qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.

Alrighty, I did have to add24k 5w resistor to get thiungs to work according the RODW's drawing above however ive run into a plasma torch problem

The torch will run just fine for 2-4 pierces then gets a tiny flake of metal which shorts the torch shield (24v+) to the nozzle (plasma cutter gnd) thus locking up the ohmic touchoff as it things it is touching the table.

Except for galvanic reasons (using a downdraft table so it shouldnt be an issue anyways) any reason why the table cant be 24v+ and the torch gnd? This would solve that issue. Just want to make sure I wont fry things. Open to other options as well

Thanks

jimmyrig wrote:

Alrighty, I did have to add24k 5w resistor to get thiungs to work according the RODW's drawing above however ive run into a plasma torch problem

The torch will run just fine for 2-4 pierces then gets a tiny flake of metal which shorts the torch shield (24v+) to the nozzle (plasma cutter gnd) thus locking up the ohmic touchoff as it things it is touching the table.

Except for galvanic reasons (using a downdraft table so it shouldnt be an issue anyways) any reason why the table cant be 24v+ and the torch gnd? This would solve that issue. Just want to make sure I wont fry things. Open to other options as well

Thanks

If you have a short in the torch, ohmic of any kind won't work.
The original reason the ohmic test was added was to check for a shorted torch.
The pulse torch was added to get preflow going to kepe debris out of the torch
There are some voltage thresholds you may be able to play with but use halshow to understand what is going on.
It is possible that you don't have a full short and the resistance is reducing the voltage below the threshold causing it to trip.
I did try inverted polarity when developing this technology but it didn't work. Too long ago to remember why. 
Maybe you could try again. You won't beak anything. But the full torch voltage is in the circuit during cutting.
 

Well...I may have stumbled into something interesting.....might have found out why people are having issues with the thcad ohmic touchoff....and how to fix them!

So i flipped the + and - so the table is now 24+ and the torch is gnd.... works just fine, still using the 24k pullup resistors on the table (was torch). Since the isolated power supply is now the same the same polarity as the plasma if any junk builds up in the torch the arc voltage will start to represent the power supply (when the torch isnt running of course) so over time you see the "torch off" voltage climb from 0 to 24 volts as junk builds in the torch. ..... This is fantastic because what it means is after the torch gets enough junk to trip the ohmic thcad we can use the arc voltage to do ohmic touchoff!!!!! Since the arc voltage drops to zero when the probe touches the material!!!

I bet we could even ditch the ohmic thcad and just use the arc voltage if we short the torch with a high enough resistor (essentially what the dust/slag is doing)


This should be fairly easy to code..... gonna give is a shot tomorrow. 
 

Funny I started doing it that way and changed it for some reason based on feedback from PCW and a problem we experienced.
There is actually an is-cutting pin (or similar name) that goes true when there is arc voltage. Does it light up for you?
The 390 k  resistor is designed so a 24v power supply will give about  4.9 v full scale to the thcad 5. So if it ever sees > 5 volts it has hit full scale from seeing an arc voltage!

Great to see somebody hae been constructive in improving the circuit instead of complining about it not being as good as a relay system.

snowgoer540 wrote:

jimmyrig wrote:

Except for galvanic reasons (using a downdraft table so it shouldnt be an issue anyways) any reason why the table cant be 24v+ and the torch gnd? This would solve that issue. Just want to make sure I wont fry things. Open to other options as well

The following post still accurately captures my thoughts after the THCAD-ohmic "hypersensing" experience:

forum.linuxcnc.org/plasma-laser/44799-ohm-sensor#231560

It will link you to another post with a pdf that has two diagrams, one shows the configuration you specifically asked about here, and the other shows how to do the same thing with relays. Side note about diodes... they're really cheap.

Also, in case you wanted them after reading the above, my thoughts on the resistor values:

forum.linuxcnc.org/plasmac/45511-ohmic-p...s-water-table#239102

Thanks for the details, will mess around with the resistor values. Gonna troubleshoot the thcad for a bit longer, too stubborn not too 

It worked flawlessly for me for a few years. But there are many variables. The whole idea was to have adjustable sensitivity...
I could actually see the voltage increase from 0v to 24 v over 0.04mm of down travel. 

rodw wrote:

Funny I started doing it that way and changed it for some reason based on feedback from PCW and a problem we experienced.
There is actually an is-cutting pin (or similar name) that goes true when there is arc voltage. Does it light up for you?
The 390 k  resistor is designed so a 24v power supply will give about  4.9 v full scale to the thcad 5. So if it ever sees > 5 volts it has hit full scale from seeing an arc voltage!

Great to see somebody hae been constructive in improving the circuit instead of complining about it not being as good as a relay system.

Thanks! I'm giving it a shot at least!
So I may have found why you were having issues. 
Below is a screenshot of an ohmic test with a slightly shorted torch. The torch is off of the table/material as noted by the 20ish volts of arc. 
I am seeing a ohmic voltage of 13ish...... should be zero. not sure why this is, may play around with resistor values to see if it changes. Turning on or off the plasma doesnt change the values. regardless it shouldnt be tripping the ohmic at 13 volts but  it is....... any idea why that could be?

Might be related to seeing some arc voltage and that's why you flipped polarity to avoid this issue? 

The issue some had previously was on water tables and water got in the torch. In these circumstances, the voltage variation between on and off was closer to 0.5 to 0.2 volts. At the same time PCW suggested the 24k resistor to extend the voltage hysterisis.

Ohmic3 incldes a moving average algorithm so instead of using a simple  low voltage threshold to sense an off condition (ohmic-low), it delares an off  codition as soon as it sees a confirmed downward trend in voltage.

If your off is 13 volts, you might like to consider changing ohmic-low to say 18-20 volts.

This is where ohmic sensing gives you flexibility vs a relay. You have flaxibility to alter the on/off hysteresis to cover differing conditions. A relay might turn off at between 5 & 8 volts which would not work for your table.