Re: [Emc-users] Looking for info about converting old Proto Trak control to EMC

[Terry N. <67...@gm...>]

This clears up some Questions that I have been wondering about. I was
referring to the accumulated error in the ball screw, not backlash. The ball
screws that I have are decent ground ones, so they should be ok. I'm not
worried about thermal expansion, that usually only comes into play on runs
longer than I will be doing.
I appreciate the info, I wanted to make sure I was going down the right path
before starting this project.
Terry

On Sat, Jul 3, 2010 at 9:28 PM, Jon Elson <el...@pi...> wrote:

> Terry Neilson wrote:
> > Gentlemen,
> > So would  it be safe to assume that the best place for an encoder to be
> > mounted would be on the screw and not mounted to the motor shaft?
> > Also once tuned using linear compensation, with other factors being even,
> > rotary encoders will be at least as accurate as linear scales. True?
> >
> >
> If you are using a belt reduction, then putting the encoder on the end
> of the leadscrew
> eliminates any error introduced by the belts and pulleys.  There is
> always some cyclical
> error in these parts.  There should be no backlash between motor and
> encoder in this setup,
> so it should not complicate servo tuning.  If you have a belt drive, the
> end of the leadscrew
> should be available for mounting a shaft encoder.  You want the drive
> pulley to be between
> two bearings, and then the encoder can be driven using a helical-slit
> coupler.
>
> If your leadscrew is a ground screw in good condition, then this should
> cause no significant
> loos of accuracy.  Thermal expansion of the screw will still affect
> accuracy, but most home
> shops don't run into that problem.
>
> If your leadscrew is a crummy rolled screw with some backlash, then the
> shaft encoder will
> not directly correct for this.  The machine error compensation table can
> help with this.
>
>
> Jon
>
>
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