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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.