Hi all,
I've been waiting for this discussion to segue to the Pi. Indeed the
Rpi4B is a decent jump up in capability. If one drives Mesa or other
hardware off the spi then the ethernet is nicely left open for
downloads, etc.With the -4b you seem to get it all. Speed,
communication, display (2), some flexibility and decent access to
Peter's or others boards for step generation. I suspect it will only be
a mater of time before someone mods the software to use both hdmi
ports. The good news I think is that the technology will just get
better. :-)
My plan is: Rpi4b - 7i90 - 7i33 (2) - sem servo motors but Mesa has lots
of solutions for steppers also.
Just a heads up; finding connectors that will allow one to directly
connect the spi to the 7i90 is not easy. The plan is to use the osh-park
40 pin to 26 pin board to as the spi bridge. Just because it is logical
doesn't mean it is possible. Still looking. ( ideas accepted ).
Comment: long ago (maybe 20 years ago) I acquired the glass scales
before I realized that they were good for position but not so good for
motion. Servo tuning with them is a bear. Not having any experience with
steppers can't comment with any authority but suspect they would be
easier since they are open loop (mostly).
Gene Heskett and maybe a couple of others are running the Rpi4b under
linuxcnc. Gene's implementation is for steppers.
Note: these comments are worth about what you paid for them. ;-)
Hang in there, stay safe!
Dave
On 3/18/20 5:40 PM, John Dammeyer wrote:
*From:*[email protected]
[mailto:[email protected]] *On Behalf Of *justin White
*Sent:* March-18-20 5:24 PM
*To:* John Dammeyer
*Cc:* Machinekit; [email protected]
*Subject:* Re: [Machinekit] Seeed to design and build Machinekit
focused Cape for BeagleBone Black/AI
Wow that has absolutely nothing to do with seeed's cape.
Yes it does!My previous posting suggested that a cape by SEED should
include the infrastructure to have a display and function as a
standalone electronic lead screw for a mill.And accept the motion
information from something like a Pi or PC via Ethernet if full CNC
was required.
That means the software running on the BBB would not be conventional
MachineKit.In fact the software might be straight C code developed
with the TI IDE as a standalone project with display that can service
as a multi-axis power feed and DRO.Or if could be a subset of
MachineKit without the HDMI interface.
So it could do motion.The idea is that it could also, via Ethernet,
like the MESA 7i92H receive information that would generate the
stepping pulses and feedback the DRO information.
It's time to think outside the box because with the much more powerful
Pi4 coupled to MESA hardware there is no way that SEED or anyone else
can produce something as good.So it has to be something different.
Seriously.Why pay $150 for a full blown BBB Cape that serves as a
Break Out Board and $50 for a BBB that then gives you sub par video
compared to a much faster quad core Pi4 running LinuxCNC and Ethernet
to a MESA board where the total price ends up pretty well the same.
Technologically the BBB is now old.Very old.And it's just plain silly
to continue to try and make it a be all end all.I'd say 99% of the
people I talk to who have CNC are running either MACH or maybe
LinuxCNC. Mostly they aren't interested in changing.The ones who have
mill but not CNC either want to add power feed and perhaps DRO but
again most of them are not interested in CNC.
So where's the market?It's easy.The DRO and Power feed controller that
can magically turn into a CNC controller.
JohnDammeyer
On Wed, Mar 18, 2020, 7:37 PM John Dammeyer <[email protected]
<mailto:[email protected]>> wrote:
Would someone perhaps be able to describe simply (like MachineKit
for Dummy's) how exactly the step pulse and direction shows up on
the Beagle pin relative to the motion command from a G00 X1.0
where current X position is 0.0.
Clearly we accelerate and move and then decelerate to arrive at
the end point of 1.0. I do this in my ELS code inside the
interrupt routine which happens at a 20kHz rate. If a step is
required I set the output at the beginning of the routine and then
clear it at the end after a whole bunch of other stuff is done.
Whether to step or not is done by the equivalent of a trajectory
planner. For example in simple terms if the motor could
accelerate instantly and the step rate was 10kHz then every second
interrupt the axis is pulsed. Also for every pulse the position
is incremented if going in the positive direction. Decremented if
going in the other direction. When it matches the end position
the stepping is terminated and we have a move to position.
Inside my ELS interrupt code I do the accel/decel too. The
interrupt routine changes the number of 20kHz intervals between
step rates and also tracks the half way point. If we're not up to
speed yet but the halfway point is reached then deceleration is
started and by the time the stepping rate reaches 0 the motor has
also reached it's desired position.
Otherwise once the step output pulse rate matches the desired
velocity the acceleration part is terminated and the number of
steps it took recorded, and now a pulse happens every second
interrupt (for 10kHz step rate). When the destination location
minus the time it took to accelerate is reached deceleration
begins. And since we decelerate at the same rate as accelerating
and we have that same distance to move, once we reach 0 speed
we've also reached the destination.
Now this could all be done outside the interrupt routine in a
trajectory planner. One could determine that it will take time X
to reach either target speed or midpoint of the distance to move.
Broken into the 20kHz steps then the information could be fed into
a FIFO so no math is done at all inside the interrupt routine.
It's only task would be to read this from FIFO one byte during
each interrupt and write it to the port. The interrupt routine
could then run 40kHz and clear the port of step signals every
second pulse. If there is no motion the FIFO, flagged as empty,
holds the last entry for the port and it's just mirrored out to
the port.
So how is it done in the Beaglebone with MachineKit. Is there a
function that reads a FIFO filled by the trajectory planner or is
it done yet some other way? I would look it up but don't even
know where to start.
Obviously there's probably more going on under the covers to deal
with hard limits. Seems like soft limits are dealt with before
motion starts with the option to not move because it will exceed
machine limits.
Thanks
John Dammeyer
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