[snowluck]

Hey! I'm building a 4axis (so far) CNC mill, but feeling a bit overwhelmed on which control mechanism to use. My motor/drives are DMM Dyna2 for the three linear axis, and a harmonic drive with a stepper for the fourth. I plan on not using electronic gearing for the DYNA2 drives, which mean 16384 steps per revolution, with a target output pulse rate of up to 500khz per axis.

My main options I'm considering are:
LinuxCNC with parralel port output
LinuxCNC with ethernet Mesa board output
Smoothieboard, using just the step/dir outputs rather than the powered
TinyG, same as above
GRBL port for teensy: https://github.com/matthewSorensen/k20-grbl-port
G2: https://github.com/synthetos/g2
Beaglebone black with machinekit: Machinekit • Moves. Controls. Things.

I've heard TinyG and Smoothieboard have better motion planning than LinuxCNC, higher order smoothing, but I don't know how signifigant the difference will be in practice (jerk vs acceleration limit, or jerk derivative, I didn't really look into it far).

Here is a CAD model of my mill roughly:



I was wondering if anyone has any stronger oppinions? I'm leaning towards the BeagleBone Black.

[109jb]

LinuxCNC got a new trajectory planner with a recent release (2.7) and I don't think TinyG or Smoothieboard are any better. I think it is simply a matter of them not having updated their website.

I don't think Smoothieboard supports 5 axis motion, but rather 3 axis and 2 extruders, or slave axes.

Th official GRBL firmware is only 3 axis

Tiny G is good and has the reguire number of simultaneous axes, but I'm not a fan of the user interface.

None of those mentioned support cutter radius compensation except LinuxCNC.

if you are dead set on no electronic gearing and having 16384 steps/rev, then you can throw any parallel port idea right out the window. Can I ask why you want 16384 steps per rev. There is really no reason to do this.

For your specs I would only consider a LinuxCNC with Mesa card option.

[snowluck]

Thats true, I'll use electronic gearing of atleast 4. Is a mesa card still my best bet? Do you have an oppinion of running linux CNC on the BBB? They have hardware acceleration through the pru coproccessors.

Does linux-cnc support definition of arbitrary transfer functions? An alternative robotic arm project of mine has a nonlinear transfer function, which I could linearize about various point and interpolate between models, but running on top of a nice motion planner would be conventient.

[jfong]

Of the controllers you listed, LinuxCnc with the Mesa board would be the only one that can output 500khz step pulse rate or more.

Beaglebone black with the fast A8 and PRU may be able to but no definitive answer I could find.

Do you really need 500khz pulse rate?

I couldn't get G2 to work reliably on a due and I really hated the interface.

I ended buying a Uc300eth motion controller today to update my router from Mach3. Its capable of 400khz step pulse rate. That's about 4 times faster than I need it to be. I'm running servos but not as high encoder ppr as yours. I think mine are 8000ppr. It was $155 with the UCCNC controller software. I ended up choosing this over LinuxCnc for my router. I still use LinuxCnc on another machine.

I use grbl but on a simple test bed setup. It has its limitations and would never really consider using it to run a CNC full time. It just lacks to many features that even mach3 (as old as it is) has.

[109jb]

I haven't followed the beaglebone stuff, but I don't really see the benefit of it unless you look at it as an interesting diversion. An old desktop with a pci mesa card is perfectly capable of running LinuxCNC, and running it well. That would be the most trouble free approach to get high stepping rates from it. There are countless people that have taken this approach so you wouldn't by any means be blazing new trails. The Tormach commercial mills essentially use this. Their PathPilot software is LinuxCNC based and they use a PCI mesa card in their controller.

Yes LinuxCNC can do non-linear kinematics. I assume you are talking about something like a scara robot. I have no direct experience with this, except for being mesmerized by the numerous YouTube videos, but here is a start of the information from the manual.

http://www.linuxcnc.org/docs/2.7/htm...inematics.html

[109jb]

I use grbl but on a simple test bed setup. It has its limitations and would never really consider using it to run a CNC full time. It just lacks to many features that even mach3 (as old as it is) has.

I actually use GRBL on my converted bench mill using my own interface. It definitely has limitations, but mine is a hobby machine, so some don't matter. I have made up for some of the others with my user interface. For example, GRBL doesn't natively support canned cycles, but I have them i fully implemented through my interface. After all, the canned cycles are just a bunch of moves strung together in the right order. My interface supports G81, G82, G83, G85 and G89. I also implemented G90.1 support for arcs. Some other things are missing, but depending on how you use your mill it may not matter. About the only thing I would like to have is cutter comp G41/G42, and I am working on it.

One thing GRBL and all of the other microcontroller based solutions have going for them is that the only thing GRBL on an Arduino does is control the machine. It doesn't have to deal with background programs, or operating system tasks, or anything else except running the machine. It is true real-time. Any solution that doesn't offload the critical timing tasks from an operating system to a smooth-stepper, UCNC, MESA card, Arduino, or other dedicated hardware is only operating "close enough to real time", and then sometimes it won't even be close enough.

[snowluck]

Mesa card + linux cnc sounds like my best option. The other project may be like a scara robot, or a continuum actuator where the "spine" is flexible, and would receive position feedback from external cameras.