If you're not aware of the ODrive then check out the Hackaday page here and their website here. The ODrive is designed to turn hobby brushless motors into accurate robotic actuators. As their creator Oskar says, such a controller should clearly exist but since it didn't, he built one.

From my perspective this device is idea because it's open source and I intend to modify its firmware in order to support joint compliance. Right now ODrive operates as a traditional rigid controller, moving either to a specific position or at a specific velocity without regard to load or resistance. I intend to add to these control mechanisms so joints can act with elasticity. 

Imagine pressure on a robot knee compressing the joint slightly as it holds the robot weight. There are a number of ways to achieve this. The passive way is to add springs or elastics to the join so they flex under load. Alternatively, we can simulate this (with Hooke's Law) by detecting the load on the joint and actively adjusting the joint position. Well, that's the theory anyway, and there are many examples of this technique in high end robots, but nothing hobby level that I could find.

So here we go, but first to get the joint built and operational.