THC not getting active even when "all seemingly needed" requirements are met

Hello All,

New to the forum and first of all would like to thank you for the valuable information you guys post, it has helped so much me moving ahead with my build. id appreciate your help with my issue where it cannot activate the thc even when all the settings (that i know of) have been properly adjusted. first my setup:
-LinuxCNC QTPlasmac table
-Rpi5 with image LinuxCNC 2.9.8 Raspberry Pi 4/5 OS based on Debian Trixie
-Mesa 7176s over ethernet
-THCAD2 + 3M resistor array enclosed near the plasma source, F+/F- routed to mesaboard encoder input A with shielded microphone cable grounded at control side, also used a more simple twisted pair for testing.
-Used Pnconf for setting thcad2 trimming and ratio
Tested for correct voltage measuring for example, hooked a power supply from 39 to 56 volts and Main screen voltage readings coincide very accurately to multimeter reading.
-Flaring 65 plasma source with no cnc interface. (HF piercing) voltage probing connected to torch (-) and clamp (+) leads.
-For testing im using a straight line of 100mm at full speed so thc is not affected by speed reduction  (tested from 400 to 1800 mm/min)
-THC is enabled
-Mode 0 is used so arc ok is derived from voltage readings
-setup plasmac.arc_lost_delay =.3 as was having some issues for getting it to start cutting due to initial spike/valley.

I took the follwing HALSCOPE trace where:

 

1)torch on triggers,
2)probe voltage goes to 260 for piercing, then falls, then recovers to around 94v
3)Arc OK signal goes to true
4)Pierce delay elapses
5)Movement starts
6)THC active never goes up and of course  no correction

as Far as I can see voltage is "stable" once cut starts and there is no variation that could lead to deactivation as the tresholds are LOW=39 HIGH 990 (for testing purposes), samples 10 and threshold voltage 10.

has anyone had this issue or any lead on what to look at?

Hello Tommy thanks for your response, yesterday in follow up to your suggestion I set the parameters as I will show in the pictures below and I have some updates:For the first time THC stayed enabled during the cut, this happened around once every 5-8 tries, dint have the chance to validate it worked properly but that is some progress.*Something that i was not doing before is to set same speed in material table as well as in the G1 command of the G code idk yet if that has something to do with it, please see the next items.*Normal behaviour in my setup is that screen always went black for a second or two right after the pierce, it was so consistent that thought it was a feature of the GUI, yesterday noticed only a few runs where the screen did not go black, some of those matched the times where THC remained enabled. I checked and seems that the black screen is rather an issue caused potentially by EMI?*Changed plasma source settings to increase pilot arc and I could see the screen going black more violently. i.e. with flickering before going black.I am making some improvements to grounding to see if this helps, do you have any recommendations to make the arc ok, feature more resilient? in case that it is emi or do you have any other idea thanks. 

 

I have a Lorch plama cutter that ignites with hf....

By me was it the monitor cable ...


MfG
Juergen

i see, will look at it too. Was the HF start giving you issues with THC?

No, it didn't

mfg
Juergen
 

Its an odd looking voltage plot.
I would look at the time after arc ok the THC is activated. To me it looks like it needs to be 2 seconds. I just can't see the setting I'm looking for.
Early activation could be disabled by the big fluctuations in voltage.

ok, found this to try those 2 seconds maybe:

Two methods of THC activation are available and are selected with the Auto Activation check-button. Both methods begin their calculations when the current velocity of the torch matches the cut feed rate specified for the selected material:

Delay Activation (the default) is selected when Auto Activation is unchecked. This method uses a time delay set with the Delay parameter.

Auto Activation is selected when Auto Activation is checked. This method determines that the arc voltage is stable by using the Sample Counts and Sample Threshold parameters.