Re: [CAD_CAM_EDM_DRO] Closed-loop steppers, was: breakout boards plus - hard choice

Michael Fagan wrote:

>
> Right. I knew about the balls (I've got a set on the BP) but I didn't know
> how they gathered data from them. I know they are advertised to have a
> faster "refresh rate" than optical ones but other than that I don't know the
> exact specs.
> The newall dro unit does have a serial port output on the back that gives
> real inch measurements for each axis regularly (don't know the exact timing)
> so I think it can be interfaced for some things.

I don't know how the Newall can have a FASTER refresh than
direct-reading optical encoders. Most can produce 100,000
quadrature counts per second with ease, and many of the models
have a high speed readout option, mostly just faster
differential line drivers. Then, they can go to 250,000
counts/second and higher. If they are comparing their
interpolator to other interpolated scales, then I think they are
playing word games, as well as comparing what they make NOW with
what others made 20 years ago. I know some of the outboard
interpolators (Renishaw, Heidenhain, etc.) can go to several
hundred thousand counts/second without introducing lag.

The serial output is definitely going to be limited to a few
hundred updates a second. 9600 Baud is roughly 1000 characters
a second. If you are reading out 3 axes with 6 digits each, or
18 digits at 1000 chars/sec, that is 55 full updates a second,
neglecting any format chars to tell which is which. Again,
totally useless for a servo system, unless your bandwidth is
about one Hz. That would be about as sluggish as your watching
a display and moving by hand.

Jon