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qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.
- jimmyrig
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25 Feb 2023 21:44 #265358
by jimmyrig
Replied by jimmyrig on topic 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
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
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- snowgoer540
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25 Feb 2023 23:06 #265366
by snowgoer540
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
Replied by snowgoer540 on topic qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.
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
The following user(s) said Thank You: jimmyrig
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- rodw
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25 Feb 2023 23:24 #265369
by rodw
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.
Replied by rodw on topic qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.
If you have a short in the torch, ohmic of any kind won't work.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
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.
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- jimmyrig
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26 Feb 2023 08:59 - 26 Feb 2023 09:03 #265392
by jimmyrig
Replied by jimmyrig on topic qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.
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.
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.
Last edit: 26 Feb 2023 09:03 by jimmyrig.
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- rodw
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26 Feb 2023 09:08 - 26 Feb 2023 09:09 #265393
by rodw
Replied by rodw on topic qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.
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.
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.
Last edit: 26 Feb 2023 09:09 by rodw.
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- jimmyrig
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26 Feb 2023 09:56 #265398
by jimmyrig
Replied by jimmyrig on topic qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.
Thanks for the details, will mess around with the resistor values. Gonna troubleshoot the thcad for a bit longer, too stubborn not too
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
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- rodw
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26 Feb 2023 10:00 #265399
by rodw
Replied by rodw on topic qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.
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.
I could actually see the voltage increase from 0v to 24 v over 0.04mm of down travel.
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- jimmyrig
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26 Feb 2023 10:04 #265400
by jimmyrig
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?
Replied by jimmyrig on topic qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.
Thanks! I'm giving it a shot at least!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.
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?
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- rodw
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26 Feb 2023 10:54 #265404
by rodw
Replied by rodw on topic qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.
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.
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.
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- phillc54
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26 Feb 2023 11:10 #265408
by phillc54
Replied by phillc54 on topic qtplasmac 2.10 arc ok, thcad thc, and Thcad Ohmic troubleshooting help needed.
If the torch is not touching the material then the voltage should be zero. If that is not the case then you should be trying to fix that issue rather than trying to work around it.
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