Help required regarding optical motional feedback theories

[Pjotr]

Yes SY, but what about resolution if you simply count up and down ¼ wavelengths?

I think it would make a dandy senior project.

Small semiconductor laser interferometers with quadrature output are readily available on the market. But not cheap!

Cheers

 

[Eva]

Just a simple tip :

If double differentiation of the measured signal reduces signal/noise ratio...

What about double integration of the input signal before comparing it to the measured signal in the error amplifier?

 

[Alexander Rice]

Okay, sorry to butt in, but anyone who has got as far as the final year of an engineering degree and who is seriously proposing to use laser inferometry as a method of measuring speaker cone position is just plain daft. For one thing the laser required to get the necessary coherence length would be pretty big - a few feet long would be typical of a frequency stabilised HeNe which is about the minimum you could get away with, add on a bunch of optics, some of which must be attached to the cone, and add to the problem that the relationship between the stationary optics need to remain constant to the nearest wavelenghth of light while there is a bloody great subwoofer nearby is plain imposible.

Actual workable possibilities are - DVC subowwfer using drone voice coil as feedback having previoulsy calibrated the interference between the two by holding the cone still and driving the other VC, single VC using the back EMF as per certain Laser graphics galvanometers, optical sensor using a graded strip, capacitative sensor - particularly good if you are using an aluminium one, accelerometer stuck on cone

Regards

Alex

 

[Jonathan Bright]

Dear "eclipse" if memory serves there was a very small article about an optical mfb system in "Wireless World" in the 70's It was not a feature just one of those 'design ideas' and it had a LED suspended above the cone, a few other components and an op-amp. I'll try and get the month/year so you will be able to track it down. Might take 24 hrs. Jonathan Bright

 

[Jonathan Bright]

Dear eclipse, that reference is the old "Wireless World" (Now called "Electronics & Wireless World") July 1982, page 49. Sorry I have no scanner or I'd put it up in a post. It uses an infa-red led and a photo-transistor with op amp compensation for ambient lighting. Jonathan Bright.

 

[jackinnj]

The proper way to do this optically is to set it up with a monochromatic source (ie, a laser diode) and an interferometer. In this case, you're just counting fringes and since it's digital, you can massage the data any way you like- there's no added noise from the two differentiations, for example. You would probably use a double beam, with one being in quadrature to the other so that you know when the cone is moving forward and when it's moving backward.

I think it would make a dandy senior project.

The low-noise HP Interferometers show up on ebay about once a week -- i haven't been shopping for laser/optics stuff in about a year, but bought a dandy Newport table, autoleveling, for about a grand last year. (Seller was an IBM lab which was being closed -- what a hoot -- the largesse of IBM shareholders -- cheaper to sell it on EBay than send it down to the Carolinas),.

 

[SY]

Okay, sorry to butt in, but anyone who has got as far as the final year of an engineering degree and who is seriously proposing to use laser inferometry as a method of measuring speaker cone position is just plain daft. For one thing the laser required to get the necessary coherence length would be pretty big - a few feet long would be typical of a frequency stabilised HeNe which is about the minimum you could get away with, add on a bunch of optics, some of which must be attached to the cone, and add to the problem that the relationship between the stationary optics need to remain constant to the nearest wavelenghth of light while there is a bloody great subwoofer nearby is plain imposible.

I had to untie myself from the straightjacket long enough to type out a reply- you'll excuse the brevity, but the nice young men in the clean white coats are limiting my typing time.

I did a design like this some years ago for a moving mirror assembly for a Michelson interferometer, exactly analogous to a woofer (magnet, motor, driving amp) using a very small HeNe laser with a beam splitter and a couple of mirrors, including a small outrigger mirror to do the quadrature. The MI operated in the IR, with its motion controlled by the accessory red light laser. The only real difference from a woofer is that instead of a spider suspension, we used an air-bearing, and the moving mirror was rather more massive than a cone. It functioned quite well up to about 100 Hz or so. Similar designs are routinely used with more springlike suspensions in commercial rapid-scan spectrometers. All I can say is that the idea may be daft, but it was used commercially for some instruments that are still considered benchmarks.

 

[Alexander Rice]

Engineering is all about compromise and trying to find the solution that meets a given set of criteria with the least possible compromise, it is also about evaluating the possibilites and choosing the most appropriate, all i was getting at was that given the good alternatives, a MI seemed a poor choice.