You have to be able to read and react quickly to +- 1 V change at the tip. So in a 50 to 200VDC range you need to pick out a 1 volt change with the tip moving 200 IPM and 20Volts of noise. More than 3 volts out and your cut starts to sputter or the tip bangs the metal. Your feedback and adjustment become a servo loop. Filter too much and you get poor response. Filter too little and it becomes a random position control. If you don't understand grounding and isolation you will burn up a lot of circuits.
If you can't afford the 350.00 THC300 I doubt you have the equipment you need to build and test a working ATHC.
It's simple until you get to the details. I spent MANY hours of engineering time to get a working THC and then many times that amount in testing and tweeking for something that is reliable and will survive the plasma cutting environment.
We are close to releasing our third generation of THC based on hundreds of installs, countless hours of testing, and even more doing customer support.
Funny you should quote the Rutex documentation. They never could make that work good enough for prime time use. It was never more that a "kit" and their approach to pierce height sensing was a joke.
Tom Caudle
www.CandCNC.com