Re: rotary encoder versus glass scales

Steven,

> 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 suggest that the glass scale, in most cases, will not work properly
with modern day servo drives. The most important reason is that
encoder feedback should be on the 'drive' (motor) side of the
mechanism to be controlled and not the 'driven' (in your case the x-
axis). In simple terms, servo motors are controlled by switching the
polarity at high frequencies back and forth between the two motor
leads and hence, bounce the motor shaft between the two adjacent
encoder signals in order to maintain position. If the encoder were
mounted on the `driven' side of the table, this bounce would be
proportional to the backlash in your drive screws etc. and would
result in the motor becoming unstable, just my thoughts.

Eric


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "alphawolf45" <haushahn@c...>
wrote:

> 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