So after 2 years thinking about this project I finally have a working system. It probably is a bit over the top but I wanted to explore a few ideas to the thing has evolved.
The scanner uses independent stepper motors for feed and takeup. A spring loaded system provides feedback for the motors and maintains a constant film tension no matter what size spool is used. The actual tension is adjustable. A third stepper drives a sprocket wheel recovered from an old viewer, this is also connected to an optical rotary encoder used to provide a timing pulse for each full rotation.
The takeup and back tensions are controlled by an arduino uno. A second uno handles the sprocket wheel timing. I could have used a single uno but having two enables me to isolate the two drive systems an simplify matters. Camera is a PiV2 mounted on a motorised focus rail and spring loaded mount to enable alignment. Light source is a led flat panel and exposure controlled by varying the drive voltage (poor colour temperature but this can be corrected). Images are collected with a Pi4, stored to usb stick for transfer to the pc. I use Adobe Premiere to edit and the built in stabilisation function to remove frame movement.
Sprocket drive. I had to change the film path so this is not great but works ok. Film wraps round sprocket and locates in 7 sprocket holes. This should even out any film with worn holes.
Rotary encoder on left, this is on an adjustable mount so the relative position of the pulse can be adjusted to put the frame in the right place. Will explain later.
Tension system. Main part is a slider pot as found on mixer desks. Bungee cord pulls film to left, pot resistance provides feedback so motor turns to compensate by tightening film and pulling to the left. Feedback has a null range to provide hysterisis and prevent oscillation. Actual tension is controlled by adjusting cord length. Limit switch stops motors.
Camera and focus rail
Film gate. Film runs on neodynimum magnets - very smooth surface so no change of scratching film. Film held flat on gate using magnetic rollers. These are plastic rollers recovered from a printer. The center pin is held by the magnets and the roller rotates as the film moves underneath. Discovered by accident but works a treat, again no possibility of scratching the film. This may be of interest…
Enough for now, I just wanted to get the ball rolling. Bottom line is that it works ok but image quality below par so have to look at alternatives.