On 11/8/23 07:25, andy pugh wrote:
On Wed, 8 Nov 2023 at 07:52, Thomas J Powderly <tjt...@gmail.com> wrote:
In this method the wire is reused,
it is wound onto a drum , which has to reverse.
A long time ago I had the use of an abrasive wire saw that worked on a
similar principle.
One of these, in fact, or the 1980s predecessor.
https://www.agarscientific.com/well-6500-vertical-diamond-wire-saw
How does Linuxcnc handle the reversing of a lathe head and chuck?
It is baked into the threading cycle code.
I think that the way to do this is probably to put the drum in charge.
When it is up to speed it enables motion and spark power, when it hits
the end stop it cuts power and motion, reverses, then enables things
again when back up to speed the other way.
I would build-in provision to keep motion and spark active to separate
drum speed limits. I can see you might want to stop the spark after
the motion, or vice-versa.
DOL reversing is probably the fastest, you have the whole power grid
to absorb the power, but I think I would use a (probably oversized)
VFD with a braking resistor programmed for active braking.
That's rather inefficient. And that is not how I do it on the 3 machines
that need it, and I am turning around a 35+ lb 8" chuck with .25 turns
of overshoot and getting back up to speed in 350 to 400 milliseconds at
100 rpms on the sheldon, And going from 3000 to 3000 in the other
direction on the go704 in that same 350 or 400 milliseconds.
A limit3 controls the rate of speed change between motion and the
spindle so the vfd in the case of the sheldon ramps the speed signal
down to zero on a direction change and a mux freezes the direction
signal at the same time.
The vfd or Jon's pwm-servo then gets a chance to prove its a 4 quadrant
controller by using the motor as a generator, dumping that energy back
into the power supply. When the edges from the encoder are far enough
apart that the spindle is stopped for all practical purposes, then the
lock on direction is released so it get to the vfd or pwm-servo, and the
speed signal is allowed to be restored at the input of the limit3, which
then ramps the spindle back up to speed in the other direction using up
the surplus energy in the caps long before they get hot and fail.
Knowing the chemistry of electrolytic caps, the overvoltage of those
surges keeps them well formed and they will actually live longer.
I haven't measured the vfd voltage but I expect its similar to the psu
on the go704, its normal 125 volts dc hits 165 to 170, quite a few volts
over the rating of the caps, but its those caps using up that
overvoltage that actually spins the motor up in the other direction. The
power line simply does to see a surge, but a null during the reversal as
it draws no power during the speedup phase until its up to 90% speed.
The mills top speed is 3000, the lathe is around 600. At 500 revs the
overshoot is around 4 turns and takes around 1.5 seconds, but the same
sequence is done by hal. And the shop lights don't blink. Machine power
draw drops to near zero during the early 90% of the elapsed time as only
the steppers are using very little with the stepper/servo's probably
stopped and drawing maybe 10% of their full power.
Both of those 2 machines have been doing that, for a decade+ in the
go704, and 8 years for the Sheldon. That go704 has a 1hp rated PMDC
motor, still running on the OE brushes, rated at 9.7 amps & 90 volts but
its getting 125 volts at a current limit of 18 amps. 3+ hp with no
complaints. No PID's in the sheldon except what's in the closed loop
stepper drivers. My 6040 has those motors for Z & B, and I have similar
motors for the go704 coming in about 2 weeks to join the A axis already
done, so when they are changed on the go704, the only PID left will be
the spindle.
I'm also rebuilding 2 bigger 3d printers with those closed loop motors.
No more "layer shifts" ever. I have the ones on the lathe and the 6040
connected into the e-stop so if they cannot get to where the TP sends
them, the machine is shut down, w/o power in the millisecond after the
driver shuts down the motor. Well tested. Funny thing though, neither
machine has ever done that mid job. Whats not to like?
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
_______________________________________________
Emc-users mailing list
Emc-users@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/emc-users
