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Stahlwerk
www.stahlwerk-schweissgeraete.de/Plasmaschneider-CUT
Good quality and cheaper than china, vertical machine torch is 40E in Germany, with 5 meter cable is 90E, bought both.
Mind the pilot arc, there are some without it so do not get those, it is easy just look for a red knob on the bottom front and usually it is written in spec info.

rodw and tommylight thanks for your answers. Although I have to admit I am stil confused.

I bought a cheap pilot arc plasma to play arround. It has an explicit connection between the tip and the workpiece. Ohmic sensing can't be done. Right?

Anyhow, I build a CNC Plasma and I am looking for a suitable frontend (plasma and torch) my wishlist includes:

1 plasma cutter (hopefully < $1000) with a european distributor
2 Ohmic or hypersensing (willing and able to build homemade electronics if I know what)
3 THC with a THCAD-x (best case with an avaibale THCAD-300)

Can someone propose a setup ?

roland206 wrote:

All of the pilot arc torch heads I know have an electrode connected to the plasma voltage (negative) and a shield connected to the workpeace.

Oh no!!!!!
Nozzle is not shield.
Nozzle can not be connected to workpiece, although in some "high voltage pilot arc" plasma's it may seems so when measuring, transformers n stuff...
Nozzle is connected to high voltage/high frequency when pilot is on
Shield is an extra layer not connected to anything internally, usually there is a retaining ring and a wire connected to it leading to ohmic sensing parts of the electronics, or not used at all electrically, just for protecting the air stream on multiple gas systems.
Pilot arc is between electrode and shield.
Cutting is between electrode (negative as you already stated) and material/table/clamp that is positive.
During cutting nozzle must be "off or electrically detached" otherwise it would burn to a crisp in 1-2 seconds.

except you are confused between the torch tip and the shield. Some torches (Hypertherm, and Thermal Dynamcs for example) Have a seperate shield that is not shorted to the workpiece.

So in general if you have an exposed tip you can't do ohmic sensing because its shorted to the material.

Some machines (ie Swiftcut) lower a seperate probe and ohmic probe off that. This can be done fairly easily in QTplasmac with a pneumatic cylinder as all the required signals are there.

I have a fundamental problem to follow the discusions on ohmic and/or hypersensing.
It would be great if someone can point to the mistake in my thinking.
All of the pilot arc torch heads I know have an electrode connected to the plasma voltage (negative) and a shield connected to the workpeace.
With a HF pulse on the plasma voltage (relative to the shield / workpeace) the arc is started.
This means that the torch head tip (shield) which is electrically connected to the workpeace can't be used for ohmic sensing because it's alrerady connected to the workpeace.
What's wrong ?

 

No special firmware is needed for THCAD (if you have a free encoder)

I'm having the same issue, but with a 7i97T + 7i78.  PWM Version is reporting 0 when doing the HMID Read.   

Could you please provide the updated firmware for this combination as well?  Additionally, I'll be using a THCAD with QTPlasmaC.  Is there any custom firmware needed?  Not planning to use an additional THCAD for Ohmic sensing currently.

JTknives wrote:

I just watched your YouTube video on you high speed ohmic and it’s impressive. 

I assume you mean this one

I had a job which I filled a 8' x 4' sheet with some flanges and small parts. I cut it in 3 sections on a 4' x 4' table and there were heaps of probing so over time I tweaked the probing routine heights etc. I was using my hypersensing which uses a THCAD for the ohmic sensing. it does not like Hypertherm machines on a water table but it would work for you by the look.

Part of the speed comes from the way it breaks contact on up travel because as soon as it senses a fall in voltage, probing finishes (before the contact is broken).
I've been sent a ohmic circuit from Proma Elextronika in Poland. Its a very nice piece of kit and have some very good reviews on it. This will by my water table solution.

Sorry for the late message. Unfortunately I had to give the project less priority. 
At the moment very busy with my full time work, my own company and graduating from my bachelor mechanical engineering next to a more than full-time job. 

I am slowly designing further on the mechanical part during my breaks. The hardware will be a pc that runs on plasmac in combination with a mesa board and a thcad. 

The drive will be a belt, I do not believe that this stretch is noticeable under the load of plasma cutting. Much cheaper and simpler in design than rack and pinion.

 

I want EtherCAT as an interface.

I forgot to mention I am planning to use Ethercat too. A Beckhoff 5151 encoder interface arrived today. I also have a THCAD10 here which may allow reading of 0-10 volt using it. I will use a Beckhoff 2502 for PWM signal for laser.

I started a build for new CNC plasma table so here's my starting point.

CPU Ryzen 5 6C/12T 3.2Ghz 32G RAM, 550G SSD, Dual Ethernet ports
Ubuntu 24.04.02 - I know it's not "debian" but is what all other machines here run. Compatibility and support are the key deciding factors
PREEMPT-RT installed, Fairly minimal system with snap completely removed.
Stepper Drives (4ea) Stepper Online DM542T - have had excellent results with these in the past
Stepper Motors (4ea) Stepper Online 23HS30-2804S - again have worked well in past
Custom built frame from 2x2 16ga steel tubing. All Linear Rails SBR20. All Ball Screws SFU1605
Motion Controller Mesa Electronics 7i95T & THCAD2 - ordered and expected arrival in next few days
LinuxCNC version 2.10 being built from source.

I can successfully build the rip version without error and it appears to work correctly however as I mentioned I do not have the 7i95T in hand quite yet and it's not listed as an option in the opening config choices.

To get the full package I'm working to compile to get the .deb version and having some make problems.

