Another plasma component...

AgentWD40 wrote:

You guys think a THCAD calibration setting calculator like this would be a useful addition to something like the configurator?
jscalc.io/calc/NTr5QDX6WgMThBVb

[edit]
I hope it works right because I can't figure out how to get back in to edit the stupid thing.. anyway I figure
scale = thcad_model_voltage * frequency_divider * plasma_divider_ratio / (max_voltage_frequency - zero_voltage_frequency)
offset = zero_voltage_frequency / frequency_divider

where max_voltage_frequency and zero_voltage_frequency are from the calibration sticker x1000

Very nice. I must confess I don't use Plasmac's scaling and have stuck with the component I wrote ages ago which does the same thing as your web calculator forum.linuxcnc.org/plasma-laser/32585-scalethcad-component. So I use scaling as 1 and offset as 0 and give plasmac the torch volts already scaled.

I certainly wouldn't argue with checking its calibration. I checked my thcad-10 with a 9.5v wall wart and a voltage divider circuit with a potentiometer. I checked it at several different voltages over the 10v range. The calculated voltage using Mesa's calibration frequencies matched my multimeter within 0.1 volts. Speaking just from my gut I think I trust mesa's calibration better than my multimeter. it's a cheapo meter, so I suppose that's the best I can do.

[edit]
Maybe another calculator for the user to plug in a series of their own frequency vs known voltage pairs and do a simple linear regression? let the user decide what they trust. I'm just thinking about convenience for the user. I would offer to do it but I know squat about gui programming in linux... maybe it's a good time to learn though

[edit x2]
Here's my test data. It was closer to +-0.03 volts, not 0.1v.

phillc54 wrote:

I still think that at the end you will need to check it against a known voltage as component tolerances will make a difference.
Although I guess if you are not intending to use the voltage from manufacturers cut charts then it probably doesn't matter too much.

Cheers, Phill.

But each board is individually calibrated by Mesa so there is no need to do this as most won't have a lab quality power supply at their disposal. Testing with a 9 volt battery is only useful as a simple check to see that you have it set up in the ball park in hal show. Eg frequency count is at 90% of the range.

rodw wrote:

Very nice. I must confess I don't use Plasmac's scaling and have stuck with the component I wrote ages ago which does the same thing as your web calculator forum.linuxcnc.org/plasma-laser/32585-scalethcad-component. So I use scaling as 1 and offset as 0 and give plasmac the torch volts already scaled.

I did install your component the other day but never got a chance to look at it or configure it as I was having other issues just getting readings (you might remember). Now that I look at it it looks like exactly what thcad users need.

That said, in the spirit of plasmac being all inclusive of all hardware, I'm still thinking it would be nice for users to have a gui to calculate their hardware's settings to work with the single generic plasmac component?

Stop me if I'm making this harder than it needs to be. I know I tend to over complicate things sometimes.

phillc54 wrote:

I believe Mesa calibrations are 100% accurate, what I mean is the tolerances of the divider components, also don't forget some users will be making their own divider as they don't have a CNC port on their machines.
I also mean a 'real' voltage on the THCAD input, not a battery on a test bench.
Rod, I seem to recall you had problems with the divider in your Everlast machine.

Cheers, Phill.

What do you mean real voltages? Calibrate with the actual plasma machine running? Or apply a known voltage to the plasma machine's voltage divider? If so is that a trivial task?

phillc54 wrote:

Rod, I seem to recall you had problems with the divider in your Everlast machine.

Cheers, Phill.

Yes but the 9 volt test would not have picked up the fault I experienced and it was not related to the THCAD at all. The main problem I had was that the THCAD saw the simple resistor divider network on the Everlast board as part of its divider network so that the 16:1 divider I was using was actually 24:1. This is not a problem with my Thermal Dynamics or I believe the Hypertherm units who have much more sophisticated circuitry on their divider board.

So if you are using a cheap plasma cutter and are getting odd results, compare the result via the divider board and from raw arc voltage with the proper THCAD scaling resistance (remember to include the resistance the plasma manufacturer includes in the circuit usually 200k). I actually did this with my Thermal Dynamics to check it was all good as it only took a couple of minutes to solder in a resistor so I know my TD divider board is correct.

I did manage to fry a THCAD at one stage but that is another story but I know for certain from that that each THCAD is individually calibrated.

AgentWD40 wrote:

What do you mean real voltages? Calibrate with the actual plasma machine running? Or apply a known voltage to the plasma machine's voltage divider? If so is that a trivial task?

Ummm see my last post. I actually only isolated the fault once I connected two 48V power supplies in series to give me 96 volts and ran it through the divider board. There was nothing on the Everlast divider board I could blow up doing this, I'd be a bit reluctant trying it on a Thermal Dynamics. I think comparing the raw arcs on a real cut vs the divided volts is a better approach (assuming the raw arc voltage is brought out onto the CPC connector).