laser control circuits

The difficulty of laser ranging problems can range from easy (across your living room with the lights turned off) to difficult (hitting the lunar corner reflector left by Apollo 11) to impossible (using a battery powered handheld unit to measure 500 yards distance to a shiny target in bright sunlight).

The difficulty generally comes from not having enough signal to overcome the noise of the background light. So first you need to do a back-of-the-envelope photon budget, to see if you have enough light for the job, and if not, how you can get more. If you don't know how to do this, I'll walk you through it if you post the following information:

1. How far are you trying to go?

1. What's the ambient light level? (Interstellar space? Living room? Bright sunlight? The interior of an arc lamp?) (None of these is necessarily impossible, and all have been done.)

2. What are you trying to hit? (Retroreflective tape? A white wall? Dirt? Raindrops? A bad guy dressed in black hiding behind a bush?)

Photon budgets are pretty easy once you've done a few, and they're very rewarding, because they actually give the right answer. A decent design will almost always be withing 3 dB of the budget, and usually it'll be within 1 dB. Seeing that happen is some of the best fun I know.

Cheers,

Phil Hobbs

It's amazing what obscure things can give engineers pleasure, like having a box deliver 3 picoseconds RMS jitter. My wife doesn't understand.

John

She needs more jitter than that ?:-)

Sorry, John, you walked right into that ;-)

...Jim Thompson

It's also amazing what engineers will accept as humor.

John

I can just see you in a bar with a floozy, John:

You: "My wife doesn't understand me."

She, scenting a profitable evening: "Ooh, poor baby!"

You: "She just doesn't understand what really gets me going."

She, leaning close: "Really?" Now whispering in your ear "How so?"

You: "My latest signal generator delivers 3ps jitter over it's entire operating and temperature range! And she just doesn't get it!"

She: "Bartender! Is there a sales guy around here somewhere?!?"

Now, _THAT_'s funny. ;-)

Cheers! Rich

On Mon, 19 Nov 2007 09:45:10 -0800, Tim Wescott wrote: ...

Do you pronounce the apostrophe?

Thanks, Rich

Of course*. I'm a disciple of Victor Borge's "phonetic punctuation".

• You tight-assed, picky b******.

Pronounced "beesplatsplatsplatsplatsplatSPLAT".

Cheers,

Phil Hobbs

Be glad. Einstein's first wife was a physicist.

Cheers,

Phil Hobbs

Anything for a sexual innuendo ;-)

Took the morning off, went to the Nail Salon with my wife... had a pedicure from a nice Vietnamese babe... amazing how petite, yet "well rounded" !-)

...Jim Thompson

Who actually did all the calculations for him, that his relativity theory was invariant under the transformations. He handed the nobel prize over to her.

Rene

Thanks Phil. I am looking for 3-10 inches, a metal (freshly machined) surface--one I've just blown off cutting fluid and chips. I want to measure with a +/- .005 in or so. Ambient light is shop level, with a little spurious fluid and mist, not too much.

I see. That sort of thing you can buy, e.g. from Keyence. I've used their stuff in the past to sort out problems with PC boards.

Or do you want to build your own? You can't use time-of-flight in any reasonable way to measure 100-micron distances, and you don't want to be limited in the kinds of shapes you can measure, so you'd be probably looking at a parallax or beam-deflection system of some sort, similar to Keyence's.

Cheers,

Phil Hobbs

You can use a mechanically scanned laser to measure profiles fairly effectively, if the edges are pristine and conditions are good (eg. for extrusion), but I think these days a high resolution sensor might make more sense. The former method is limited by mechanical factors associated with the scanning polygon, IIRC.

Best regards, Spehro Pefhany

Thanks again, Phil. I can use the Keyence unit or Pepryl + Fuchs has one too. I am interested in the technology to do the measurement to roll my own in an OEM venture. I wonder if, since most of these folks copy off each other, PLUS I saw a ???Cooper tools??? version lately at Lowe's to do the same thing (albeit +/- 1/4" resolution) if there is a commercially available chip that these three groups are using on their boards.

So, had anybody run across one?

(of course, it would have to be non proprietary...)

One approach that might work is a kind of self-oscillating continuous wave system, in which the modulation frequency of a laser source is determined by the delay interposed by your optical path. You measure the modulation frequency to determine the distance. For the 3" range you mention, the minimum delay would be about 500 ps, so you'd need to be able to measure frequencies up to 1 GHz (or possibly 2 GHz, depending on how you do the feedback). I believe there's a patent on this type of approach, but it may be expired by hnow. For these short ranges, you might consider MHz ultrasound. Paul Mathews