OpenPnP with LinuxCNC via linuxcncrsh?

I see what you mean about the Z drives, but I'm actually not keen on the belt drive arrangement (as per the video). The problem is that the first operation when homing is to home the Z, but with this arrangement it is possible that one nozzle could plunge into something it shouldn't. I haven't looked that hard into an alternative, but I can't really see a way to make it fool-proof without using individual Z drives. 

The Odroid's are fantastic little boards, I run a C4 on my desktop plasma cutter (also running LinuxCNC). I've written the LinuxCNC driver for the I/O pins on the C4 and hence don't need any interface boards (well, perhaps some interface circuitry, but nothing with any intelligence). I did at one point make an ethernet driven interface out of a STM Nucleo board, it worked well, but getting hold of the main chip proved impossible, so gave it up. 
 

plhsystems wrote:

I see what you mean about the Z drives, but I'm actually not keen on the belt drive arrangement (as per the video). The problem is that the first operation when homing is to home the Z, but with this arrangement it is possible that one nozzle could plunge into something it shouldn't. I haven't looked that hard into an alternative, but I can't really see a way to make it fool-proof without using individual Z drives
 

That's a valid concern. I typically see OpenPnP used on machines that the nozzle can't actually reach the platform, everything is raised up from it. Haven't used OpenPnP yet but if it can just call LinuxCNC's homing routine, then it is fairly simple. Just needs to call the "home all" and setup the routine to home Z last and set X and Y home coordinates to some spot where you'll never put anything. Technically not foolproof but pretty simple to mark a spot on the platform to never put things

The only problem with that (and this is true of any machine that doesn't use absolute encoders) is that the system doesn't know where any of the axes are and therefore any movement on the X and Y with the Z in an unknown position could cause it to crash into an object. I know it's unlikely, but if you hit the stop button just as the nozzle was picking up a part from a feeder, any movement after that that isn't homing on Z, risks damaging the nozzle and the feeder. Z homing must be done first to avoid any collisions, I know it may seem slightly paranoid, but if there is potential for a machine to damage itself through misuse, someone will misuse it.

Homing Z first is probably the worst idea. if the machine has no idea where X Y or Z are then poking something sitting on the platform by homing the heads is pretty likely to smack a part.

Sure if the heads are not retracted then you can have a bigger problem by homing X and Y first but the simplest thing to do on a machine like this is find out if the heads are retracted.

For one, using a cam style Z will by design retract the heads anytime the steppers are disabled and only requires 1 Z for 2 heads.

For two, just using a potentiometer on the Z shaft will give you it's absolute position. Pots have been used in industrial machinery for many years and while I'm typically not a fan of pots for positioning, they would be perfect for just checking if the Z position is in center range before homing.

As long as you know the heads are retracted it's far better to home X and Y first.

Right, I see, you consider home in Z to be fully extended, I think opposite, so I see why you think homing Z first would be wrong.

-Why would anyone assume the Z axis home to be down? It is up, always and on every machine i came in contact with.
-Even if using pneumatic Z axis, it should have homing disabled if no air pressure
Or am i missing something?
I have yet to see a machine, any machine that does not home Z first.
I am sure there might be some strange machine that does that, but i can not think of one.