Re: DIY CNC Machine How Hard Can It Be?

Jonathan Kirwan wrote:

On Sun, 17 Jun 2007 19:18:24 GMT, D from BC <myrealaddress@xxxxxxxxx>
wrote:

However, I do wonder if accuracy gets effected by sudden stops and
starts.

With stepper control, at least, my experience has been that there is a
controlled rate of speed increase (starting to move) and decrease
(slowing down towards the end of a move.)

Jon


And if you want the stepper to be able to go at more than a tiny fraction of
its speed capability, it will get up to a speed where a momentary (well say
millisecond) interruption to the stepping signals will result in it losing
step. Basically, stepper motors cannot stop suddenly without first ramping
down to a low speed. Either you need to always be running at a painfully
low speed, or you need a control computer that does not go AWOL for a
millisecond here and there.

I remember seeing a PC-controlled circuit board milling thing that used
stepper motors. Because it used some M$ operating system or other (can't
remember the exact vintage), sometimes the OS would find something more
important to do ... Step X Step Y Step X Step Y.... See if the network card
is doing anything interesting.... ooh let's try the screen saver... let's
page some virtual memory to disk...where was I ....Step X Step Y.... The
motors would lose step and half of the tracks and holes on the PCB would be
in the right place, and the other half would be shifted off to one side by
a few millimetres, sometimes cutting through the first tracks. Completely
useless. You really need to be in full control of whatever software is
controlling the motors, even if you're only using stepper motors. I
haven't tried that "EMC" software, (supposedly running some real-time linux
variant), but the only other alternative I can see would be to use a
microcontroller for the time-critical stuff.

Chris

.

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