[Coco] George's CNC Machine (WAS: Something else)
gene.heskett at verizon.net
Wed Sep 24 14:34:02 EDT 2008
On Wednesday 24 September 2008, Chuck Youse wrote:
>On Tue, 2008-09-23 at 22:24 -0600, Ron Bihler wrote:
>> The speed all depends on how fast you want it to interpret the moves and
>> how accurate you need this to be. Trade off, accuracy requires more math
>> and processor time hence slower movements.
>Is it not possible to precompute the math? I.e., do something akin to
>compilation - take the time up front to do all the calculations, before
>the CNC machine gets any commands at all? Or does it require feedback
>from the machine?
The output files used as cache would be quite large, and the bandwidth from
the disk to the driver interface could still limit things. The main loop in
my emc install, runs at a period of no slower than 50 microseconds per pass
through the loop. The loop itself takes about 15 microseconds on that box,
and emc runs its other, not so time critical control loops run in that spare
time and do the rest of the calculations on the fly. There is also a display
loop that runs at about 10 hertz to keep the realtime display reasonably well
up to date.
In my present configuration, there are 2, 8 bit wide outputs to the parport,
and at least one read, just to monitor whatever the machine may be reporting.
That is 40k a second going out the parport, and 20k coming back in. I don't
think the coco can manage that data rate since its best bandwidth is 11
seconds for a megabyte moved. Bear in mind it has to keep the data flowing
even when it is also refilling the output buffer, no pause in the data flow,
or even a bit of jitter in the timings can be allowed else the max attainable
speed must be reduced until it can manage a steady data flow AND do the
With steppers, there is no feedback to the program, so if the stepper misses a
step, it doesn't know about it, and the part may well be wrecked if the
operator doesn't hear it. Usually, if it misses a step, it will stop, and is
not able to accelerate enough to get back to speed, so its stalled, and the
operator can't miss hearing that unless the spindle is screaming too loud.
I think George is doing amazingly well, given the limitations of the coco's
speed. And he is doing it for the sheer love of doing it with a coco. And
that folks, IS what its all about.
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