Help required regarding optical motional feedback theories

[eclipse]

Hi all,

It's my first post and I'm asking for help already!

I'm doing my final year project on motional feedback loudspeakers and as an alternative / comparitive to the system I'm building (based on Jeff Macauleys work) I'd like to mention an optical alternative to an accelerometer.

Is there such a thing? I've seen mention of it on one other thread here but I can't find anything anywhere else on the subject?

Anyone got any ideas or links to ideas? Its not the concept I need help with but it would be interesting to know of any existing examples. I'd be very grateful for any help!

Cheers
Tim

[Svante]

Originally posted by eclipse
Hi all,

It's my first post and I'm asking for help already!

I'm doing my final year project on motional feedback loudspeakers and as an alternative / comparitive to the system I'm building (based on Jeff Macauleys work) I'd like to mention an optical alternative to an accelerometer.

Is there such a thing? I've seen mention of it on one other thread here but I can't find anything anywhere else on the subject?

Anyone got any ideas or links to ideas? Its not the concept I need help with but it would be interesting to know of any existing examples. I'd be very grateful for any help!

Cheers
Tim

I can predict one problem that you might run into. The sound from a driver is proportional to cone acceleration, not cone position. So, in your feedback loop you will have to take the second derivative of your transducer signal and regulate on this rather than the transducer signal itself. This will amplify the noise from your transducer, and probably you will have to limit the useful frequency range of the system because of this. This is also why accelerometers usually are preferred (at least I would guess so).

[Mudge]

I've had thoughts about doing this, the best way I could see to do it was to measure the doppler shift (a non-trivial task) of the reflected light and take the first differential of that.

[zelter]

Hello Eclipse,

I saw test of a T&A loudspeaker in a german hifi magazin several years ago. They used an optical position sensor in the woofer.
Infact it looked like a moving piece of plastic connected to the membrane. The end of the plastic had a triangular shape.
This "moving triangle" was situated between a optical sensitive resistor and a light source. The whole sensor was placed on the backside of the speaker magnet. The moving plasitc piece was connected to the membrane through the hollow pole piece.

Because I'm just working on a speaker feedback projekt using the voltage of a secondary voice coil I'm very interrested in all available material concerning speaker feedback. You mentioned the work of Jeff Macauley. Are copies of his work available in the web?

Greetings from Germany

Zelter

PS All native speakers - please excuse my English

[Al3x]

Originally posted by zelter
Hello Eclipse,

I saw test of a T&A loudspeaker in a german hifi magazin several years ago. They used an optical position sensor in the woofer.
Infact it looked like a moving piece of plastic connected to the membrane. The end of the plastic had a triangular shape.
This "moving triangle" was situated between a optical sensitive resistor and a light source. The whole sensor was placed on the backside of the speaker magnet. The moving plasitc piece was connected to the membrane through the hollow pole piece.

Because I'm just working on a speaker feedback projekt using the voltage of a secondary voice coil I'm very interrested in all available material concerning speaker feedback. You mentioned the work of Jeff Macauley. Are copies of his work available in the web?

Greetings from Germany

Zelter

PS All native speakers - please excuse my English

Yeah, that T+A design is the only one I've come across...

I would also be very interested in any info regarding the use of a secondary voice coil as the feedback sensor or if anyone has any copies of Jeff Macauley's design.

thanks

[GRollins]

You don't need to make it so complicated. The absolute position of the cone isn't relevant, per se. If you use any form of sensor on the motion of the cone, you'll end up with something proportionate to the signal. Just put that in as negative feedback, whether it comes from a mic, optical read, second voice coil, etc.
Optical sensing of the cone is a little more tricky than it appears at first, although it has its good points. Incidentally, the second voice coil strategy is convenient, but you're also going to get crosstalk from the driven voice coil so the feedback signal isn't as clean as you'd like.

Grey

[eclipse]

Thanks for all your responses - all helpful!

If anyone wants info on the Jeff Mcauley system I can sort you out there - just email me.

Cheers

Tim

[substrate]

Originally posted by Svante


I can predict one problem that you might run into. The sound from a driver is proportional to cone acceleration, not cone position. So, in your feedback loop you will have to take the second derivative of your transducer signal and regulate on this rather than the transducer signal itself. This will amplify the noise from your transducer, and probably you will have to limit the useful frequency range of the system because of this. This is also why accelerometers usually are preferred (at least I would guess so).

You're right that the sound from the driver is proportional to the 2nd derivative of the cone position. However, every signal can be considered as an infinite sum of sinusoids, and we all know that the 2nd derivative of sin(x) is simply -sin(x) [noting the negative term]. This implies that you can simply invert the position signal and you are given acceleration (out by a scaling factor).

As for the position sensor itself ... I wouldn't imagine that you'd be using this system on tweeters, rather large woofers with decent excursion levels ... thus I'd suggest a thin plastic rod with a small white dot on the end. A CCD (of sort) reading the position of the white dot, and inverting it, would be your feedback.

[Svante]

Originally posted by substrate



You're right that the sound from the driver is proportional to the 2nd derivative of the cone position. However, every signal can be considered as an infinite sum of sinusoids, and we all know that the 2nd derivative of sin(x) is simply -sin(x) [noting the negative term]. This implies that you can simply invert the position signal and you are given acceleration (out by a scaling factor).

Yes, for a single frequency, that would work. If several frequencies are involved, the amplitude relations between them need to be maintained, and to understand how this relation is affected by a double differentiation we need to take the 2nd derivative of sin(wt) would be -w^2 * sin (wt).
So, in order to acheive a flat response, you'd need a double differentiation of the position signal, otherwise your regulator would make the cone *amplitude* constant (independent of frequency), and the sound pressure would go as w^2 or +12 dB/octave. A simple sign shift wouldn't do.

[annex666]

I am currenly studying my second year of Electronic Engineering at York university and I was considering this very idea for my final year project.

I had found a company that makes laser-based optical distance measuring sensors, although the sampling frequency of these sensors is limited therefore the frequency at which you could apply this type of system is also limited. The error is also non-linear as they use a system whereby a mirror tracks the laser dot on the object they are measuring and the relative distance is based on the angle of the mirror.

Without looking into the matter further it would appear that using an accelerometer would be the most sensible thing to do, unless of course you intend on pioneering overcoming the problems that come with using lasers?

Best of luck with the project.