Re: [CAD_CAM_EDM_DRO] Re: Re: smdromod.o was no reply/this is a test

>
> From: ccs@...
> > Subject: Re: no reply/this is a test
> <S>
> > ObCncContent: with smdromod closed loop steppers under EMC, do they
> > hunt? �Ie, would a servo loop closed via a linear encoder across a
> > screw with substantial backlash be stable? �

To which Ray replied

>
> To prevent hunting between steps you would need to set deadband to more
> than 1/2 of one step. This is true with any of the stepper motion
> modules. Overshoot with a lot of backlash is a different problem. Dave
> Engval has scales on his mill and insists that backlash and overshoot with
> servos is not a problem.

I would guess overshoot could be reduced by keeping the acceleration
limit fairly low, so as to insure that it is friction and not the
motor that is doing the braking as the desired position is reached.

Obviously this is not an ideal setup, but many manual machines work
quite fine with lots of backlash. I was thinking about what would
happen if one were to inexpertly mount a stepper to say a lathe
leadscrew possibly having to use a long timing belt and make other
compromises that would make various errors between the motor position
and the machine motion. A couple years ago I saw what a cyclic error
could do to a tapered part - not pretty. For many hobbyist
applications, an inexpensive linear encoder might provide enough
feadback to linearize such an axis.

I think I'm actually going to use step servo (G3xx series drive) with
a rotary encoder on the screw - provided I can figure out how to mount
the wheel in a way I trust. But I wanted to consider a backup plan.

Chris

-------------------------------------------------
Christopher C. Stratton
Engineer, Instrument Maker, and Horn Player
ccs@... 617 628 1062
http://web.mit.edu/~stratton/www/brassbuild.html