This is a series of very rough sketches of how SEA work and how to design one, based on a cheap servo (or two).
Just a visualization (a poor one) of how the inner wheels work and how to attach an elastic loop.
Actual prototype (but I will call it version 2), with a place for a servo and two potentiometers.
Just one bearing, to hold the front of the elastic part.
Version 3 lands with a custom bearing with 4mm balls (later on I'll do it with more available 6mm).
Encoders are no longer on-axis, I moved them to the side, they are multi-turn now, so extra computation is needed at the start to get the hold of relative position.