Ohmic Sensor not operating as expected

[ChrisO]

I installed an OS3T-01 Ohmic Sensor http://robot3t.com/en/san-pham/os3t-01- ... ma-cutting to a retrofit build plasma table. ( new install) The issue is the Ohmic Sensor output light comes ON when connected to the Torch Tip. I can rotate the sensitivity knob(counterclockwise) to reduce its sensitivity until the output turns OFF, however when the torch tip is grounded to the workpiece the Sensor output will not turn ON.
I checked the consumables in the torch tip to ensure its not grounded. I also measured the resistance between the TorchTip and Workpiece ground clamp and get around 150K ohms (with the plasma cutter off).
If bench test the Ohmic Sensor by touching the Sensors TorchTip and Workpiece wires together, the sensor works perfectly, and does so through most of the sensors adjustable range. In addition, I get -11.7 volts DC between TorchTip wire and Workpiece wire. Is this normal to have this voltage? The Sensor is very sensitive, it will respond to very high resistance values in the millions of Ohms( it can turn ON by sensing ground thru the human body). Is this to be expected?
The Plasma cutter is a Thermadyne Pak Master XL100 with PCM-100XL Machine Torch.
If you have any ideas about how to go about diagnosing this issue, it would be greatly appreciated.

[acourtjester]

You did not state which software you are using, did you enable the Probe signal path in the software. Here is where it is in the Mach 3.
Your install instructions should say how to hook it up.

[ChrisO]

Thanks for the reply, I am using Mach4 however I have not gotten that for in my install. The Ohmic sensor itself has a Yellow LED output and that is what I am using at this time to determine what it is sensing.

[WyoGreen]

Ohmic sensors are pretty much just a continuity check. One side of the circuit is connected to the torch shield, which is isolated from the rest of the torch. The other side of the circuit is connected to the plate you are cutting. When the shield touches the plate, the circuit is completed. So you should be able to connect a jumper from the torch shield to the plate and have the sensor light up. If it doesn't then probably you don't have continuity on the plate side of the circuit. If that side is just connected to the gantry, it may not conduct clear thru the bearings, table structure, slats, etc. to the plate. On my table I had to run the wiring thru the cable chain and connect it to the table structure to get it to work reliably.

Steve

[ChrisO]

Thanks for the reply, I too have run a 8AWG ground wire to the Gantry and connected the Ohmic Sensors workpiece ground to it. I confirmed that I have 0 ohms resistance between there and ground clamp I am using to touch against the Torch tip.
When testing this after many repeated attempts I found I could adjust the sensitivity knob ever so slightly, where the output light comes on when the workpiece touches Tip(Its working!), only to find out that when I unground the Torch tip, the output light stays ON (uggh). (If I then, disconnect the ohmic sensors TorchTip wire from the torch, the output light will turn OFF). It seems that the sensor sees enough ground signal from the torch to hold the Sensor on).

[WyoGreen]

It sounds like the circuit works fine without the torch shield being involved. If you have a voltmeter, you could set it to measure ohms, and then measure from the torch shield to the plate and see what you read. It should read nothing, as the shield should not have any path to ground. If you do read something, then the shield is not isolated from ground as it should be.

Edit: You should also check for voltage, again, there should be none.

[ChrisO]

With the Plasma cutter and ohmic sensor turned off, I get about 155,000 ohms, between torch shield and ground clamp. The table has a Main ground plate that all grounds are connected to,(star configuration) including a earth grounding rod. If I disconnect the plasma cutters work ground from the tables Main ground plate, I continue to get 155,000 ohms. I then unplug the plasma cutter from the wall outlet, the reading changes to 0 ohms. I then connect the plasma cutters work ground back to the Main ground plate, I get 155,000 ohms.
With Plasma cutter turned off, Ohmic sensor turned on... (and all grounds properly connected back from previous test) I can't get a resistance reading, however voltage reads -.6 VDC. (probably because the ohmic sensor is providing -11.7 VDC which when connected to the torch shield drops -.6 VDC)
When I turn on the plasma cutter power supply in (standby mode) the digital multimeter ohm reading constantly bounce around 7K but I expect this is not accurate. When I try to measure voltage, either AC or DC, the meter bounces around and reading can not be obtained( it seems to be in a millivolt area however).
This plasma cutter uses is a High Frequency Pilot Arc. Inside the torch, the pilot arc is initiated in the gap between the negatively charged electrode and the positively charged tip. I wonder if some level of resistance is typical for this type of machine?
I appreciate any thoughts you have to offer...

[acourtjester]

I don't think you will have success with a HF start with a ohmic sensor. The shield on a Hypertherm is completely isolated from any other circuit except the Ohmic sensor input lead.

[Rodw]

Sounds like you do not have a shielded torch and the electrodes are already connected in the torch. That can't work. YOu could look at other options such as a seperate probe on an air ram...

[ChrisO]

I have come to the conclusion that the Pilot Arc Circuitry, that is connected to the Torch Tip(Torch Shield) is the source of the resistance to ground. This causes Ohmic Sensor's Output to come on. One option I see is to install a relay with the Torch Tip on the relays com and the Pilot Arc wire to the relays NC and the Ohmic sensor on the relays NO and then...somehow... during Probing, activate this relay... This may be something I can entertain in the future, but I am going to shelve this idea for now.
Thank you ALL for your inputs as it helped me identify the issue.

[acourtjester]

Not saying this will work but you may get inspiration by looking at 3-D printing, they use a sensor to detect the print bed. This a called a BL touch, it senses the surface and retracts the tip so it is out of the way after sensing. It is retracted when the led is on inside the BL touch. This would require a offset from the torch tip for location purposes, and maybe a shield to protect for pierce ejection metal.

[ChrisO]

Interesting, Thanks for sharing.. and again thanks for helping identify my issue.

[Greolt]

I have come to the conclusion that the Pilot Arc Circuitry, that is connected to the Torch Tip(Torch Shield) is the source of the resistance to ground. This causes Ohmic Sensor's Output to come on. One option I see is to install a relay with the Torch Tip on the relays com and the Pilot Arc wire to the relays NC and the Ohmic sensor on the relays NO and then...somehow... during Probing, activate this relay... This may be something I can entertain in the future, but I am going to shelve this idea for now.
Thank you ALL for your inputs as it helped me identify the issue.

I use the same ohmic sensor on a pilot arc cutter. My torch has no shield, just the electrode and nozzle. Pilot arc wire is connected to the nozzle.
So I put NO relay between the sensor and the nozzle as well as a NC relay to disconnect the pilot arc from the nozzle during surface probing.
Works OK.

[ChrisO]

Thanks Greolt for your thoughts on it. Sounds like using a relay strategy will work. Thanks for your insight.
ChrisO