Re: [CAD_CAM_EDM_DRO] Just How Accurate Is CNC?

Just as a side note to the accuracy discussion, you might be suprised to
learn by far the most precisely measurable dimension is in fact time. The
new high accuracy clocks are orders of magnitude ahead when compared to the
mechnical measuring devices we can produce.

High accuracy, reasonably cheap timing systems have almost certainly come
about solely due to their role in obtaining useful computing power.

So, without looking futher into it, I'd be kind of interested and hopeful
that perhaps something could be done with a time based feedback system.

However, if industry isn't already using it, it's almost certainly not going
to be as simple, cost effective or effective to implement as standard
encoders.

Best wishes,
John

> 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-
>
>
>
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