Re: [CAD_CAM_EDM_DRO] more on separate positioning and metrology

Elliot Burke wrote:

> Chris replies:
>
> >> 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.
>
> Do you know of any examples? I think that for this to work the objections
> you mentioned below must be overcome, what has been done to do this?

All the high-end machines use this, or have it as an extra-cost
option. Machines like Fadal, Mazak and up.

> > 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).
>
> The common servo motor/ballscrew type drive probably has very limited
> bandwidth. Anyone care to make a guess what the 3 dB point is? 5 Hz? 50
> Hz, 500 Hz? Also has anyone measured the frequency where the phase shift is
> 180°?
>
> This could get a bit tricky, as the length of the ball screw between motor
> and nut changes, so the resonance properties should change too.

Yes, but a stiff enough ball screw (perhaps 4-5" diameter) like used on
Cincinatti machines, would help keep the resonances high.
Linear motors are now becoming the "in" thing, probably this has something
to do with it. When they are talking about 1800IPM cutting feedrates,
they need to up the bandwidth a lot.

Jon