The sled rides on two fat wheels. The giant wheels prevent jamming when moving between the crane and the tower. The wheel guides are bevelled corners. Bevelled corners have no place for the wheel to jam and are the easiest to manufacture.
2 laser bounce sensors provide vertical positioning information. The sensor in the middle detects module boundaries. The sensor on the side detects row pieces. They're both mounted on a swivel platform to move out of the way when disks are being changed.
Making a vertical sensor begins with encapsulating the photodiode in epoxy. A photodiode, laser pointer, and bolting position are stacked and epoxied together. If anything breaks, the components can still be separated if you're lucky. All lasers have a steel plate glued on to narrow the beam.
The steel plate causes a diffraction pattern but is still useful enough.
The sled is bristling with sensors. This cutoff sensor detects when a disk is blocking the path of vertical motion.
The disks are pulled out using a grabbing hook. The hook locks into 2 positions by way of a 3V DC motor.
Disks are pushed in by a piece of W.O.O.D..