3 Achs Fräse mit MESA 7C80 und Raspi 4

I got the smallest Gravograph machine (UNICA) up and running.
Thanks once again for your input and this amazing forum in general

Now, the larger machines have 1-phase AC motors and a separate speed control PCB that's actually a single-phase frequency converter. So far so good, there's also an encoder and a potentiometer to set speed, BUT:
The potentiometer is 500 kOhm and voltage across the potentiometer is merely 0,22 V.

Is there any way to get this to work with LinuxCNC?

[EDIT]
I'd like to apologize for crashing Hans Peter - HPWs thread.
Would it be better to move my issues to a new topic instead?

Hans Peter, verzeih mir bitte, dass ich mich hier hineingedrängt habe

Kein Problem