Re: Homebrew STG card - 8254

[Jon Elson <jmelson-at-artsci.wustl.edu>]

Ray Henry wrote:

> At 03:37 PM 3/2/2000 -0500, Jon E. wrote:
> >
> >Also, with a DC tach in an analog computing loop, the velocity
> >bandwidth can be GREATER than the computer can provide, giving very
> >smooth velocity, down to rediculously slow motion, like .01 IPM.
> >If the encoder resolution is coarse, it can't give good velocity info near
> >zero.  Again, a bandwidth vs. resolution problem.  My analog loop servos
> >don't jitter, AT ALL!  A servo based on the encoder would likely jitter
> >badly, unless the encoder had VERY high resolution.
>
> These kinds of problems must be why some machine tool companies are moving
> from 125k ppr to as high as 1m ppr.

Gulp!  I thought I was hot shit when I got 1000 cycles/rev to give 4000
counts/rev, on a 5 TPI screw, giving .00005" resolution.  Whew!  1 Million
PPR?

> I did read about some kind of velocity from encoder system that switched
> over from computing velocity from pulses per unit time to time between
> pulses to get a little better resolution of velocity at very low speed.
> But such a system would still be dealing in digital rather than analog.

Yup, you want to have fast updates when the servo is moving at moderate
speed, but you don't want the speed signal to bounce every time it crosses
an edge when it is just creeping, or holding position.

Jon