Laser Instead of an Accelerometer? Is it possible ?

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OK...I'm in way over my head here, but that hasn't stopped me from spouting off before, so why not now :)

Howz about a triangular strip of reflective foil that is as wide as the beam at one end , and diminishing to zip at the other that is the length of the throw of the woofer (more or less, depending on the angle of the dangle) with a photo receptor receiving the reflection…Voila…amplitude modulation.

Whatcha think ?

-Casey
 
poobah said:
.... Just pick reasonable numbers: 6" driver, 400 Hz, at say 65 dB SPL. What's the peak to peak peak displacement?...

According to J.Borwick in Loudspeaker and Headphone Handbook:

x(peak)=(1180*10^(SPL/20))/(f^2*a^2)
at 1m
f - frequency
a - cone radius (mm)

for 6'' driver, a=65mm, f=400Hz and SPL=65dB,

x(peak)=3um (for the theoretical "rigid piston")

poobah said:
... about 1 um resolution? Is that enough for a LF driver?

The resolution depends on the LF driver's max frequency, cone diameter and minimum SPL we wish to measure/record at max. frequency.

Regards,
Milan
 
>I am so confused according to >http://www.rythmikaudio.com/servo_survey.htm accelerometer is not the best, they claim that their technology is better.

Our technology indeed is sensing coil based. I have a lot of reasons to believe accelerometers and laser are the best. We call our technology directservo because it is direct a lot of things. First, it is a direct conversion sensor. That is the sensor signal is derived directly from velocity. Acclerometer on the other hand, as one post mentioned before, does not directly react to acceleration, nor velocity. It actually reacts to deformation of the piezo material in there. So there is a small conversion mechanism based on mass loading to convert the the acceleration into "deformation" that the piezo material can react to.

Then there is this issue of single axis vs multi-axis sensor. Cone can only be controlled to move in one direction. All movements in other directions are not controllable or correctable with servo loop. sensing coil is single axis, it will not be affected by lateral movement or vibration. Accelerometer is not. So it may pick up movement that the subwoofer cannot correct at all. We know what would happen then.

Our sensing coil also colocates with the driver coil where the force is applied. No latency issue and there is no lag at all.

Directservo is also DC coupled. The servo feedback loop is a straight wire. What can be simpler than that? The feedback goes all the way to down to DC. Most accelermeters have limited bandwidth below 10hz. If sensor cannot see distortion, the servo loop cannot correct it.

Brian

Rythmik Audio
 
I know this is a super old thread, but I had great results using radar. It also has output down to DC with the combined doppler phase result being a direct proportion to displacement. The only downside is the wavelength of 10.525 GHz which is 28.2mm. If the throw goes outside a wavelength boundary, distortion is high and placement of the waveguide is super critical. As I was only using the signal as a key input to a limiter, proper tracking wasn't needed and I didn't even consider using the signal for feedback.

http://www.diyaudio.com/forums/subwoofers/153359-reading-driver-displacement-radar.html
 
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