math model. All sorts of bad things can happen here, like noise gets blown all out of proportion and the like. Bad things don't have to happen, but often do.

jduncan,

Really what we want is position feedback (ignoring that acoustical info could theoretically be better). To get this from an accelerometer, we need to integrate twice. This typically makes the noise situation worse. This can be overcome, but I think that some people (myself included) would like to get direct feedback on what we want to control, cutting out the middle man.

Grey,

I was asking because I teach and troubleshoot control systems in the semiconductor industry. You had piqued my intrest on this topic I'm very curious to see what you'll come up with!

Erik
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Old 10th December 2001, 09:03 PM   #25
Won is offline Won
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Accelerometers can be made to be quite ridiculouly accurate. I believe ballistic missiles use accelerometers (doubly integrated) as a means of guidance for their entire trajectory. DC noise in accelerometers is rather trivially rejected, and white noise will average itself out upon integration (aka low-pass filtering).

Then there's always that observation that the integral of the integral of a sinusoid is that same sinusoid with inverse phase and some gain, so integration can be avoided entirely (I saw this in one of the accelerometer-sub articles).

The speaker driver itself "differentiates" the signal because it is essentially an inductive AC motor interacting with a mass-spring system.

Naturally, all of this must be properly modeled for an accelerometer-based subwoofer to be successful, but I believe it to be very straightforward. jduncan and I have spoken at length (off-line) about this...

-Won
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Old 10th December 2001, 10:15 PM   #26
jduncan is offline jduncan  United States
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"Then there's always that observation that the integral of the integral of a sinusoid is that same sinusoid with inverse phase and some gain, so integration can be avoided entirely (I saw this in one of the accelerometer-sub articles). "

The problem here, of course, is that the "some gain" varies with frequency and the superposition of the frequencies in the audio signal will not have a uniform gain you can just correct for.

This doesn't rule out compensating for this seperately in the circuit but I think it then becomes much simpler to simply do the double integration.

As also posted, this isn't incredibly difficult to do with very low noise.
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Old 11th December 2001, 02:27 AM   #27
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Samuel,
I'd thought about the second voice coil option, but my objection to that is coupling between the two windings. If there's any significant amount of flux bleeding from the driving coil, then you're not getting an accurate measurement of what the cone is doing. That's why I brought up the possibility of an entirely separate, self-contained sensor coil.
Besides, I want something that can be done--after the fact--to any driver, anywhere, with minimal modification to the driver(s). The minimal modification stipulation tends to knock out a number of potential schemes, like epoxying a straw or magnet to the cone.

Grey
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Old 10th January 2002, 12:18 AM   #28
Jacob is offline Jacob  Sweden
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Default Loudspeaker feedback

Quote:
Originally posted by downhere
what about measuring current & voltage used by the loudspeaker, and using a programmable chip to provide the correct feedback?

theoretically, the excursion should be able to be calculated if you know the current and voltage going through the speaker.

I may be wrong, this is wayyy out of my depth.
If you measure the voltage over the speaker (using a diff. amp)
you will get a voltage that is proportional to the back-EMF, but this voltage is alsp an unlinear function of the cone excursion. This is due to that BL-factor decreases with increased driver excursion. If one could linearize this function, a voltage proportional to the actual cone movement could be easily measured and fed back into the amplifier.

Not an easy task...

Jacob
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Old 11th January 2002, 07:04 PM   #29
weeghel is offline weeghel  Netherlands
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Default philips motional feedback

Have you guys ever thought of studying the principle used in the philips motional feedback range of speakers?

a search on google will turn up many results
this one is nice to start with:
http://www.homestead.com/whaan/files/pagemfb.html

I'm guessing you're already passed this stage, so don't shoot me for giving obvious info
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Old 11th January 2002, 07:12 PM   #30
subwo1 is offline subwo1  United States
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I can't believe how loaded that thing is with circuitry. That could speak against the use of accelerometers in the motional feedback system.
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