Re: [CAD_CAM_EDM_DRO] Just How Accurate Is CNC?

At 9:49 PM +0000 11/9/04, Major A wrote:

> > I can well understand the cost issue, but motion measurement has
>> changed in the last few years. Much has been done with inexpensive
>> lasers. I do not know a whole lot about it, but it seems that if I
>> can buy a optical mouse for $5, there might be a more precise version
>> of a laser measuring device that could be mounted to the bottom of a
>> table to precisely read table movement. The optical mouse is not that
>
>The motion tracking of an optical mouse is nowhere near linear, and
>certainly not reproducible. You'll need a high-quality high-resolution
>reference which spans the entire length of the linear actuator, and
>new detection software as well. It gets very tricky to say the least,
>especially if you're looking for resolutions of 0.001" or better.
>
>> surface could be much more consistent. Or you might use a laser in
>> the saddle reflecting off the end of the table and measure the
>> trip-time per pulse.
>
>Not really. To resolve 0.001" of saddle movement, you'd need a time
>resolution of about 150fs (1fs = 0.000001 ns). Good luck...

So 150 fs = 0.000150 ns

A crystal 20 MHz PIC can run at 5,000,000 instructions per second.
Doesn't that give a headway of 75 instructions per required resolution?
Perhaps my math is off, but if not, with a PIC chip, you can do a
lot in 75 instructions. If you wanted to drop it a notch tighter,
15 fs would give you 7.5 instructions per second. That might be
pushing it.

I doubt that I have enough knowledge to do it, but I think that
someone will be doing it soon. Too many of the pieces are becoming
available at low price points.

Seems, even if you wanted to laser engrave a scale on the bottom of a
table, you could read it much the way we read a micrometer. You
could have a set of LEDs that read the course scale and another set
matched the fine scale. The two readings together would give you
a pretty fine resolution.

>You can use interferometry, but that gets very expensive because you
>not only need a very good laser for it (coherence length much bigger
>than saddle travel) but a complementary method for course measurement,
>unless you always want to move your saddle at about 0.1 IPM or
>slower.

Interferometry is outside my my knowledge, but it should be easy to
reduce resolution. You can do a lot with PIC processors now and if
you step up to something like a ColdFire processor you would have
a lot of time to do measurement calculations. -Chuck-