Originally Posted by
jimcolt
Ohmic contact is used primarily for initial height sensing (IHS). IHS is used for finding the surface of the plate before the arc fires....so the the THC can retarct the torch to the proper pierce height (as recommended by the plasma manufacturer).
Ohmic contact is usually acheived bu using the ohmic contact retaining cap on the torch, which allows a wire to connect....which makes contact with the shield on the front of the torch. When the shield contacts the plate the connection is sensed through the THC....which should then start retracting the torch to the pierce height. Once this height is reached then the torch should fire....and arc voltage control will then take over controlling the height (see my previous post).
Some THC systems also use this ohmic contact function as a collisions sensor. In this case, if the shield of the torch contacts the plate during steady state cutting....the torch will rise rapidly to 1/2 the pierce height...then will reactivate the arc voltage control after a few seconds. This is a great function that minimizes torch crashes that cab disrupt a cut. It is available on many industrial grade THC systems.
Jim Colt
Hypertherm