CNC Plasma Proma torch heigh control step dir

Good evening,

I built a CNC plasma with Mach 3 control. I realized the height control via a Proma THC SD, which measures the voltage of the arc and sends an up or down signal in the form of a frequency to the stepper motor of the Z axis. Now to my problem: With optimum cutting height, the THC reads a voltage of 70V. As soon as the voltage rises to 71 V, it does not regulate in time and the arc gets bigger and bigger until it is gone. As soon as the voltage drops to 69 V, the nozzle moves into the plate.

Would be very happy if you could help me to complete my project.