Re: [CAD_CAM_EDM_DRO] PC based CNC system architecture

Jeff -

I think what you say below is correct. I also think
this is exactly the thing Jon Elson is working on.

Here is a quote from a post Jon previously made:
(Jon - I don't mean to throw you in the middle of this)
******************************
What I am working on is a 4-channel programmable pulse generator
that would actually send the step and direction pulses to the stepper
driver. Each channel would be given a velocity value and a direction.
It would also generate quadrature pulses that followed the step and
direction signals, and could be used to drive old phase-drive type
stepper drivers, as well as be counted by an encoder counter, for
open-loop systems. For closed loop, the encoder counters would
read actual encoders on the motors, or on the machine itself.

I am running EMC with a 100 MHz Pentium, and updating the
servo loop at a 1 KHz rate, with plenty of CPU power to spare.
This type of device would look about the same to the CPU,
encoder values are read in, servo calculations are done, and a new
velocity command is written out.

A 24-bit counter/comparator circuit running at 10 MHz (100 nS
resolution) could output up to 5 Million steps / second (although the
jump from 3.333 to 5 million is obviously too big a jump).
But, reasonable step rates would be well in excess of 100,000/sec,
which might be needed for people using high microstepping
division ratios. And, the timing resolution there would still be
100 nS, giving extremely fine velocity control, no large jumps
in velocity, and smooth acceleration and running at any sped
desired.

The low end is one step every 1.6 seconds. The code could either
stop the counter if no steps are needed, or just let it bump every
1.6 seconds, first one way, then the other. This would cause
some VERY slow and small hunting, and might not be objectionable.
If it is, the code could shut off the pulses when the commanded
velocity is below some threshold, like .001" / sec.

Jon

***************************************

It sounds like this "black box" is already being done and drivers
for EMC are also in the works, from what I have read on the EMC List.



Jeff Barlow wrote:

>
> Hi again,

(SNIP)

> We seem to be in general agrement that hooking motor drivers directly to
> the PC printer port doesn't really cut it. We also seem to agree that
> there is a missing piece of hardware that would go between the PC and
> the motor drivers. It is, in my view, the exact form that this "black
> box" might take that is the primary subject of debate, here.
>
> It is tempting, for many of us, to try to go off by ourself and design
> this "black box". If we do, we will, most likely, get tripped up by the
> usual tendency to view a task in terms of the tools with which we are
> most comfortable. I'm trying to resist this temptation.