I started with with the OpenTorque Actuator. As this concept was initially done for an only 3D printed solution (most accessible production solution for all of us).
Using this actuator for a robot it would look like this as a scheme:
Starting to connect individual components with the target to make it as compact as possible we get following:
The black parts are the components fixed to the robot chassis. The Red is the hip followed by the knee in blue.
Squeezing out here and there some space we get to here:
The components are topology optimized. Looking now in a complete assy for the rear end of the robot:
it started to be very ... yeah... pretty big... so that was the kickstart to focus on the actuator first....