THC ADC resolution? about plugin_plasma

[bLouChip]

@phil-barrett ; So my experience with using a Proma Compact THC SD is that it appears to have a 1V / 0.2mm sensitivity, that's 0.2mm increase or decrease of arc length during cutting. You are correct that 100VDC is a fair nominal cutting arc voltage, range being 50 - 130VDC. 120 - 150VDC is a nominal pilot arc/non-cutting voltage, and 150- 250DC is a fair range of open circuit voltage (not even a pilot arc), although this latter case would be a misfire or erroneous assembled torch tip. These are values are for blowback pilot arc power units, not High Freq pilot arc units which are not preferred in hobby/small business plasma tables due to higher than normal EMI transmission.

An ARC OK signal from the power source should negate the need to monitor/measure for anything other than cutting arc voltage. Regardless, a delay time control value in the sw to delay Z motion control after an arc is first detected can mask the need to pilot arc monitoring. Also, a max voltage deviation value from monitor voltage value could suffice as an indicator of cutting-to-pilot transition if the torch runs off the metal.

So to answer your question, I believe 10 bit ADC could work in the case of the circuit having a bias of +40VDC or so, so the 100VDC monitor range is shifted to an actual arc voltage of 40VDC - 140VDC.

[phil-barrett]

Interesting. A 1V sensitivity tells me that 8 bit resolution would be fine, though 10 bit is probably to lowest I would go. The current ADC design doesn't do an offset - probably isn't necessary with 12 bit. Maybe not needed for 10 bit either.

The current ADC design simply monitors a range of 0-200V so my numbers above are off by 2 with out any offset circuitry. I think the 10 bit ADC still is ok though modifying the divider to have 150V (or 140) yield 5V would help.

[terjeio]

Cheaper converters has less stable output in the lower bits than more expensive ones requiring averaging of samples to approach the specified resolution?

[TorbanP]

Ive used the TI ads111x series for many projects, even through the part shortage they have been available. its also what I was using with my external THC attempt with fluidnc. jlcpcb can PCBA them as well.
ads111x

[terjeio]

Driver support for additional converters is something I'll look into, I am thinking about adding an additonal API level for ADCs and DACs since the low-level device code can be pretty simple.