Problem 1
I get lots of errors during the build of man pages. Some work but many report this error and often multiple times.

Making manpage sampler.9
<unknown>:1: SyntaxWarning: invalid escape sequence '\S'

I suspect this has to do with the fact that Ubuntu 24.04 uses python3.12. I haven't dug into to it but have had many problems in many projects with python version compatibility and this is known issue from 3.11 to 3.12. Any suggestions on how to fix this are welcome and sooner or later it'll have to be addressed anyway.


Problem 2
This is the end summary of the build

Compiling module_helper/module_helper.c
<command-line>: error: "_FORTIFY_SOURCE" redefined [-Werror]
<command-line>: note: this is the location of the previous definition
Compiling localized message catalog ../share/locale/ar/LC_MESSAGES/linuxcnc.mo
Compiling localized message catalog ../share/locale/bg/LC_MESSAGES/linuxcnc.mo
cc1: all warnings being treated as errors
make[2]: *** [Makefile:278: objects/module_helper/module_helper.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory '/home/plasma/Documents/linuxcnc-source-dir/src'
dh_auto_build: error: cd src && make -j12 build-software returned exit code 2
make[1]: *** [debian/rules:55: override_dh_auto_build-arch] Error 2
make[1]: Leaving directory '/home/plasma/Documents/linuxcnc-source-dir'
make: *** [debian/rules:43: build] Error 2
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2

The _FORTIFY_SOURCE error shows up many times as well and probably isn't the only problem.

Again I know it's not debian but Ubuntu is derived from debian and the rip version compiles and works so I've very optimistic once I work out a few kinks this will too.

cc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
Copyright (C) 2023 Free Software Foundation, Inc.

Thanks for any and all suggestions. Mike

 Great! Let's move ahead.

Let's talk about 0-10 volt range first.
The Mesa THCAD is a voltage to frequency converter. The THCAD10 has a 10 volt full scale. The THCAD 5 has a 5 volt full scale. The newer THCAD2 can be configured to either have a 10v or 5v full scale range. For your hardware, the 10v range sounds best.
We read the frequency using an encoder A input (usually on a Mesa card) and then scale it internally back to a voltage. Usually we set the THCAD's on board frequency divider so we read 1/32 of the frequency when sending it to Linuxcnc.

To receive a frequency, we would simply connect your hardware direct to an encoder A input and not use a THCAD.

It would be easy to apply a mathematical function to the encoder frequency to convert to a voltage in a real time component written in C.

There are other options to read both voltage and frequency. For example, appropriate Ethercat slaves for both voltage and frequency are available.
I think for Ethercat I would prefer 0-10 volt.

Finally, I might mention that for plasma torch height control, we adjust the height using external offsets (this offset is external to the motion controller). A scaled count (much like a MPG) allows us to take the input and output a height adjustment of the error. But because you output a distance, it might be possible to manage height control via one of Linuxcnc's PIDS.
 
Anyway, if you have a contact us option on your website, let me know, and I will reach out to take things further.

fantastic.

There is still a possibility of implementation, so I would like to adapt it to LinuxCNC as much as possible.

I am not sure if it makes practical sense to process data multiple times, which loses measurement accuracy and introduces additional errors at each stage C/f -> U -> f ...

i can made frequency output directly in sensor.

I see that the THCAD5 card processes voltage to frequencies in the range of 100 kHz - 1 MHz, but I can adapt to any frequency (lower frexuancy could be directly monitored in many "less efficient" controllers and send via cable easyer ), it can also be a PWM output or serial communication, which I can send a ready-to-use height without the need for the CNC system to process the signal, which in my opinion would be the best option.

In your opinion, what would be the best option?

Send one to Rod, her can make it work easily as he has THCAD that turn 0-10V to frequency, and he is very detailed when testing stuff.
I do not have a laser (i do have a big box of lasers, but He-Ne ones at low power) so i can not help with testing it. I can attach it to a plasma torch and see how long it lasts with all the interference plasma causes, but that is it. Oh yeah, Rod can also test this, as he has plasma machines.

Hello LinuxCNC community,We are a group of engineering students working on our semester project – building a CNC plasma cutter using LinuxCNC as the control system.Since we are developing the machine from scratch and learning LinuxCNC at the same time, we would greatly appreciate the opportunity to discuss and receive guidance from experienced users. We are particularly interested in understanding potential challenges, key considerations, and best practices for setting up LinuxCNC for our machine.Brief Overview of Our Machine:

• CNC plasma cutter with X, Y, and Z axes
• Z-axis with a ball screw and linear guides
• Floating head with a proximity sensor as a probe input
• Plasma cutter with a voltage divider and THC (Torch Height Control)
• LinuxCNC control, likely using Mesa hardware
• Torch Height Control (THC) integration via Mesa THCAD-2
• Voltage divider (50:1) for plasma voltage feedback
• Probing system for automatic height sensing before cutting

We have already explored various existing solutions, but we would love to hear from those with practical experience in setting up and running a CNC plasma cutter with LinuxCNC.Some key topics we would like input on:

• Choosing the right hardware (drivers, Mesa cards, sensors)
• Configuring LinuxCNC (HAL, INI, probe input, THC integration)
• Common challenges and pitfalls when setting up a plasma cutter
• Best practices for calibration and fine-tuning

If anyone is willing to share their expertise or discuss this with us in more detail, we would greatly appreciate it. A virtual meeting would also be very valuable for us.Thank you in advance for your time and help.Best regards,
Ibrahim Al-N and the 4.semster students at Absalon Kalundborg (Denmark)