DIY Ohmic sensor

[acourtjester]

Some time you see a slick trick somebody come up with and you need to pass it on. I have been working remotely with another DIY guy to get his CNC Plasma going. He sent me a video and he had a slick way to have an Ohmic sensor for his hand held torch. Looks like a copper wire attached to the torch mount bent so it has a flexible contact to the metal. It is bent so it has 2 contact points on either side of the torch tip.

Johnny's ohmic sensor contract.JPG

[weldguy]

Pretty clever and I bet it works very well. Thanks for passing this on Tom.

[BrayD]

Ohmic sensing has my interest. I'm realizing the limitations of my torque sensing arrangement on thinner sheet. Thanks for sharing creative solutions like this.

[acourtjester]

I have used a very simple Ohmic sensor circuit, and small module very cheap works great. happy to share if you would like, they come in 5,12 and 24 volt packages.

Ohmic sensor 12 volt.JPG

Ohmic relay box (1).JPG

[BrayD]

Please do! I'm not even sure what voltage I can pull from my THC, but I'm happy to poke around with a mulitmeter.

[acourtjester]

Here is a eBay post with the module you can buy either voltage (5, 12, 9, and 24 ) I am using LinuxCNC with a Mesa card and it runs 24 volts so that is the module I use. On other taboes I have used different controller cards and have use both 5 and 12 volt module they all work the same. Go to eBay and type in the number shown for the post for what ever module you want. Here is a video showing it for router touch off, I will post the G-code later. I also think this is the module I used for torch fire relay too, so you could buy a 2 channel module if you wanted to do that.
The router touches off once then backs off and touches at a slower speed, backs up and resets the Z at 0.750" ready to run a routine.

ohmic module ebay 2.JPG

ohmic module ebay.JPG

[acourtjester]

BrayD
When using a Ohmic sensor you need to wire the contact to the Probe input connection as it is different then a Floating head switch. And you need to use a Post processor that has probe commands so it will put them into the G-code. Many have a floating head switch being used as a safety backup if the ohmic sensor misses the contact (crud, rust, or dirt on the metal surface). And that would be wired into the limit signal string so it stops movement if tripped. Depending on your control card and the software used they can use different G-code commands. Mach and similar software will use a G31 and LinuxCNC use a G38 command. Here are 2 examples.
G31 Z -4 F25
G92 Z 0.0
G0 Z 0.160
Or
G38.2 Z -4 F 25
G92 Z 0.0
G0 Z0.160
The Z movement should be up to home or in the positive direction, and the command has a - number to move down toward the metal. The F valus is the speed the Z moves and the G92 to set the Z DRO to 0.0 then the G0 Z 0.160 tells the Z to move to 0.160" for the pierce height.
For my router I have a short G-code the will do the touch off shown in the Video above, used when I start a routine or change the bit to find the surface then back up to a safe height to move around. Some have this build into their system as a tool change routine, the tool are in a holder and the routine stops cutting and exchanges the tool, sets the Z height the starts the next cut.

[BrayD]

Good stuff! I was able to find those modules and have saved them for future use. I have a standalone THC, so this may take some experimenting to find an arrangement that communicates with everything properly. I have hope that it's possible though. Sounds like a good project when I get some down time!

[acourtjester]

So you understand the Ohmic sensor and the THC are separate functions and are not related. A floating head switch and Ohmic sensor are actions to find the metal surface before the torch fires. And the THC only comes into play after the torch fires and starts to move (cutting).

[BrayD]

Yep. I'm still learning my system, but I'm pretty sure my THC controls all torch height functions, not just the auto height control after the cut has started. There are no Z values in my G code. I set pierce height manually with a potentiometer on the THC.

No floating head either, so it's not operating with simple limit switches. There's a 'smart motor' on the Z axis which communicates the increase in torque (current perhaps?) back to the THC to tell it that it hit material. I need to figure out how all of that works electrically, then I can see about integrating another switch (ohmic sensor).

[acourtjester]

Ok maybe another member can jump in as I am not familiar with Red AVHC. I have used different software and controller electronics but yours sounds very different for what I have used. have fun

[tcaudle]

tom, its a stand alone THC that was sold by Torchmate. There was the red readout version and the Blue readout. There was very little exchange between the control, the control software, and the THC. There is not a "Z axis" but rather a "torch lifter" Leagcy technology and on par with the Promo 150 units . If I remember tha only singlas were a torch fire and a HOLD back to the motion control. I don't think they had a g-code or parameter setting to disable the THC .
The THC was designed and built by a third party and when TM sold out to Lincoln that design, and then the whole control, system, was abandoned and the third party ( i think) sold remaining stock and the buyer still works on the units and repairs them . It Used a "smart" motor and is often the point of failure. Repair is expensive.
TM had a low cost hobby grade table they sold without a THC . It cost almost 2K less so it seemed like a luxury until you realize you burn though consumables at a mind boggling rate without some kind of THC.