Hypersensing with THCAD - better way to do ohmic sensing

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18 Apr 2020 15:04 #164468 by PCW
OK you can lower the sensing resistance to lower the sensitivity of the probe
_BUT_ beware that the resistor must withstand full plasma voltage and the 390k
resistor must remain in place since it sets the full scale THCAD range (= 24V)

A resistor to raise the sensing current would go from +24V to the probe
Note that 1K is probably way too low because it could dissipate ~100W
at 300V, 10K 10W might be a more reasonable compromise (roughly 50X
higher sense current)
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18 Apr 2020 15:17 #164469 by tommylight
Hmmmm that is new to me, thank you PCW.
I was under the impressions that the probing is switched off before firing the torch.
Adding a relay will solve that issue.
Now i have to wonder, if the probing THCAD is on all the time, the use of another THCAD seems redundant.
Will think about it a bit, while waiting for my THCAD's to arrive.

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18 Apr 2020 20:15 #164502 by rodw

Hmmmm that is new to me, thank you PCW.
I was under the impressions that the probing is switched off before firing the torch.
Adding a relay will solve that issue.
Now i have to wonder, if the probing THCAD is on all the time, the use of another THCAD seems redundant.
Will think about it a bit, while waiting for my THCAD's to arrive.


Yes the arc voltage is seen by the ohmic probe sensor.
I don't think a THCAD calibrated to read the arc voltage will be sensitive enough to read a 24v (or even 48V) voltage.
One of the advantages of this circuit is that it removed all relays and additional components. IF you are going to add them back, it makes no sense to use the THCAD.

Remember, we are talking about a use case here that is an issue for any ohmic sensing circuit.

I think its premature to be modifying the circuit at this stage as we have not exhausted software solutions.
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18 Apr 2020 23:00 #164535 by tommylight
Water is not a software issue, and there is nothing in software to fix that.
And i do get that using relays will defeat the purpose of THCAD, so back to the drawing board.
Also i am sure it is an issue as i have seen it used only on downdraft tables, never on water tables. But that will not stop us from trying to get around it and find something usable and reliable. Oh i miss my THCAD's......... :)
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18 Apr 2020 23:31 #164540 by phillc54
An interesting quote from John in this post

I know there has been a lot of discussion at times about "false" triggering of ohmic sensing. I can say with certainty that I have never had a problem with this that was a true false trigger. Yes, I have probing errors all the time but they are always caused by a flooded torch end or dross stuck in the torch end. So the probing errors are always cause by a true shorted torch which is why you check for that condition before the start of probing..

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19 Apr 2020 00:09 #164545 by rodw

An interesting quote from John in this post

I know there has been a lot of discussion at times about "false" triggering of ohmic sensing. I can say with certainty that I have never had a problem with this that was a true false trigger. Yes, I have probing errors all the time but they are always caused by a flooded torch end or dross stuck in the torch end. So the probing errors are always cause by a true shorted torch which is why you check for that condition before the start of probing..


I'm pretty sure that is why John made you add the probe test button to the GUI so he could check for a shorted torch before running a job. Press the button and if the ohmic light came on, he had a problem.

I've made some progress recoding using a moving average to hopefully account for the noise observed which worried me.

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19 Apr 2020 00:11 #164546 by rodw

Water is not a software issue, and there is nothing in software to fix that. :)


Tommy what about softwater instead of software????

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19 Apr 2020 00:12 #164547 by phillc54

An interesting quote from John in this post

I know there has been a lot of discussion at times about "false" triggering of ohmic sensing. I can say with certainty that I have never had a problem with this that was a true false trigger. Yes, I have probing errors all the time but they are always caused by a flooded torch end or dross stuck in the torch end. So the probing errors are always cause by a true shorted torch which is why you check for that condition before the start of probing..


I'm pretty sure that is why John made you add the probe test button to the GUI so he could check for a shorted torch before running a job. Press the button and if the ohmic light came on, he had a problem.


Exactly...

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19 Apr 2020 01:21 #164557 by tommylight

Water is not a software issue, and there is nothing in software to fix that. :)


Tommy what about softwater instead of software????

None of that around here, we use plain hard water from the surrounding :) mountains, it has plenty of calcium and other minerals in it! :)
Back on subject, or subjects as this is going on on two separate threads and causing a lot of chasing around, can you do a plot of the THCAD5 raw voltage from the time a probe is issued till it finishes, please ? I would like to see the change throughout the process with a dry and a wet torch. Also a plot from the torch on signal to arcok from both THCAD's.
Both of you, please, as it looks one is having much more issues than the other, and he might be onto something regarding his wiring.

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19 Apr 2020 09:28 #164619 by rodw
Here is something interesting about ohmic sensing and watertables

Ohmic sensing does not work on a water table with a Plasmacam despite what people say. The water continuously goes back into the nozzle and shorts it out causing a "torch hit material" error. The machine continues to blow air after it is done cutting and this causes water to spray up into the torch, shorting it out.


See: plasmaspider.com/viewtopic.php?f=3&t=30183
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