Rods "Spaceship" Scratch built Plasma Cutter build

Rod

You may want to build a separated electrode for your torch to use for ohmic sensing. On my torches the ohmic sensing is connected to the shield (an electrically isolated part) and not the nozzle (tip for for some manufactures). I don't know the torch connections on your system or how the arc is started but connecting to the nozzle may be a problem. Normal relays will not withstand high frequency starting voltages. Look at how a Hypertherm Duramax torch goes together on their web site or how a ESAB/Victor/Thermal Dynamics S100 torch goes together on their web site. I have successfully used ohmic sensing with both of these torches.

Looking at your circuit you may want to rethink the sensing arrangement. You have a relatively high impedance sensing circuit (100K series resistor). Both of the ohmic sensing circuits I have used (one from CandCNC and my own) are relative low impedance needing several mA of current to operate.

John

islander261 wrote: Rod

I figured you'd get to the point of wanting ohmic sensing once you start cutting thinner sheet. I just use plain 12VDC out wall warts on both my present working system and my unfinished LinuxCNC controller for isolated power supplies. I disconnect the ohmic sensing with relays when not probing and use an Opto22 module for isolation to protect the 7I76e. I also use steering diodes to protect from actual plasma power supply voltages. I have extensively tested my LinuxCNC ohmic probing and it appears to work better than my present setup. I can post circuit if you want.
John

John thanks. I would appreciate a circuit.

Yes I was going to turn on a 7i76e pin before probing and the torch has to be off as well. I thought the input needs a high voltage blocking diode inline to protect stuff in the case of a catastrophic accident. I think now an opto makes sense instead of R3 becasue of the 100k resistor built into the plasma machine. I figured I used a 2.2k current limiting sensor to use LEDS on 24V to limit current to 10 mA so adding another 1.2 k Ohm resistor (total of 2.2k) would work for an opto quite nicely.

A mate has suggested a mains powered 24v LED power supply which cost only about $10 or so.

Rod

Please find attached the drawing that has my ohmic sensing on it. You are interested in zones B2 and C2. The Opto22 IDC24 is on the G4PB4 mother board. The relays are on the generic Chinese 8 channel relay board. My isolated power supplies are wall warts I picked up at the dump for free.

The use of a mains power supply is good idea to get the needed isolation.

I think there may be some pin number mistakes on the 7i76E block ( I haven't updated the drawing since wiring the system) on the drawing but that doesn't effect the part you want.

John

John,

Thanks for your help and advice. I had a good look at your diagram and checked out some of the plasma torch diagrams. I had not realised that I had a "naked" tip that was exposed when others have a metal shield.

I don't have to worry about High Frequencies as this is a pilot arc/blowback torch. From what I can work out the tip and the electrode are touching when the torch is off so it should be fully discharged of any residual capacitor charges. Other than that what I'm planning, is very similar to what you've done. I promise I will see if it pops before I connect it to the 7i76e :S

I dug around in my junk box and found some 12 volt solid state relays that switch up to 50 volts. They have a maximum power draw on the input side so it looks like they are a perfect match for the 100k built in resistor which will give me about 10 mA current at 12 volts. It has opto isolation of up to 250 volts. A little bit light on for torch voltage, but it should be OK. I also found a 12 volt plug pack which will be fine for testing. I'm sure plug packs breed around here.

So the relay can replace John's opto22 module which are a bit hard to find down under (well $150 was a bit steep) so I just need to grab a couple of diodes because the IN4001's I have are only rated for 50 volts.

Data sheet for the relay is attached.

Rod

The relays you posted are output devices. I understand your intent to use them "backwards". You really need to use real input modules. You have to use the right one for your field voltage on the 7i76E. The real input modules have some protection and noise cancelling features which are nice in this application. I got mine from Ebay because of the high cost of the 24V rated modules. The Crydom brand modules are much cheaper as are the European ones. You can probably use a lower voltage module with the correct resistor (the output is open collector).

John

Module data sheets.

John, I don't understand the difference between input and output modules and why I need them other than they are more robust. When I look at the opto22 schematic, its identical to a simple opto isolator switching circuit and my opto isolated SSR is similar. Its no different to using a 24v input relay to send 5 volt to the stepper drive disable signals which works fine.

In any case, I don't think it will work as I stuffed up the maths. They need a 1k current limiting resistor and the plasma circuit has a 100k one. I think I might have to bypass the internal wiring in the CNC port and just earth it to the table frame which is somewhat annoying. I don't think this plasma signal wire is designed to read low voltage signals as I want to do.

Rod

Every ohmic sensing system I've seen (mine as well) uses a direct connection to the tables star ground point for the work connection.

I questioned your calculation for the current limiting resistor but figured you'd find out soon enough that you can't drive the opto with 12V through a 100k series resistor and have it turn on hard or fast.

Beware that the simplified circuit diagram on the data sheet is not exactly what is in the module. Years ago I had occasion to make some input modules with a custom input function. They had work perfectly in existing module racks. So I "disassembled" and reverse engineered the interface circuit to make sure I got it right. Did the same thing with an output module at the same time. They are actually quite a bit different. Some of the differences are about manufacturing costs but some are ones you'd only do to fix a function problem.

John

Thanks John,

I think I've got a plan now. I did think of using one of these which a few people have used for torch height control
www.fruugoaustralia.com/kemo-m158-water-...ule-912vdc/p-5626024
But, I'd have to wait for it to come from Germany or the UK so I think I'll mount up the relay I have and see how it works. The only added safety feature I could think of is adding a delay on some signals in Hal to account for switch hysteresis. I did see the Opto22's were quite slow (5ms) vs some other things I've looked at.