Closed-Loop Amplification with Driver Cone Position Fedback - Page 4 - diyAudio
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Old 4th July 2013, 11:56 AM   #31
oshifis is offline oshifis  Hungary
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There is also the bridge technique by getting the motion induced voltage from the voice coil (no second coil needed) and creating an error signal.
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Old 4th July 2013, 01:27 PM   #32
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Some have claimed that they have used it successfully and some say that it didn't fair well.

jer
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Old 7th July 2013, 03:37 AM   #33
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Quote:
Originally Posted by oshifis View Post
There is also the bridge technique by getting the motion induced voltage from the voice coil (no second coil needed) and creating an error signal.
Works great. Works on all woofing drivers. Simple. Coherent. No little gizmos with their own distortions to glue to the dust cap.

Anybody can mock-up a bridge and derive an error signal and see if they'd like to correct it.

Ben
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Old 7th July 2013, 11:02 AM   #34
xrk971 is offline xrk971  United States
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More info on the bridge technique please. How do you separate voltage from amp vs motion induced voltage?
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Old 8th July 2013, 08:15 PM   #35
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By negative output impedance...
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Old 8th July 2013, 08:38 PM   #36
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More info on the bridge technique please. How do you separate voltage from amp vs motion induced voltage?
A good question. Essentially, you don't.

If you read the signal across a small resistor in series with the driver, you are looking at the impressed voltage less driver static impedance, less the driver motional impedance (AKA back EMF).

Reading it in a bridge (balanced for static impedance (not really correct to call it DC resistance)), you remove the static component and the impressed signal and therefore enhance the motional component. See first diagram:
Patent US5542001 - Smart amplifier for loudspeaker motional feedback derived from linearization ... - Google Patents

The reason you don't see anything special eyeballing the final signal is because that's how feedback works. You take the whole final signal and subtract it from the source signal (way, way, upstream). The final signal doesn't look much different to your eyeball but when subtracted, the magic happens.

Back to your good question. So the problem is that it looks on the scope only wee bit different than the impressed voltage unless you have sky-high distortion in the driver. So you need to analyze it by removing the fundamental with a spectrum analyzer, distortion tester, XY plot, or what I used for many years, home-brew twin-tee filter.

I'm just an amateur and maybe others can do a better job clarifying.

Helpful?

Ben
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Last edited by bentoronto; 8th July 2013 at 08:43 PM.
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Old 8th July 2013, 08:59 PM   #37
xrk971 is offline xrk971  United States
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Ben,
Thanks for link to patent. With Dsp all this can be done digitally - may be a great application for a mini DSP in addition to the usual digital crossover function folks use it for.
X
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Old 14th July 2013, 07:11 PM   #38
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Are you still looking for a way to sense the position of the cone directly?

Here is my idea to do this using an LED emitter of the right peak wavelength and several simple CdS photoresistor detectors. Place the LED on the underside of the cone near the VC former connection and position a "vane" or "light shield" a small distance away. Position the array of photoresistors so that at rest half of them have the LED light blocked by the vane. As the cone moves up and down the position of the edge of the shadow will change, and more or less total area of the photoresistors will be illuminated. The outputs from all photoresistors can be summed together to give a composite "total" voltage that will go up and down with cone position. This needs to be correlated with actual excursion, but should be linearly related. At maximum excursion at least part of one photoresistor must be exposed to the light source.

The photoresistors must overlap so that there are no zones where a change in the position of the shadow from the vane does not change the total summed output. This can be done by using two nesting columns of photodiodes, one positioned down by half the diameter of the photoresistor, like the ROWS in this image:
Click the image to open in full size.

-Charlie
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Old 14th July 2013, 11:06 PM   #39
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Charlie - yes, advantages to working with light but all kinds of mechanical complexity involved in your mask and matrix-board as well.

Do you suppose your matrix of photodetectors will have and order of magnitude less distortion (that is, will be more precisely linear) than a $200 woofer that is the product of 75 years of ardent development to make it linear?

On the other hand, easy to imagine capacitative sensors that only need to sense distance and the linearity resides in properties of the air gap!

Ben
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Old 15th July 2013, 12:16 PM   #40
Calvin is offline Calvin  Germany
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Hi,

the problem with such photo-sensors would probabely be to achieve a sufficient dynamic range -or, the other way round- sufficient resolution.
T&A tried it with optical Sensors (kind of optical fork sensor) and a wedged or conical bezel fixed to the speaker dustcap, protruding through the magnet core to the inside of the casing. A hood shielded the structure against air pressure from the casings inside. Depending on the cones position the bezel shaded the sensors light source and such a signal proportonal to the displacement/position could be generated. They later omitted with this, probabely due to complexity and resolution issues.
Another method is to measure position with a very fine light-dot passing over a PSD (position sensing device) which is basically a flat light-sensitive resistor which detects the position of the dot over its sensing area. Again resolution/dynamic issues are seriously limiting factors.

jauu
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