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50:1 worked well you can use the THCAD10 without any modification, 
Some good practices are:
-Place THCAD10 as close as possible to the Plasma power supply.
-Use a shield cable and connect the shield to GND on one side only.
-Use a ferrite bead on the control side to reduce induced noise 
 

Omit the offending line and try again
from
setp hm2_5i25.0.pwmgen.00.pwm_frequency 1000
to
#setp hm2_5i25.0.pwmgen.00.pwm_frequency 1000
---I am pretty sure that is for plasma systems, so the spindle encoder inputs are NOT A/B/Z, but are A0/A1/A2 for 1 to 3 of THCAD.
Anyone, do correct me if that is wrong.
Thank you.

Gents,

I'm slowly preparing items and knowledge to make my plasma cutter based on Hypertherm Powermax 45 XP (just received CPC port upgrade kit). I have a specific conditions - at work for 4 weeks and away from home/garrage and i could only take limited set of toys with me:
- Lenovo T470 with Linuxcnc installed
- Mesa 7i95T
- 2 x THCAD (for THC and ohmic)
- some limit switches
- 3 x H-bridge and 3 x small PM DC servo motors
- bunch of wires
- old laptop power supply to power those servos.

That's all i have to play with LinuxCNC and learn configuration, etc. Ideally i'd like to make those servos live on my bench.
I couldn't take "real" servos and drivers due to weight limitation - my only option was to take those small servos and h-bridge modules. 
If i understand things correctly, out-of-the-box mesa cards are preloaded with firmware containing Step/dir generators and can't be used to control -bridge based servos it this state. So i need to build a specific firmware to replace step gens with pwm generators, right?
I'm stepping here into very dark area for me - creating custom bit file.
I heard that custom bit file are now easier to create with the new FPGA chips on Mesa cards - is it true?
I case of my failure - would it be possible to get some help from you to create custom bitfile? The only source i found is talla83 videos on yt.

Thank you for inputs and have a great weekend.

You could get rid of the short and try adding an isolated 24v power supply (Meanwell hdr-15-24) that is connected to the plasma inputs on the thcad. When cutting, nothing will get damaged. The hypersensing component can see full torch voltage when you are cutting  and it does not care... so this should give you the confidence to do it

You might need to and2 the probe signal so it will only turn on when actually probing.

Here is a quick test where I jumped the shield to the nozzle and then feed voltage from my ohmic into it and then ground the nozzle and the voltage drops like a stone. My thcad-10 is using opto22 and relays to isolate everything so that’s why I have to trigger the ohmic button to send voltage to the nozzle.



youtube.com/shorts/fuJnQ76LXl8?si=pr583jeB8bvqNj0g

Yes, that was one of the original reasons the ohmic test was included. It was intended to check if there was a short like yours before starting a job. It does not happen very often. I only ever experienced it once and I found a bit of rubbish inside the tip,  much like you did. I have thought of setting it up to use one thcad. I'm sure it could be done fairly easily.

Oh ok so you’re not waiting for the voltage to go to zero to trigger the contact. Your looking at any voltage drop and once it does it’s triggering contact. That make perfect sense to use a thcad and watch the frequency output. Then you could set the trip frequency on the encoder input on your mesa board. I wonder if there is a way to double duty a single thcad-10.

You know I stumbled upon something the other day by accident that could work. I was using my fine cut consumables. And I was testing my retraction to make sure my actual retraction was what the display was stating it was. I held the ohmic button down and moved z down till it stopped from the ohmic trip. But I noticed when it touched I seen a +10v on the cut voltage. I did it agian and it was still there. So I started looking around and found a tiny slag bead stuck up in the nozzle bridging the nozzle to the shield. This was back feeding the ohmic voltage into the nozzle and it was showing up on the cut voltage.

Would there be a way to use this to sense ohmic. I don't know the ramifications to running the nozzle bridged to shield but it had been like that for awhile as I had just been cutting.

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.