Passive dynamics is a good basis for robot design and control. Not.

Many airplanes can, or nearly can, glide stably without control. So it seems natural that the first successful powered flight followed from mastery of gliding. Many bicycles can, or nearly can, balance themselves when in motion. Bicycle design seems to have evolved to have this feature. Also, we can make toys and ‘robots’ that, like a stable glider or coasting bicycle, stably walk without motors or control in a remarkably human-like way. Again, it thus seems to make sense to use `passive-dynamics’ (uncontrolled dynamics) as a core for developing the control of walking robots and to gain understanding of the control of walking people.

That's what I used to think. But, so far, this passive approach has not led to robust walking robots. And when looked at more carefully, the airplane and bicycle motivations are not as suggestive as they seemed. What about human evolution? Unlike what would be indicated by the passive-dynamic paradigm, we  didn’t evolve dynamic bodies and then learn to control them. Rather, people had elaborate control systems way back when we were fish and even when we were worms with no skeletons. 

Now, instead of thinking of good powered walking as passive walking with a small amount of control added, I think of good powered walking, human or robotic, as highly controlled, while optimized mostly for avoiding falls and, secondarily, for minimal actuator use.  However,  if control is paramount, why is it that uncontrolled passive-dynamic walkers walk so much like humans? It seems that energy optimal, yet robust, control, which is perhaps a proxy for the evolutionary development of human coordination, arrives at solutions that have some features in common with passive-dynamics. Such optimized control, when well done, has most of the motor effort, always at the ready, titrated out. Thus, highly controlled systems can deceptively-look almost “passive”.