On Dec 17, 2023, at 8:52 AM, gene heskett <ghesk...@shentel.net> wrote:
One of the not so quiet revolutions taking place in the 3d printer landscape
over the last couple years is called input shaping, something probably done
best in the cards like Peter Wallace makes.
A simplified explanation is fitting an accelerometer to to tool head, the
exciting each axis it turn withe an audio sweep of a small amplitude, sweeping
from 10 Hz to about 250 Hz while measuring the resulting movement with an
adxl345 recording the data it outputs for each of the axises tested. Since a 3d
printer works in slices, the z axis is generally much slower on the 3d printer,
so is left out in some versions.
This data is then run thru a forrier or butterfly transform to develop a
compensating acceleration curve that does not effect the overall speed, but
does reduce the fine detail or boost it, to essentially cancel the machines
natural vibrations. On a 3d printer, given enough heat to supply the head with
continuous hot plastic, the use of this compensation has taken the 3d printer
from 50 or 60mm/second maximum speeds to 200 mm/second over the last 2 years. I
can now buy a printer with this built into its OS for under $1000.
There is one obstacle, most 3d printers do not have an interpreter that knows
about G2/G3 and its ilk, so most slicer's converts those to tiny straight line
moves that look like a circle in plastic.
This is done in the default interpreter, Marlin, but is done better by klipper
in the better printers but can be reflashed into 99% of the controller cards
out there just like we can do with Peters cards.
This interpreter runs on the controller cards, often stm32 based cards that
sell for, in the ones BTT makes for $59 for a low end octopus card, which can
drive up to 8 motors, 5 fans, 3 heaters and all our limit/home switches and
probing gizmo's. Some of these even include the G2/G3 stuffs. These cards are
nearly all designed to handle nema-17 motors at 24 vols and maybe 3 amp max
motors, but one line of the octopus family of cards has a separate motor supply
input that assumes 60 volt rated drivers so even those tiny motors can be moved
at amazing speeds.
Top that with signal stealing plugins that fit the driver socket of these
boards I'm rebuilding 2 bigger printers with nema-17 versions of the closed
loop servo/steppers with optical encoders that use drivers like we use with
linuxcnc, 2m542 sized stuff, but now rated for 90 volts and up so they can be
driven at unreal speeds w/o losing home. The PID is in the driver, linuxcnc
just tells them what to do and they do it. And if they can't do it, they will
tell linuxcnc, stopping it by linking that signal into the F2 of linuxcnc.
Doing the stop quick enough on my Sheldon the I can position a chuck jaw in the
way, and jog a carbide chipped tool into that jaw at 20mm a second, it hits the
jaw, the driver shuts off the motor drive at the same time it tells linuxcnc to
stop, the release of the motor drive lets it spring back away from the jaw by
10 thou or so. The carbide chip is not damaged and the chuck jaw is not marked.
Tested many times, but has never occurred wile running a job on either machine
where I've put those motors. No PID's in the lathe config, no PID's in the 6040
config. Its all in the much smarter drivers. They use the PID error to control
the motor current so if conditions are low load, very little current or motor
heating, but it it hits something, it will use every amp the supply has to
prevent a step loss, so home is maintained under downright abusive conditions.
In short, they Just Work and work well. I'm so sold I have 3 more motors,
drivers, and higher voltage power supplies to convert the G0704's other 3 main
axises to them, the A axis already is. With the PID's in the drivers, there
will only be one PID left in the G0704 when I'm done, in the spindle circuit.
It is the "input shaper" thing I think could improve the finish linuxcnc can
do, effectively driving the machine to cancel its vibrations as it changes speed and
directions. Doing it at ridiculous speeds. And that pays the bills in a for profit shop.
Something to investigate, for 3.1 maybe?
Cheers, Gene Heskett.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
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