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Bit of an overdue update ....

Electronics cabinet has come a long way and know looks like mostly respextable.

With THCAD for ohmic sensing installed
 

I setup ohmic sensing on shielded and unshielded consumables. As I was not able to easily source a machine shield for the PTM80 torch I got a gouging shield, mount it in the lathe and turned the end down so there as 0.5mm gap back to the nozzle.  I did a similar approach to the hand torch standoff.  I had to make a mandrel to mount it on and clamp with a hose clamp. But once that was done turning it down was easy.
 

 

Next I made a "fence" on X0 (X is my long axis compared to the YY one).  This was done using 3adjustable triangles which i then aligned using a DTI.  Accurate to within 0.02mm over the whole travel.
 

Final update is some work I have been doing to enhance the YY axis travel.  My machine had been 610 x 1240mm on YYX config.  This allowed for indexing full 2400x1200 sheets through the machine even though I did  not have room for a machine of that size (hence the fence).  But I wanted to try and improce on the YY travel. I have figured out how to get it to 700mm.  Stage 1 gets me to 690mm and these new over travel flags support that extended travel.
 

 

Finally the machine making parts for the machine:


Cheers - J.

 

RDA wrote:

tommylight wrote:

I do not have any torch with shield, so i can not test although i do have 2 of THCAD for now.

 
I assume you mean that the tip ”sticks out” from the torch? You could always wire the sensing between the tip and work.

I have my 30XP setup like that but I use a cheap solid state relay rather than a THCAD.

RDA wrote:

If you have an HF start plasma, the HF is created between tip and electrode, you might still not believe me but anyways.
There could be some HF in the tip so keep that in mind.

Yes, that is the reason i never advise using such torches with ohmic.
And all the plasma sources i have are HF/HV start ones.
I am sure i can cobble something together to avoid the high voltage but not worth it, i like my THCAD's not crispy
Oh, i do have a Hypertherm MAX 43 that has blow back start but i have no consumables for it, despite ordering them several times.

tommylight wrote:

I do not have any torch with shield, so i can not test although i do have 2 of THCAD for now.

 

I assume you mean that the tip ”sticks out” from the torch? You could always wire the sensing between the tip and work.

I do not have any torch with shield, so i can not test although i do have 2 of THCAD for now.
I have to go pick up a Hypertherm 3070 as i do have it's torch and cables in my shop, but the transformer was to big to fit in a Skoda SuperB!

Tommy, We use a 24k 5W resistor as suggested by PCW a long time ago.
You need to calibrate the adjustable voltage on the power supply to be 24 volts.
And you need to monitor the ohmic voltage when configuring it and ensure the ohmic threshold is above the off voltage when the torch is wet.
Initially, Steve had this set too low even though it worked for me.
I was going to ask Steve to try a threshold of 24.9 volts to see how reliable it is as an experiment. I have always seen a full 24 volts on contact.
There are so many advantages of using a THCAD for Ohmic sensing, it would be good if a few like minded people could experiment further.
 

In regards to Ohmic triggering, did anyone try to add a resistor in parallel to THCAD input?
That should raise the current required to trigger, and might improve usability a lot.
A 220 ohm 5W should be a nice value to try, it raises the reuqired current to roughly 120mA, instead of uA. Should be more than enough to eliminate water induced triggering.
470 ohm will be under 50mA, if that proves to be problematic on dirty and rusted sheets, although i doubt it.

SteveatSteelCut wrote:

increasing the ohmic threshold to 23.5 seems to have solved this issue for me.
After the nozzle got wet from the watertable the ohmic voltage was showing at above 22volts.

