[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

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