Help With Ohmic Sensing

[surefire_fab]

Hello all.

I recently finished building my second combo Co2 laser \ plasma machine. The first one was geared more toward the laser side, and did not have THC, but this one I have geared more towards the plasma side and, as such, this is my first dive into the world of THC. I am running both machines off of ruida offline laser controllers. That posed several challenges, but once overcome, I feel that the benefits outweigh the compromises.

I am using a somewhat custom THC from my friend at Aqutronic (this guy, I highly recommend BTW). I have chosen ohmic sensing, as my bread and butter work is ductwork (ie very long cuts on mostly 26 gauge gal). Mechanical switches just won't cut it. Just to clarify, I am using a single wire connected to my shield for probing. I have a constant 5V to my +Probe input at the THC and the shield is connected to the -Probe input, so it goes low when it touches off on the metal. With brand new consumables it works great. Ever so barely deflects the metal when probing. Something happens, however, way too soon, and the ohmic probing stops working. I can check my shield to GND with a multimeter and have no continuity, but the THC thinks there is contact. Played around putting all kinds of resistance values in series using a decade box, with no luck. I have to change my consumables to get it working again. To put that in perspective, I would say that a fairly complex program (like one I recently tried which had 60-70 separate vectors- most of them small wouldn't make it on a set of consumables. This was with a pierce height of about 5mm and a cut height of about 2mm. My torch is an x45 machine torch from plasmadyn, with the consumables also coming from them.

I expected to have to be diligent with keeping the shield clean, and have been from the beginning, but this seems excessive. I have to think maybe I'm missing something. If anyone with more ohmic sensing experience can set me straight I would greatly appreciate it! Below you can see the simple ohmic sensor setup.

20230322_184103.jpg

[acourtjester]

What you may have happen is using +5 volts for your Ohmic sensing is it is burning the contact point of contact. I use a small module that senses the ground side of a circuit with an optocoupler for the ohmic sensing on my table. This is used for both a plasma torch and router bit contact, with out problems. Here is the vendor on eBay they have 3 voltage types I use the 5 volt one. I assume there is a connection to the shield on your plasma torch, I use Hypertherm which has the connection that way. the connection is made to the Probe input of the controller I use.
https://www.ebay.com/itm/401188750996?var=670761872997

[surefire_fab]

Thank you for your reply. Interesting. I had thought of an isolation relay but drifted away from the idea as my probe is a single wire on the 0v side.

It connects to my thc. It has +probe and -probe inputs. Probe is connected to -probe. Would an isolation relay still be advisable for this scenario?

Yes. I had attached a picture of the connection at my shield. Not sure if you can see it on your end.

I actually have some of those Chinese relay boards on hand. Not sure what voltage they are though.

Thanks again!

[asuratman]

New THC, what type of Aqutronic you use ? does it work well ? I looked on utube looks good.

[surefire_fab]

The thc i have is a prototype he is working on. He isn't really advertising it yet, as it's still in beta.

Before this one, I had another of his thc. That one was the jetcut sd. That one doesn't handle the probing, which you might not need if it's part of your gcode.

I had a quasi solution for probing on the ruida but it didn't turn out to be very elegant, so he gave me the beta model at a heavily discounted price along with a ton of help tweaking the firmware and getting everything working as per my needs. Can't recommend this guy enough!

[acourtjester]

Another thing to look at is the pierce height, you may be getting to much blow back which may dirty up the contact point. Or is your metal galvanized which will also dirty up the contact point. I would try sanding the shield off as a test to see if this is your problem, this is normally not needed but maybe that is your problem.

[surefire_fab]

I guess I want to understand what is even happening.

As I said in the op I mostly work with 26 ga. Galvanized and I've been diligent with keeping the shield clean. Sand it clean after every job.

I wasted more time than I should probably admit looking at other possible problems since when I would check with a multimeter I did not have continuity between my probe wire and GND. The consumables were so new. Only that one project I mentioned. But since it did have so many separate vectors, your point about pierce height is worth investigating. As I said, I am using 5mm for piercing...would I be trying to increase that height?

Thanks again for your thoughts on this!