RE: [CAD_CAM_EDM_DRO] Laser Linear Scales

> From: Harvey White
> ....
> From what I understand, the laser beam from the average diode
> is coherent only for the first few inches, if that. Without
> that coherency, no interferometer. I understand that
> (perhaps) the higher priced diode lasers might be ok, and
> that the gas lasers are coherent enough.

Yes, generally true. I had done a Google search on "Laser Interferometry
measuring short distances" and had turned up a reference where there are now
reasonable cost laser diodes that have a reasonable coherent length beam.

> From: David A. Frantz
>
> ....
> The number one issue is the high data rates generated. This depends of
> course on the exact type of interferometry used but is related to the
> wavelength of the light used. Thus for very small movements you get
> alot of data real fast. The really fast data can be an issue.
> Tracking the resultant counts becomes a problem which is usually
> overcomed by moving SLOW. Todays electronics may offer the
> possibility for faster data rates but do realize that in part
> you are dealing with analog circuitry.

Yes, I knew that the data rate would be high but I wondered if modern,
inexpensive components would be able to keep up with the data rates.

> Now for the smaller machines often talked about here I'd have
> to wonder why you might think that an interferometer is called for.

Well, I wasn't looking for interferometer resolutions because I knew that shop
vibrations would swamp that level of resolution but one could ignore the 'noisy'
Least Significant Digits of the data and still have high resolution compared to
other inexpensive methods. Of course, this assumes that it could be none
relatively inexpensively.

> Simply going to higher resolution encoders can often bring much
> better results, though you do need to address the quality of
> the rest of your electronics.

I'm thinking of DROs in this case. So I was envisioning the laser setup as
replacing high precision, expensive scales.

> From: Jon Elson
>
> .... Also, there is the problem of
> air temperature changes affecting the accuracy, and thermal
> expansion of the interferometer reference-path components. I
> tried to build one of these years ago, and was never able to
> even get any visible fringes due to vibrations. (I don't
> have an optical bench supported on air bags at home.)

Again, I was thinking that, if you can inexpensively track the fringes, even
though they are jittering like crazy, you can drop the 'jittering' digits and
still have higher resolution than other inexpensive methods.

The bottom line is that it's only practical if you can build it from inexpensive
components and be able to reliably track the fringes. But it sounds like we
haven't made it to that level in 'Scrapbox Technology". :-)

Cheers,

Chuck Hackett
"Good judgment comes from experience, experience comes from bad judgment"
7.5" gauge Union Pacific Northern (4-8-4) 844
http://www.whitetrout.net/Chuck