Proximity sensors

I am looking for info on inductive proximity sensors, particularly
the electronics (schematics) to make them work. They are generally
divided into NPN vs PNP and into analog and normally open/normally
closed switches. They have application particularly with positioning
plasma cutter and monitoring for metal that might tip up toward the
torch during cuttinig. I am aware that commercial products for
plasma cutters are available but cost $$$ (approx. $1600-1800).
Thanx....Richard.