ohmic probe?

[agrifabs]

evening all, just a quick question, whats an ohmic probe sensor? and how will it benefit me on my cnc plasma? theres one on ebay for £55 from romania

[sphurley]

Search this forum. There is a lot of info about it.

[jimcolt]

Here is what Ohmic contact is, does:

It is important for a plasma torch to pierce at a higher height than it produces its best cut quality at. Generally the height for piercing that works best is 1.5 to 2 times the recommended cut height. Piercing closer than this can damage the torch nozzle orifice on the first pierce, and all cuts after that will have poor edge angularity.

Height control systems for plasma cutters need to accurately locate the surface of the material being cut before each cut......this allows the system to more accurately set the pierce height.

The different methods used to find the surface (on many different height controls over the years) have been:

- torch cut gas back pressure (old Hypertherm technology from the 1970's!)
- retractable inductive probes (old Hypertherm technology for underwater applications)
- torque sensing, monitoring the current of the z axis motor. (originally from InnerLogic in the late 1980's, now commonly used)
- stall sensing (similar to above, using stepper motors instead of servos) originally from Hypertherm in the 1990's.
- Limit switch sensing (also known as "floating head) originally ESAB technology from the 1970's, now commonly used on stepper z axis.
- Ohmic sensing, using the metal shield on the front of torch to sense the plate with electrical contact (Hypertherm technology from 1990's)

All of these methods above except for the inductive probes and ohmic contact put some downward pressure on the material being sensed, often causing thin materials to flex downward a bit before being sensed (due to the force sensing method used). This, on thin materials often cause the pierce to happen too close to the material (as the material that flexed downward will follow the torch back up as the torch is seeking the pierce height)....and with a pierce too close nozzle damage will occur.

The ohmic surface sensing can be tuned to be extremely accurate with no deflection of the material, and can allow the torch to retract very accurately to the correct pierce height, providing much longer plasma torch nozzle life.

- Ohmic requires a shielded torch where the shield (electrically isolated from the nozzle) is used as the electrical sensing circuit between the torch and the material surface. On non shielded torches some sort of conductive probe can be fabricated that extends very slightly below the exposed nozzle so the probe can contact the material for sensing.

- Ohmic sensing works very well on most material surfaces....even if lightly rusted or oxidized.

- Ohmic sensing generally does not work well with painted or primed or heavily rusted materials as it cannot establish electrical contact.

- Ohmic sensing does not work well in many water table cutting applications......as the water becomes highly conductive (suspended solids of metal)...and the ohmic circuit will sense the water before sensing the material.


Hope this helps! Jim Colt Hypertherm

[East German]

Hallo

here is a small price Version, works fine on every years by my plasma table.

Cost in Germany 11.37€.

Best Regards Peter

[acourtjester]

Will not ship to the US may be OK for Dave in the UK

[Larry83301]

Will not ship to the US may be OK for Dave in the UK

Try this http://www.ebay.com/itm/Brand-New-Kemo- ... SwxH1T9DJ0

They ship to the US, 'cause it starts in the US.

Larry

[acourtjester]

I looked in the wrong spot on eBay (water detector) On Amazon is where the said no US shipping and another web page too.
Old story "Its who you ask" thanks

[agrifabs]

thanks for replies ! i have just purchased this one off ebay http://www.ebay.co.uk/itm/Ohmic-probe-s ... Swqu9U562Q

will the ohmic probe and mach3 sense the height of the material at the start of each cut on the same piece of work?
at the moment i dont have any sensors on my table so i have to manually zero the z axis, but when cutting the plate sometimes distorts and the tip becomes either too close or too far away from the piece

