Re: [CAD_CAM_EDM_DRO] Re: Comments from expert PID programmers?

Dennis Schmitz wrote:

>This is absolutely true assuming a requirement for continuous motion
>through a thrust reversal, but when I thought about a mill, I didn't
>really see any requirement for dead-zone backlash to happen in zero
>time, leaving the controller free to ease through the entire thing. So
>if the controller knows the thrust reversal is coming, it can ramp the
>speed down (even a very steep ramp should work), proceed across the
>dead zone at a pre-determined safe speed, and hit the other side
>gently, watching the current draw to know when it was there. Screw
>torsion would actually help here to make a soft landing.
>
>So what would happen if you were milling a circular channel for
>example? That's four thrust reversals. If the milling head pause for a
>couple of milliseconds at the 0, 90, 180, and 270 degree points, you'd
>see a change in the finish where the milling head got to rotate freely
>for a while -- a smooth spot in a field of little tool marks.
>
>
>

If you do climb milling, which not only vastly improves surface finish
but reduces
tool wear dramatically, the cutter is always trying to pull the work
toward the
cutting tool. If the machine has much backlash, you get jumpy motion of the
slides, and a step-like edge.

>But it would be accurate, and the results would still be better than
>no compensation. So I think this effect might lead to people wanting
>zero-backlash for speed and finish considerations, but not really
>needing it to make accurate parts.
>
>
>

Maybe, but my limited experience with sloppy machines is that it really
doesn't
work, unless the only work you do is drilling holes. The backlash is a
discontinuity
in the transfer function of the system, and normal linear algorithms
don't really
handle this well. I'm not convinced ANY algorithm actually handles it
well, for
that matter.

Jon