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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

 

I love these pendants
 
But they take some wiring in.
 
I tried some complex hal trickery to get eveything working but gave up and wrote a custom  component
In addition to that, we are using my Lam stepboost component to get higher performance out of the Lam drivers.
And My Ohmic3 Hypersensing component that uses a THCAD which will be a bug upgrade for Steve.

It was pretty hard to wire up a plasma cutter without a table, so I made a couple of errors to make Steve's job a bit harder.
Shh don't tell anybody but I had to get some hellp to set up  my own ohmic3 component!

I forgot to mention the control box uses a safety relay for the estop circuit. Thats the red relay with the row of lights on it.I elected to utilise an external estop reset button,so we just use the QTplasmac estop as an indicator.

To the left of the safety relay are a couple of force guided relays which add some redundancy to the connection back to linuxcnc that tells us that we are awake! Thse have a plastic pin that forces the relays open to prevent any possibility of the contacts being welded shut. These are the only relays used in the build as the torch is turned on by the 7i76e's onboatd spindle SSR.

So that is a brief summary of the build. Next post will show the after.

I built a second plasma controller utilising what I learnt making the spaceship plasma a few years ago.
 
This still uses a Mesa 7i76e and a couple of Thcads for torch voltage and Ohmic sensing
Additionally it has a 433 Mhz Wireless pendant which has a wired in receiver. This is the second one of these I have used and I like them a lot
Steve in Melbourne saw some of my videos and asked me if I could help out with his plasma table which was  a bit tired and in need of a refresh.

 
So a deal was struck.
The table was using a 24 volt power supply to drive the motors which was well underpowered. My controller includes a 60 volt AC toroid I had custom made which powered 5 x Lam Technologies D1076 6 amp drivers.
We replaced the motors with industrial quality ones befitting the Lam drivers from Moon's Industries. I wanted to use NEMA34's but we could not fit them in. We also added 5:1 reduction planetary WUMA gearboxes. These needed some customisation to the output shaft. We had to use an 8mm shaft. The 14mm standard shaft just would not fit into the exisring pinion.

 

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