Positioning three phase induction motors.

Continuing with my Fehlmann mill-drill conversion, I have been sorting out what might be referred to as the W axis, that is the coarse positioning of the milling head. The machine as originally designed uses a microswitches and stops to obtain three user settable positions of the head. This vid from a couple of years ago shows the head and stop arrangement https://youtu.be/EafSH0RX1Lk?t=20

I've had this working with the stops but they only appear to be repeatable to about 0.1mm - which will mean tool height setting every time the head is moved.

I was intrigued to find that Fehlmann continued developing my machine until about 2011 with the later Picomax 54 and 55 being almost identical but with CNC control of the W axis. Seen here:
https://youtu.be/HR-zZuK0Tkk?t=98
https://youtu.be/IC6KW4-KDgY?t=65

The Picomax 54 uses the same motor as in my P50 but has a rotary encoder on the end of the pinion shaft - I'm sure Fehmann's rack and pinions are very accurate but it doesn't seem like the greatest way of obtaining positional info. The motor is controlled by a VFD and somehow this controls the position of milling head using info from the encoder.

I think that the Picomax 55 is doing the same thing but with a glass scale - I'll check with Fehlmann.

Can anyone illuminate me about how it is possible to use an induction motor, inverter and encoder to obtain absolute position? I see this clever chap manages to achieve it: https://youtu.be/3oTJNEVpvYY

Quote:

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Can anyone illuminate me about how it is possible to use an induction motor, inverter and encoder to obtain absolute position? I see this clever chap manages to achieve it:

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This is a vid by Andy Pugh He is on the LinuxCnc forum and I think he is also a developer.

Thanks Clive, I'll drop him a line.

David