Re: laser control circuits

On Nov 26, 7:08 am, Howhurley <howhurl...@xxxxxxxxxxxxx> wrote:
On Nov 20, 12:22 pm, Phil Hobbs <p...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
wrote:





Howhurley wrote:
On Nov 19, 10:45 am, Phil Hobbs <p...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
wrote:
Howhurley wrote:
Hello,
I am looking for laser control circuits to use as distance measuring
applications. A set frequency output with a detector circuit might do
the trick. Any suggestions?
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?

2. 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.)

3. 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

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- Hide quoted text -

- Show quoted text -

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...)- Hide quoted text -

- Show quoted text -

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
.

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