@mcdanlj There seems to be a linear magnetic encoder with a linear magnetic strip you can lay down next to a rail. I’ve seen one mention, you may try Allegro.
I also saw a German reference to a delta robot that was calibrated using lasers. No idea how they do it other than perhaps a rotating beam and a target.
OpenCV and a camera?
The list goes on, which is why I am trying to do baby steps. Most of my throw-aways are from my print head hitting a bubble in the previous layer.
If I can at least alarm and pause on that I am much further towards reliable printing. Auto correcting is a good second option cause many of my prints are purely functional, so a misplaced vector or two is not catastrophic.
From there - there is so much uncontrolled metal and plastic in a Reprap its frightening - Unless of course you consider send-and-pray protocols as GOOD.