Re: [CAD_CAM_EDM_DRO] Separate drive and metrology structures

> Has anyone here built a machine with separate drive and metrology
> structures? I was just reading about a diamond turning lathe at Livermore
> labs of quite large size, > 1 m swing, I think. It had separate structures
> for the drives and laser metrology.

It is quite common to use a linear encoder for feedback rather than a
rotary encoder on the screw. Not seen too much on low grade machines
due to expense, and also because it is hard to make a servo loop
stable if the screw has any backlash (servo loops tend to hunt between
encoder counts - if there is backlash between the motor and a linear
encoder, it will hunt across the entire backlash distance).

> This makes a lot of sense to me. If the feedback was quick enough it could
> act even though the drive structure was flexing, if the metrology structure
> maintained its accuracy.
> So vibration, constant load, and thermal strains would be less of a problem
> with this sort of design.

The problem is that making actutors which can operate at the
frequencies necessary to combat vibration is really pushing the state
of the art and currently more expensive than the amount of cast iron
or polymer concrete necessary to stabilize most machines of the size
that we tend to talk about.

Correcting for bed twist and stuff should be possible, assuming that
vibration doesn't render the laser measuring setup inoperable. And
also assuming that you can model the mapping of bed twist to cut
diameter error - not trivial as it depends on the cut diameter, tool
shape, tool height, etc.

Chris