Re: [CAD_CAM_EDM_DRO] Laser Linear Scales

Hi chuck;


There are several problems to consider, the diode or other laser is
really only a small part of the equation.

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.

Now for the smaller machines often talked about here I'd have to wonder
why you might think that an interferometer is called for. 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've worked on systems where, after hardware
multiplication, we where getting 500,000 counts per revolution. That
high resolution though increases gain in the system which brings about
its own problems.

Now considering the expense of high quality rotary encoders I have
wondered what it would take to make one based on laser diode
technology. In the end it does seem to be simpler to just buy high
resolution encoders at the resolution required. The required
resolution brings up an interesting question, which is how much
resolution do you need in your system. You will simplfy things if you
stay within 4 to 6 times the number of counts required to resolve the
smallest step you want to operate at. In otherword if you want to
work at 0.0001" then select an encoder that gives you a count every
0.000025". But really think about the resolution you need, in effect
increasing the resolution of the encoder increases the gain of the
system and increase the data rate from the encoders. This then
impacts your CNC loop.

Thanks
dave


Chuck Hackett wrote:

>This may have been covered before but ...
>
>With the availability of good quality laser diodes these days wouldn't it be
>possible to build a low cost/high precision linear position sensor based on
>Laser Interferometry?
>
>... I assume not, or I'd see them everywhere. What's the sticking point?
>
>I suspect that one of the problems might be the ability to track fringes caused
>by normal machine vibration.
>
>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
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