encoder resolution vs final accuracy - was ... USDigital new resolutions 250cpi x 4

Russell Shaw wrote:

>
> Why does anyone need that resolution? Wouldn't backlash
> be greater? I'm figuring out what
> resolution to put into the router i'll make.
>
> >
> > Hi
> > USdigital just came out with 250 lpi EM1 module and linear strips, so
> > the x4 mode should yield the .001" resolution people have been asking
> > for.
> >

One huge reason is that:
"1 encoder count" does not equal "your final accuracy"

Many people use the rule of thumb that:
"ten times the encoder count precision" equals "your final accuracy"

So if you want 0.001" accuracy, you really should be using
0.000 10" feedback step-equivalent.

You might be lucky and get away with "five times",
but then ..... you'd be lucky!

That is why Dan Mauch is keen for a resolution a bit
higher than 1/1000".

Lastly, because typical 'industrial' leadscrews (ballscrews &
acme) have accuracy specs of being off 1-3 thou/foot, that is
another source of error. That is why I am keen to see leadscrew
compensation in software (only EMC has it now, as I am aware).
'Precision' and 'Lab' quality leadscrews are of course better
at a much higher cost, but still not perfect. Using leadscrew
compensation in software theoretically means you could use
hardware store readi-rod thread and expect better than 1/1000"
positional accuracy.

Doug Fortune
http://www.cncKITS.com