[acourtjester]

there are 2 way you could use the ohmic sensor and both are accessed by G-code commands. This will be part of the post processor you use.
1) the ohmic sensor is wired so the output is wired to the Z home pin and post and the G-code would have commands with G28 like below
2) the ohmic sensor is wired to the output is wired to the probe pin and port and the G-code would have commands with G31
this is what the G-code may look like at the start of each cut. it may not be exactly but it is close. But you should also have a floating head switch as a safety backup if the ohmic sensor fails.
G28.1 Z0.02 (Start Touch-Off ) this line drives Z down until the ohmic touches
G92 Z0.0 this line zeros out the Z dro
G00 Z0.001 (Switch Offset Lift) this line gives a slight offset for Z axis
G92 Z0.0 this line re-zeros the Z dro
G00 Z0.150 this line moves Z to pierce height
M03 lights the torch
G04 P0.6 pierce delay
G01 Z0.060 F60.0 drops to Z height for cutting

For the probe the G-code would used G31 in place of the G28 command

[Greolt]

here is a small price Version, works fine on every years by my plasma table.

I would be very interested to see how this is implemented to be a touch sensor. And how it is protected electrically from the plasma.

[East German]

Hello,

the power supply fore the Sensor not from the CNC! Use another litle power supply!

This is my circuit runs since years.

Regards Peter

[East German]

I use this Macro m3 /m5 fore touching.

Postprozessor mach2 noz in Sheetcam!

[Greolt]

Hello,

the power supply fore the Sensor not from the CNC! Use another litle power supply!

This is my circuit runs since years.

Regards Peter

Thanks for that Peter

Here is my attempt to translate for my own interest.

Although it only makes sense to me if the 230v relay is 2 NO and 1 NC contacts.

The 2 NO connecting the two probe contacts only when probing, leaving them disconnected all other times

and the NC locking out the plasma-on switch while probing, to prevent accidental firing, which would fry everything.

.

[East German]

That's right it should work

Peter

[acourtjester]

I have a much simpler way to set the switch up it is how I have my feather touch wired and think the Kemo switch could be hooked up the same.
I am using a Hypertherm machine torch with shielded tip contact and work ground contact for the sensor inputs.

KEMO switch.jpg

[East German]

Hallo,

is not working with 12 Volt from Computer or CNC !!! The Power Supply not a protective conductor.
Otherwise creates a short circuit.
Use another litle power supply!


Sensor lines without a relay is not good.

Regards Peter

[acourtjester]

not a problem for me

[vmax549]

Your Feather touch HAS built in ISOLATION . This sensor obviously does NOT so you have to provide it or protect against the problem (high voltage )

Otherwise you could easily SMOKE your entire electronics package including teh PC.

Seen it done with plasma . Seeing it once was enough for me (;-). The electronics side made a poor welding rod . For some odd reason it could not handle 300 volts @ 100 amps very well.

(;-) TP

[davek0974]

I would certainly add more isolation to that install, never a good idea to use the pc supply for anything outside the pc or the isolated side of the BOB, just begging for trouble IMHO

[East German]

Hallo

I built in the last few weeks, a test board with smooth stepper from Warp9.
I have the motors from my old system connected to my test board.
The cut rule of Sheetcam the M10P2 and M11P2 be tested for switching the signals from the Proma THC.

Here is a Touch Test with the Waterswitch:
https://www.youtube.com/watch?v=VzyLPQikAJw

Test the SC Cutting Rule in the Corner is THC Signal off, the lamp over the Proma THC is out:
https://www.youtube.com/watch?v=A1HF7nWbujA

First Cut with the Testboard:
https://www.youtube.com/watch?v=snMvi9BDSnY


Regards Peter
Happy New Year

[jimcolt]

Normally the connection to the plasma torch shield has isolation through a high voltage relay. The shield is in contact with the ohmic sense circuitry only when the plate surface is being sensed, and isolated (open relay contacts) before the torch is fired and during steady state cutting. There are conditions that can apply relatively high DC voltage to the shield, and on high frequency start plasma's there can be some serious RF voltage levels as well. Hypertherm height control systems use a 10kV isolation relay to disconnect the ohmic wiring under firing and cutting conditions.

Jim Colt Hypertherm

[acourtjester]

Peter your setup works very well. It is another plus for the DIY group

[East German]

Peter your setup works very well. It is another plus for the DIY group

Thanks

Homebuild is funny

Regards Peter