Since I started posting my Arduino-EMC interface work I'm getting an email
every week or so from someone asking how to use this to do step generation.
I've generally given the WC Fields "go away kid, you bother me" reply but at
some point my day-job "if the users keep asking for it maybe I should
listen" mentality kicked in.
I've been reading the wiki and mail archives about previous assaults on this
particular windmill and want to throw an idea out there for criticism (feel
free substitute your preferred microcontroller for the Arduino):
1. EMC would generate control signals as it does for software step gen now,
except instead of bit-banging them out to the parport, would marshal them
into six arrays (assuming step and dir for three axes) and throw them over
the wall to the Arduino board.
2. The Arduino would buffer control sequences and latch them out to the
motor driver pins as necessary. The Arduino would be set to generate steps
on the same base frequency as EMC is set to "think" so that the commanded
feed rates come out right.
3. The Arduino would also maintain an internal absolute position counter for
each axis, kind of like a virtual encoder. This would be squawked back to
EMC as time permits. EMC would use this to update its internal state of the
machine's position.
The obvious catch with this (or any outboard step-generation scheme) is the
phase shift introduced between commanded, actual, and reported positions.
Obviously this would preclude any type of closed-loop control, but I am
wondering whether it could work with open-loop control where EMC is just
spraying control signals out to the world without real knowledge of the
machine's position. Likewise, I don't know whether the
virtual-encoder-counter idea adds anything useful, or whether it's better to
just accept that what the EMC GUI displays for the machine's position and
the actual position will be up to N milliseconds out of sync whenever the
machine is in motion.
While there are some significant limitations (I think USB bus speed would
limit the step rate non-trivially as well) in this, it would still seem to
be a useful option for the lowest end of the entry level like a tabletop
Dremel router or mini-mill. I'm wondering if something like this would allow
to run on machines without a parport and with uncertain latencies?
What I'm really not yet qualified to judge is whether there's some other
gotcha in the architecture of EMC that means that this approach would
fundamentally *not work*, or whether it's doable so long as one can live
with specific shortcomings.
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