forum.cncdrive.com

OK I found a post of yours that explains it a bit more

If you have an ohmic touch off + floating head then you can modify the code to test for which one has been triggered and set the right offset.
I normally do this via a normally open relay so that both the ohmic and floating head are wired back to the same G31 probe input.
At the start of the G31 routine, I close the relay, and test the input (via code) and if the input is triggered,
I then release the relay and then use only the floating head input, otherwise I probe with the relay enabled until the G31 skip (probe) input is triggered at which point,
I then break the relay signal and test the input, and then set the offset according to the ohmic or floating head input.

This gives me a better idea. Will have to think about how to write the macro to do this but I think I can work it out given time.

Thanks again Rob.

example macro for you....

Thanks Rob

I will study that.

OK got my head around your macro. Lots of good ideas in that.

I have yet to receive the ohmic sensor unit, so this is preparatory thinking. Hopefully this week.

I will go with an optoisolator.

The mechanical/electrical issues are a little different with my cheap Chinese cutter. It has no shield. Only electrode and nozzle. SG55

Fairly certain the pilot arc will destroy the ohmic unit if the nozzle is used as the ohmic contact point.

Not sure what voltages are used in pilot arc, but am guessing quite high. So isolating the unit becomes a challenge.

First idea is to use a separate contact point. A stiff contact fixed with a hose clamp to the side of the torch. Protruding just a smidge lower than the nozzle.

This may introduce issues catching material etc. but that will have to be seen.

Thanks again Rob for the assistance.

This is what I do for mine....

I've got a steel wire armor shielding / earth tag (brass) that I over sized the hole + thinned it down a little so that it does not affect the clearance for the cap and the internals.



other possible solutions are:

https://images-na.ssl-images-amazon.com ... SX342_.jpg

https://encrypted-tbn0.gstatic.com/imag ... ETv5bj95iQ

https://i.ytimg.com/vi/AxuuuQ9hEWc/hqdefault.jpg

https://www.bakersgas.com/media/products/220061-.jpg

Some nuggets of info on ohmic sensors (all same principle):
https://neuroncnc.com/products/004
https://minithc.com/pub/Ohmic_Sensor_EN.docx
http://candcnc.net/images/Manuals/Hooku ... s-REL6.pdf

You will have to forgive me if I have this wrong but;

Most if not all of those examples have a shield which is electrically isolated from the arc. Most blow back start.
That sheild can be connected to the ohmic sensor without too much concern.

My torch does not have a shield. Just a nozzle and electrode.

What's more is that the nozzle is elctrically connected back to the cutter. This is the pilot arc circuit.

The maker of the ohmic sensor I intend to use says it will withstand 600volts.

My understanding is the pilot arc is more than ten times that voltage. Again I could be wrong.

Pics are the best I could find of my style of torch.

The pilot arc is much higher than the cutting voltage.

It depends I suspect on which ohmic sensor / its associated circuit you have.

The neuron can be directly connected to the HF side of the torch

https://neuroncnc.com/products/004

Specification
Power - 12V DC, 0.1 A, with integrated reverse voltage protection
Over Voltage protection 15 V
Sensing range - (0 - 1) MOhm
Output - isolated Normal Close relay contact
Life of used relay with a capacity of 12V/10mA - 1*108 ops. minimum
Ability to adjusting the ohmic sensor sensitivity
Ability to eliminate HV/HF ionizing voltage
Ohmic sensing is very easy to accomplish with normal air plasma system, the issue is the Arc Start on high frequency systems.

Neuron Ohmic sensor has high protection and ability to eliminate HV/HF ionizing voltage. Ohmic sensor may be connected to torch shield or direct to torch nozzle.

Wondering if isolating the sensor with a relay when not needed could work.

With pilot arc purpose being to jump an air gap, not sure if a normal relay could succeed in isolating it.

1 relay may not do it, but two in series will, given the air gap would be much greater as the link in the middle will be floating.

OK that should be easy to do.
Thanks Rob