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Ohmic sensing question

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schmaus
Posts: 27
Joined: Sat Feb 12, 2011 1:35 pm

Ohmic sensing question

Postby schmaus » Mon Jan 22, 2018 1:26 pm

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

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acourtjester
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Re: Ohmic sensing question

Postby acourtjester » Mon Jan 22, 2018 2:32 pm

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.
DIY 4X4 Plasma/Router Table
CandCNC DTHC-IV & Feather Touch
Hypertherm PM65 Machine Torch
Drag Knife and Scribe
Miller Mig welder
13" metal lathe
Small Mill
Everlast PM 256 tig

schmaus
Posts: 27
Joined: Sat Feb 12, 2011 1:35 pm

Re: Ohmic sensing question

Postby schmaus » Mon Jan 22, 2018 4:13 pm

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.

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acourtjester
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Re: Ohmic sensing question

Postby acourtjester » Mon Jan 22, 2018 4:47 pm

Your machine but not needed, maybe some one else with comment
DIY 4X4 Plasma/Router Table
CandCNC DTHC-IV & Feather Touch
Hypertherm PM65 Machine Torch
Drag Knife and Scribe
Miller Mig welder
13" metal lathe
Small Mill
Everlast PM 256 tig

jimcolt
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Re: Ohmic sensing question

Postby jimcolt » Wed Jan 24, 2018 10:33 am

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



schmaus wrote: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
Posts: 27
Joined: Sat Feb 12, 2011 1:35 pm

Re: Ohmic sensing question

Postby schmaus » Wed Jan 24, 2018 9:30 pm

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
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Joined: Tue Feb 28, 2012 6:47 pm

Re: Ohmic sensing question

Postby tcaudle » Thu Jan 25, 2018 1:02 pm

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
Posts: 27
Joined: Sat Feb 12, 2011 1:35 pm

Re: Ohmic sensing question

Postby schmaus » Fri Jan 26, 2018 5:40 am

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
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Re: Ohmic sensing question

Postby tcaudle » Fri Jan 26, 2018 10:34 am

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
Posts: 27
Joined: Sat Feb 12, 2011 1:35 pm

Re: Ohmic sensing question

Postby schmaus » Mon Jan 29, 2018 7:43 pm

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
Posts: 3
Joined: Sat May 19, 2018 10:35 pm

Re: Ohmic sensing question

Postby Zero6200a » Fri Aug 03, 2018 5:40 pm

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


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