polishing stainless steel machine

[EddyCurrent]

I've worked on a grinder for steel that was controlled by PLC but don't you just need something that goes back and forward, side to side ? this could be done with relays and limit switches.
On the PLC grinder, a microphone located near the grinding wheel detected when the wheel had made contact with the metal and stopped it's downward travel. An electronic notch filter was used to reliably detect the right frequencies, surprisingly it worked well.