On Saturday 03 July 2010, Terry Neilson wrote:
>Jon,
>That was a good explanation of the possible problems that can occur when
>using linear scales for position feedback. What I still don't understand
>though, is how the knee milling machines in the shop that have linear
>scales, continue to run every day with very good accuracy, and no problems
>with servo hunting issues. I would think that a rotary encoder reading the
>rotary travel of a ball screw would cause a cumulative inaccuracy that
>increases with increasing travel of the table.
If the rotary encoder is solidly coupled to the drive screws so there is no
place for friction caused slippage, then any errors are in the math you used
to set the scale factor, and possibly thermal errors from the screws
expansion and contraction with changes in temperature. Scale factors can be
set in very high accuracy floating point format in the individual axis
section of the .ini file. Eg, 40.0000000001312 is, I am told, a valid
number. My Z axis has such a number in it, and appears to be as close as I
can measure with a digital caliper over at least 6" of travel.
>A linear scale is measuring
>the actual movement,so in theory should be more accurate especially at
>greater distance.
>Can you tune out errors in ball screws using EMC, and get repeatable
>accuracy for most of the length of travel?
Generally, yes. I have never done it but I have been told that screws can
be mapped for error, if one has the ability to make that fine a measurement
over the range of travel. That of course assumes your screws have
essentially zero backlash.
[snip]
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
In the land of the dark the Ship of the
Sun is driven by the Grateful Dead.
-- Egyptian Book of the Dead
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