ohmic sensing

[stirling]

can anyone explain *exactly* what this is? particularly with reference to initial plasma torch "touch off" i.e. how does it work?. I can find any amount of explanations on the webnet re: capacitive sensing, but nothing on ohmic sensing.

Thanks

Ian

[keebler303]

I think it just means measuring the voltage drop across the arc. Basically measuring the resistance of the arc. Too high resistance means the torch should be lowered, too low resistance means the torch should be raised.

Matt

[jimcolt]

"Ohmic Sensing" ( A Hypertherm term!) is simply an electrical connection between the torch height control on a cnc cutting machine and the shield (front most part on a shielded plasma torch).

You normally will see a wire attached to the retaining cap on the plasma torch...that connects back into the THC. The primary use of ohmic contact or ohmic sensing is for finding the surface height of the plate before piercing. This allows the torch height control to retract accurately to the manufacturres suggested pierce height before each cut cycle......extremely critical to ensure the best consumable life as well as to ensure the the height control has an accurate positioning of the plate to allow for indexing to the correct cut height after the pierce is complete.

Some torch height controls also use this connection during real time cutting to detect a collision with the plate. A plate collision can occur from thermal stresses (warpage) in the plate or from inherant stress in the plate that occurs from changes in structure after a few pieces are cut off.

On industrial systems....ohmic contact is the primary initial height sensing system, and stall force feedback is secondary. In the event that the top surface of the plate has poor conductivity (rust, paint, primer, underwater) then the THC senses an increase in motor torque when the torch bumps the plate (while indexing down to find the plate)....and uses that form of initial height sensing when ohmic does not work well.

Ohmic contact is necessary to get the best possible consumable life with cnc plasma cutting applications.

Matt's response about measuring the voltage drop across the arc is in reference to "Arc Voltage Control" (AVC) which is the method for controlling the torch to work distance after the arc has started and the machine is moving in x and y directions.

Jim Colt Hypertherm

Originally Posted by stirling can anyone explain *exactly* what this is? particularly with reference to initial plasma torch "touch off" i.e. how does it work?. I can find any amount of explanations on the webnet re: capacitive sensing, but nothing on ohmic sensing. Thanks Ian

[stirling]

Hi Jim - thanks for your clear reply. I was kind of guessing that it was simply detecting a "made contact" between torch and work - but thought hmmm - Ohmic sensing - that sounds posh - must be more to it.

So - presumably there is no need for a floating head arrangement if this method is used for IHS - OR - is a floating head still needed to "take up the slack" between the torch touching the work and the Z decelerating to a stop?

I've been reading a bit about some suppliers and their "capacitive sensing" technology - can you help me out with this one? - just trying to understand the options/pros/cons.

Thanks again

Ian

[jimcolt]

Capacitive sensing is ok as long as it can be accurate to +- about .005" of the desired height. The issues we have found with both capacitive and inductive sensing (Hypertherm formerly used inductive sensors) is with different accuracies on different materials.....they settle at different heights on aluminum, stainless and carbon steel....and also are affected by moisture and surface finish of the material. I know Dynatorch uses a laser IHS sensor.....although I suspect it could be affected by smoke/steam and certainly would not work underwater either.

With ohmic and torque sensing you would not need a "floating head" as long as your z axis decel was fast.

Jim

Originally Posted by stirling Hi Jim - thanks for your clear reply. I was kind of guessing that it was simply detecting a "made contact" between torch and work - but thought hmmm - Ohmic sensing - that sounds posh - must be more to it. So - presumably there is no need for a floating head arrangement if this method is used for IHS - OR - is a floating head still needed to "take up the slack" between the torch touching the work and the Z decelerating to a stop? I've been reading a bit about some suppliers and their "capacitive sensing" technology - can you help me out with this one? - just trying to understand the options/pros/cons. Thanks again Ian

[stirling]

Thanks again Jim. So from what I've read and from what you've told me I'm thinking:
use a floating head and a switch - fair?

[jimcolt]

I prefer using the ohmic contact.....but a floating head and switch works almost as well for accuracy..if you have minimal hysteresis in the switch......and works better on rusty painted surfaces.

Jim

[stirling]

Thanks for all your help Jim - much clearer to me now. I'll have a think and a chew...

[meff]

Can someone put a wiring diagram for ohmic sensing?
Can it be used with any type of torch?

[jimcolt]

Ohmic contact is a method of initial height sensing (IHS) that is used to find the surface of the metal before each cut cycle.....by accurately locating the surface, the z axis can then retract to the manufacturers specified pierce height for the material being cut....this dramatically lengthens nozzle life and improves cut quality on a plasma torch.

In order to use Ohmic contact....an electrical connection must be made from the front-most metal part of a plasma torch.....not all plasma torches have this capability. The Hypertherm Powermax series (Powermax45, 600, 1000, 1250 and 1650) have special retaining caps that have a spade terminal for attaching a wire....this terminal electrically connects the wire to the sheild (on the front of the plasma torch).....when this shield touches the plate during the iHS cycle...contact is sensed through this wire to the plate...indicating to the torch height control system that the surface has been located.

As far as where the other end of this wire attaches....that varies depending on the torch height control system manufacturer.

I have a PlasmaCam DHC2 machine....and the ohmic contact wire disapears into the wire harness that goes back to the operator control panel. Many height control systems do not have the ability to use the ohmic contact function....many use a limit switch that senses contact with the plate...and performs essentially the same function as ohmic contact...except that on very thin materials the limit switch type IHS system tends to deflect the material...then the material sprinds up toward the torch while retarcting to the pierce height allowing the torch to pierc in contact or too close to the plate...not good for nozzle (tip) life.

Advantages of ohmic contact:

1. accurately senses the surface of the metal being cut without deflecting the metal.
2. also can be used as a collision detecter....if while traversing the torch contacts the plate...the motion of the machine can be stopped to avoid damage.

Disdavantages:
1. does not work underwater (senses the surface of the water), does not work on non conductive surfaces such as heavy rust, primer, paint, masking, etc.

If you look in the attached picture you will see the ohmic contact wire atteched to the front of the Hypertherm Powermax45 torch....this is on a PlasmaCam cutting machine.

Jim Colt