Re: rotary encoder versus glass scales

Steven:

Most CNC controllers these days use the signal from the motor encoder
as both position information AND rate feedback. They thus avoid
having an added tach on the motor. A system is easier to stabilize by
using the motor encoder as a tach instead of getting data from the
table. However the best *position* feedback comes directly from the
table position via the glass scale; this is the most accurate as the
lost motion as well as the *springiness* of the mounts, bearings, etc
are all included within the feedback loop. Therefore, if you want
the best accuracy, the glass scale on the table is best. Also, if
the provious user managed to operate with that configuration, it must
have worked! A previous respondent indicated that a tight
recirculating ball preload did perform well on a PC drill.

I think I'd give the table encoder a try!

Regards,
Jack C.

> > I am in the process up gathering info and parts to upgrade an old
> > Hurco Bedmill to PC controls.I have ripped all the old

electronics

> > off the machine cept motors and limit switchs ..I have new servo
> > drivers in hand and just gotta get a coat of frsh paint on the
> > machine and will be ready to wire something up....Cleaning on the
> > machine today I found that the X-axis motor's rotary encoder had
> been
> > disconnected and somebody had installed a glass scale on backside
> of
> > the bed for that axis.Reckon I should try to find a new rotary
> > encoder or is the linear scale gonna be alright? I been reading
> about
> > rotary optical encoders but havent a clue at the electrical
> > differences between them and the linear scales..I want to keep
> this
> > simple where I can and save the sweat for the worthy
> > problems..Hehhehehheh
> > Then another question if somebody wants to take a stab at it

is

> > why did the old controller make use of a tach signal from the
> > servomotor and these PC run conversions dont use that signal?--Or
> > maybe they do and I havent got to that part of the reading
> > materials..This is my winter project and I got a lot to learn

yet.I

> > figure this isnt the last time I'll ask questions here.
> > Thanks
> > Steven