Re: Poor Mans DRO

Jim,

It's the other way around...

Most DRO's use LINEAR scales, for their better accuracy. Many less
expensive CNC machines use rotary encoders to close the loop to the
servo. And yes, with the less expensive servo systems; the addition
of the axis variability due to mechanical issues DOES work against
using linear scales directly in the motor feedback.

The better CNC machines use rotary encoders to stabilise the servo
motor and close that "loop"; AND use linear scales for position.
These systems close both loops in the computer/control, NOT at the
drive. I suppose you could set up a step-servo(gecko) to the motor,
and use linear scales for position in a second servo loop with EMC,
but I think you'd have to do some tweaking...

Ballendo

P.S. I'd sure like to see a renishaw ballbar test of a Gecko (step-
servo) machine... And compare it to a "real" servo setup, like Jon
E's...

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jim_stoll" <jim_stoll@y...>
wrote:

> Why is it that almost all DROs seem to use rotary encoders vs

linear

> encoders? (at least that I've heard of - bearing in mind I'm new to
> this and haven't actually ever _seen_ a DRO... :-) Is it a matter

of

> it being easier to match the rotational step of a motor directly to

a

> a rotary encoder - vs trying to get a rotational step, once driven
> through a lead/ball/etc screw arrangement, to match up to a linear
> increment?
>
> Thanks for any input!
>
> Jim
>