@friolator thanks for the transport of the Magnatech, interesting design.
Had the opportunity to do an initial testing of the potentiometer dancing arm, it worked well. It is powered with the 3.3V output from the Pico board, and initially the ADC values were jumping around a bit. For minimum conditioning a 1 uF capacitor was placed in parallel to the potentiometer, and also a 100nF capacitor to quiet the middle leg into the ADC, and that basic conditioning was enough to get good stable readings.
The proof-of-concept algorithm advances the corresponding steps for one frame (measuring the radius of the pickup reel manually), the frame is moved in 4 steps, and after each quarter step, the supply returns the tension to a preset range.
The purring sound is the supply-stepper doing short ramps with acceleration/deceleration until the set tension is reached. The tension is maintained well throughout.
Again, this is a proof-of-concept first run, the coding needs lots of improvements.
The missing link to make this work precisely is a good way to measure the size of the pickup reel as the scan progresses.
I think my next step is to implement a pure-math pick-up driver and see how many frames before it needs significant correction. Basically the problem to solve is a work around to the failure shared, the ToF inaccuracies.
Thanks again for the feedback and ideas, all well received.