Ohmic sensing question

[schmaus]

I am building a new plasma table and have a question about DIY ohmic sensing. this was talked about before and I didn't want to bring up an old thread.
I was thinking about using one of these and wiring it up like the picture. I want to use the normally closed contact on the torch fire relay. so it is active when the torch is off and inactive when the torch fires.
Does this give enough protection from the torch votage? It is a Hypertherm Powermax 1000 if that matters.

KEMO switch.jpg

Sensor.jpg

[acourtjester]

As long as you use the correct tip on the hypertherm. The circuit does not fire the torch that is a separate circuit on the BOB board. this does not care if the plasma is on or off it is a signal to show contact between the metal surface and the tip, to tell the software it has found the metal surface. If you wire it like the image you need to have a g31 in your g-code for the Z axis travel, this happens before the torch is fired. The box with torch on relay is incorrect it should not be there just the green wire directly to the metal plate.

[schmaus]

I was going to wire it to the existing torch relay so it was disconnected when the torch was on just for extra protection. I already have the electronics done for the build and have tested them on my friends table. right now I am using a limit switch to find the top of the plate but i want more accuracy (less plate deflection).
I am using g28 but I can change that if i need to.

[acourtjester]

Your machine but not needed, maybe some one else with comment

[jimcolt]

Under certain failure modes with consumables in the plasma torch there can be DC voltage in the 300 VDC range. You should have an isolation relay that opens the ohmic circuit immediately after sensing the plate so that there is no possibility of torch cutting voltage levels causing issues with your lower level control voltage. Industrial torch height controls used on high end machines have had this type of isolation for over 20 years....unfortunately I don't see it on low cost machines. Jim Colt Hypertherm

I am building a new plasma table and have a question about DIY ohmic sensing. this was talked about before and I didn't want to bring up an old thread.
I was thinking about using one of these and wiring it up like the picture. I want to use the normally closed contact on the torch fire relay. so it is active when the torch is off and inactive when the torch fires.
Does this give enough protection from the torch votage? It is a Hypertherm Powermax 1000 if that matters.

KEMO switch.jpg

Sensor.jpg

[schmaus]

Jim, I plan on using the second set of NC contacts on my torch fire relay (which is using NO contacts) that way there is no possible way of the torch being on when the ohmic sensor is on. It is a standard ice cube relay with multiple sets of separate contacts all switched with the same coil.

[tcaudle]

The Feather Touch Ohmic sensor si totally isolated from controller inputs and protected from forward or reverse voltages up to 1000 VDC. Its even epoxy potted to prevent flashover. Not having to turn it off when the torch is fired offers a way to sense a tip crash during cutting as well as conventional collision detection during non-cutting moves. Since inputs SHOULD be isolated then having double isolation makes having something in the controller damaged extremely remote. It can survive the CD start ESAB machines so I think a conventional HF or blowback start presents even less exposure.

[schmaus]

Tom, Will your feather touch unit work with only one input as I have a G540 so with the THC that is all I have and it would have to share the input with the Z float switch. currently I am using a UC100 but that is probably going to change as it started causing problems with it and the THC. I should of done more research before I bought it.

[tcaudle]

The FT-01 is engineered to work in conjunction with a mechanical Z switch as a backup. Typically as a separate probe. As such it comes prewired to plug into our Table I/O card for both its power and signals. You cannot rely on an ohmic sensor only unless you have a breakaway torch with E-stop sensors. Ohmic is great but you will get false senses from debris or water in the shield to nozzle cavity or total misses form metal with poor conduction (surface rust or heavy oil, etc). There are various ways to provide for a backup but it for sure needs to be implemented. Having very limited inputs on a control system causes some issues. While some inputs can be "shared" it needs to be for the same function. You can define an input to be both X and Y homes since the system does one at a time and assumes the signal is from the axis it is moving. Mixed functions start to cause a problem.

[schmaus]

I think I will just order one from you and adapt it to my machine when the time comes. seems like it would be easy to do. I didn't know it was so inexpensive.

[Zero6200a]

I used the cand cnc omic I used a 5 volt opto trigger relay on the out put it works fine. I used a relay from amazon . just be sure to wire a backup switch to a torch slide so when the ohmic fails the other input stops the machine .. I used the water switch also you need to put a relay to open the tip and ground wire when the torch fires or it will fry if tip voltage gets in . I have a vital systems controller the initial hight sence input if active when cutting like a torch crash or a flip up part it will stop the controller .. this also allows for use of the round torch mount . so it can wire in series with the ohmic relay I use nc input for my controller for this . I used a switch to switch from ihc to ohmic my settings are on the screen so its easy change

[Nacs Fab]

I’m about to assemble my ohmic setup using a G540 and LCTHC with interface card. I’ll tie the input into one of my limit inputs on the card. I’ll add the relay on the triggered output of the Kemo Switch. I have the post that acourtjester wrote and will modify mine to include the G31 code. Hopefully I don’t burn up my THC because Tom doesn’t make it anymore. I think I’ll do some testing to see if the torch will stop at the material prior to even firing the torch. I do have a floating head for backup and it’s tied into the same “input bank” on the LCTHC interface board. I’ll try to make a diagram prior to powering up and post it to see what you guys think.

[msimpson99]

I have been using the M158 for years with my PM65. Works great. Never had any issues. It does require an isolated power supply. I use a generic 9V wall wort to power it.

When I use the this sensor, I use the floating head as an estop in case the tip fails, IE paint or rust. The sensor output is a relay so the sensor its self is isolated from your electronics (if you use an isolated power source).

[msimpson99]

_MG_7638.jpg

My sensor is mounted on the back of my X carriage.

[xnaron]

I have been using the M158 for years with my PM65. Works great. Never had any issues. It does require an isolated power supply. I use a generic 9V wall wort to power it.

When I use the this sensor, I use the floating head as an estop in case the tip fails, IE paint or rust. The sensor output is a relay so the sensor its self is isolated from your electronics (if you use an isolated power source).

Do you use a relay on the water sensor side of the kemo to isolate it when the torch is on?

[acourtjester]

Not needed it is a separate circuit the only connection to your controller electronics is the relay contacts. Mine is actually inside my controller

[xnaron]

Not needed it is a separate circuit the only connection to your controller electronics is the relay contacts. Mine is actually inside my controller

Thanks. Just wanted to make sure I didn't need to protect the Kemo from the plasma cutter voltage at the water terminals.