Re: [CAD_CAM_EDM_DRO] Comments from expert PID programmers?

Dennis Schmitz wrote:

>Regarding tuning:
>
>We've found several approaches to tuning and after having checked them
>out, they all look like kludges to me. Every one we've looked at has
>been a trial and error kind of thing with no real analytical solution.
>
>
>Did I mention "rock solid backlash comp"? This potentially means
>replacing expensive zero-backlash stuff with less expensive backlash
>prone stuff, doing more with less, etc.
>
>
>

I have a couple problems with all this. In some machines, wear has
caused friction to
vary with position, in others, changes in the balance of overhanging
tables does the
same. The torsional spring in small leadscrews varies with the length
between
motor and nut. Some of this could severely affect a system that was
finely tuned
at some different position. Finally, you have to account for
intermittent and
unexpected impacts from the cutting teeth of the machine. This is in no
way a
steady-state system, and any steady-state tuning will fall flat.

Second, backlash can't be "compensated", due to the finite time it takes
the motor
to get to the other side. if you don't know the amount of free play at
that position,
then the motor slams into the end of the backlash at high velocity, and
you have a
severe hammering. Even if the code knows the amount of backlash to be
compensated
out, the motor still has to accelerate like mad, and then brake so as to
just nudge the
machine from the other direction. Immediately after the nudge, it then
has bumped the
table in that direction, and may have to run to the other side to nudge
it back. There's no
free lunch here, no matter how much digital signal processing you throw
at it, hardware can't
be "in two places at the same time", unless the backlash is already very
nearly zero.

Jon