Re: Re: Cheap Encoders

Alan Rothenbush wrote:

> From: Alan Rothenbush <beer@...>
>
> Does anyone have any thoughts about using these low cost rotary encoders
> from Goldmine attached to the TABLE ( or quill ) instead of the SCREWS.
>
> Maybe by some sort of belt or wire driving a pulley with a spring to
> keep it all under tension ?
>
> By attaching it to the table, any lash in the driving mechanism would then
> be accounted for automatically, similar to using linear encoders.

If the leadscrew has backlash, then the servo system has to jump back and
forth across the backlash just to move the table even a tiny bit. This leads to
a lot of banging around, and jerky motion. Of course, having that inside the
positioning loop improves accuracy, but it leads to motion that is not smooth.
So, removing the backlash is of great importance if you are going to be
doing any smooth surface cutting, circles, etc.

There are problems in connecting a shaft encoder to linear motion accurately.
You need drums that are very concentric, and wire or bands that are very
uniform. A mediocre rolled ballscrew might be more accurate than anything
you can cobble up with drums and steel cable.

Jon