Great news. All it took was a little tuning. Perhaps the settings that worked for me were not appropriate for a default setting. Also, many may have forgotten that the reason why the pulse torch feature was added to Plasmac was to get air flow running before the first pierce to keep water off the material on the initial probe. This was added for islander261 who adopted plasmac during the early development days. Islander261 also contributed the original ohmic relay circuit detailed here linuxcnc.org/docs/stable/html/plasma/pla...r.html#ohmic-sensing

@snowgoer, thanks for the refresher. Its been a long time since we did all the experimentation on THCAD ohmic. There are many people using it sucesfully including James in New Zealand, Joey in Ireland, Santiago in Portugal  to name just a few recent users. I can't help but think making the same change Steve did would have solved your problem. There is enough data on the forum that proves the voltage caused by a water induced short is lower than full contact. Your video shows instant failure when probing starts so the threshold is clearly too low. We need to set the threshold high enough to be above the wet voltage. 

 

SteveatSteelCut wrote:

Yes it is usable.
When it let's me use ohmic probing it is working great.
It's a lot faster than the float switch, doesn't put pressure on any adjustments, and it doesn't push the material down. It seems like a much more accurate measure for pierce height.

Deleting the var file was a placebo. When you receive that error, it's because the ohmic probe tripped prior to reaching probe height. Generally speaking, this is due to either the torch being wet, a piece of slag completing the circuit, or in the case of my Powermax 65XP, when I used the THCAD Ohmic sensing the voltage used to accumulate for no apparent reason (though I was told this is impossible (by those without a Hypertherm)). After months of testing, I did away with THCAD ohmic sensing in favor of a few diodes, relays, and a resistor or two which sounds complicated but is much cheaper and easier to implement and has required 0 dickering, I can find a post with the diagram if you're interested. You can decrease the sensitivity of the THCAD ohmic by adding resistor(s), but ultimately I still had the issue of the accumulating voltage. You can plot this and see if it might be your issue using HALSCOPE. After a few cuts my voltage would hover near the trip voltage even when the torch wasn't wet.

PCW wrote:

Funny aside: We had to change the name of the THCAD on a invoice for a
shipment to Brazil as anything with the letters THC gets held up in customs...

I wonder why, they should check for stuff containing THC LEAVING the country!

One big issue with reading the plasma voltage is that the GND connection
will have large (possibly 50V or more) high frequency transients when the
torch fires. This makes is very easy to damage connected devices. This is why
the THCAD is fully isolated and has only a few pF of capacitance between the
plasma side and the control side.

Funny aside: We had to change the name of the THCAD on a invoice for a
shipment to Brazil as anything with the letters THC gets held up in customs...

7i84 should also have 4 analog inputs that can do up to VCC, so yes you can use it to measure 9V, but at low resolution and at low speeds.
Although speed might not be much of an issue for slow cutting with plasma, the resolution is as it will cause big jumps when correcting.
Buy a THCAD.
Or make a THC using a "voltage to frequency converter" chip, plenty of those around and quite cheap, until you figure out you need high voltage isolation and care with wiring and stuff, so it really does not pay off to build one, given the price of THCAD is really low for what you get.

1. Thats correct

2. You need to set the encoder in counter mode

3. Yes, you need to use the encoder velocity signal as the feedback source
for the Z axis position loop, you may also need to offset and scale the feedback
because the THCAD2 has an inherent offset (since it has an about 100 KHz signal
at 0V)

Thank you very much for your reply. I will arrange to purchase the THCAD2. Just a few more questions, please:

( 1 ) - Are the electrical connections between the THCAD2 and 7i77 electronic boards: FOUT+ with QA0, FOUT- with /QA0, +5V with +5V, and GND with GND of TB3 of the 7i77?

( 2 ) – Since the QB0, /QB0, IDX0, and /IDX0 connections of the TB3 connector of the 7i77 will be left without any connection, do I need to update it so that the 7i77 recognizes only the A signal channel?

( 3 ) – Is there an update for my configuration that allows the 7i96s + 7i77 to recognize the THCAD2 as the Z axis encoder?

2 options I can think of would be to add either
a 7I87 (8 channel  sserial analog +-10V input card)
or a THCAD2 (0 to 10V voltage to frequency card)

The THCAD2 frequency would be read by a encoder input