About 0.5sec + 1 move.

Mhmm you're right in saying that's going to be a problem.

My plasma is remarkably forgiving in terms of crashed into the part so perhaps I'll begin with a sprung torch (to ensure it doesn't crash too hard) and focus on the other axes while I see how either modifying the baby steps/DDA goes.

Thanks once again for the help

@mhackney said in Advice on adding plasma torch height control:

David, in thinking about this, could the M290 baby stepping be used to inject the Z height variation as an option #4?

This sounds like a pretty good entry point for me, as getting into the nitty gritty of the integral code base, especially given you intend to soon rewrite is somewhat daunting.

You said:

To make it work well I would need to rework the babystepping code to make adjustments more quickly than happens now, but that's desirable anyway.

How slow is it currently?

EMI Testing

Just a quick update on this one. I've done some testing very specifically on the EMI induced in a shielded wire taped adjacent and in very close proximity to the plasma cable. This test cable is a CAT 6 Ethernet cable, where each pair is foil shielded and the whole cable is braid shielded.

Also note that I didn't actually conduct everything in the following order, but I thought it made more sense to present it this way.

Test Setup

I got some interesting results (and way, way more noise than anticipated).

Control

No wires connected to the scope (except the power cord)
Thanks dad for suggesting this one after a mammoth amount of head scratching

At a (extremely short) peak of ~-40v, I was already surprised by how intense this noise is. Note the scale of the chart is 10v/div and 250ns/div. This pulse however only lasted for less than half a microsecond.

Unshielded Cable

The cable's shield wasn't grounded and instead connected to the signal line of the scope. The scope's ground was earthed.

Peaks are well above 100v and noise is at least 3 microseconds

Shielding

Grounding the shielding substantially improved the noise. Apologies for a lack of photo, I will repeat the experiments tonight and upload another image of the shielded cable for comparison.

The shielding reduced the peaks to ~30v, but this also changed dramatically depending on whether the torch was grounded when the arc was initiated. I believe that preliminary a set of small ceramic capacitors to ground should suitably protect the drivers, but I think I'll be testing it with a stand along step stick and cheapo arduino first!

Thanks @OwenD !

I haven't (yet) had much experience with CNC plasma systems, but really appreciate your input on the matter.

I hadn't actually at all considered that cut speed would vary the voltage so that's certainly something to look out for. Perhaps I can do that based on the move feed rate change or add additional M3 Sxxx codes in these sections. I'll look into it though and your advice is really appreciated.

As for the separate controller, that was my original intention (though with only a couple of GPIO lines), which perhaps would still be simpler given the difficulty of independent Z moves in the current setup. I do like however having a tightly integrated controller, which allows for things such as automatically varying the expected cut voltage based on the current program. It also means I can use the (already plentiful) stepper drivers etc... on the board.

I actually have another technical concern you could perhaps help with. I am worried about the EMI from the plasma damaging the controller and was wondering the level of shielding/protection used on industrial machines. I have heard of swathes of hobbyists having issues in the past with this one and damaging one of these isn't an inexpensive issue. What are your thoughts on the matter?

Great, thankyou. I'll take a look in the whole back end of things!

Can you point to anywhere beyond the code for any documentation/diagrams etc... that may be of use?

Thanks @dc42 , that's the workflow I expected. I'll double check the M454 with Marlin

Regarding the gap, it depends slightly on the type of torch, but for mine and other cheap torches:

1. The torch begins at z top (for arguments sake ~30mm above the work piece and the Z axis is low-load so high speed)
2. M3 is called to start the torch
3. The plasma is turned on. Vref will immediately go to max, ~90v (which will be read through a voltage divider, low pass filter and probably some other protection)
4. The torch lowers via the PID until the an arc at is established at the desired voltage (which will be set in M3 using the S(?) parameter)
5. Once the voltage is deemed stable etc... M3 continues, allowing the program to run normally
6. A dwell should be included after the M3 to allow for piercing of the work-piece, but this can/should IMO be a standard dwell for the sake of tuning for different materials

My apologies, I wasn't very clear by 'preference on how I can add this functionality', but I meant how you would prefer/recommend the functionality to be implemented. After having a look through the code (well commented I must say!) this is my current plan:

• Add 'plasma' to Enum MachineType (https://github.com/dc42/RepRapFirmware/blob/dev/src/GCodes/GCodes.h)
• Add configuration basically the same as laser and cnc are handeled
• Add M3 Sxx command for plasma MachineType. I'm currently expecting this to add the waiting functionality using a GCodeResult::notFinished similar to these !!!(https://github.com/dc42/RepRapFirmware/blob/dev/src/GCodes/GCodes2.cpp#L3330)

The other thing I'm having a little trouble with understanding is adding the motor control for the Z height. You've already obviously got a reasonable step generator which I'd ideally reuse. Can you recommend the best place to tap into it to? Do I need to schedule a new task in the RTOS?

Thanks once again

Matt

Hi all , awesome work here! One of my good friends recently bought a duet and it has sparked my interest in them as a controller.

For a little context, torch height control uses the arc voltage from the output of the plasma cutter to approximate the air gap between the torch and work piece. This voltage is then used as the input to a PID loop to maintain a constant height of the cutting torch, ensuring clean cuts.

You've obviously already got a PID library, Tool library etc... and wanted your advice/preference on how I can add this functionality to maintain the direction and structure of the project as you want.

Looking forward to working with you!