26 Jul
2015
26 Jul
'15
10:33 p.m.
On Sun, Jul 26, 2015 at 01:45:42PM -0400, Gene Heskett wrote:
There is no buffering in a cnc setup. The computer
expects to get
uptodate data from the machine which I have done at rates to 4,000 times
a second, but normal is 1,000/second. Ditto sending data back to the
machine, that data cannot lollygag around in a buffer someplace for 20
milliseconds while USB is searching for a suitable sock to put it in.
Even 2 milliseconds will likely put the resultant part out of tolerance
bad enough its a mantle decorator.
Are you saying there is motion feedback from the machine and the
PC is supposed to 'close the loop' ? In that case indeed any delay
due to buffering would needs to be taken into account, or could
(depending on required closed loop BW) render things impossible.
But *none* of the machines I've ever driven (that includes all
sorts of milling machines, and tangential knife, laser and water
jet cutters) required feedback to control the motion of the tool
as function of time. They did only for things like changing tools
and/or workpiece, or to signal general state. If there were feedback
loops involved in controlling motion then these were local and
certainly not implemented in the controlling PC (which in those
times only had RS232 interfaces, orders of magnitude slower than
anything USB).
And anyway, what is the BW of any heavy piece of machinery ?
Probably at most 10 Hz or so, and that you can control using
a feedabck loop even with 5 or 10 ms of latency.
--
FA
A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)