Birds make better bipedal bots than humans do

That's been done before. It's just new for Pratt, who previously used series elastic actuators.[1] Those don't do energy recovery; they fake it using a jackscrew and a stiff spring.

Running humans have about 70% elastic energy recovery. Cheetahs have around 90%.

What you want in a leg muscle is a spring whose spring constant, damping constant, and neutral point you can control. That can be done pneumatically (you need compressed air), hydraulically (you need hydraulic accumulators), mechanically (two springs operated by separate actuators), or electrically (a fast back-driveable motor drive that generates power during compression). All those options have been tried. There's no really good way to make a muscle-like actuator with power to weight ratios comparable to biology.

Systems with a fixed spring work, but under a narrower range of conditions.

[1] http://www.ai.mit.edu/projects/leglab/robots/Spring_Flamingo...

It's interesting that most fictional depictions of legged robots (outside of explicitly humanoid robots) use digitigrade legs. Did the artists model their art after animals which happened to have these legs, or did they subconsciously realize that this would be more effective?

It really does look exactly like a BattleMech from one of the MechWarrior video games doesn't it! Thats fascinating. Reminds me most of the Locust:

https://www.sarna.net/wiki/Locust

https://sites.google.com/site/fineartofmechcombat/_/rsrc/146...

ED209 too. But how will they handle stairs? https://youtu.be/mRDl5_-wJ0Y

Another thing that we are going to see in combat vehicles in the next decades, IMHO, is something like OmniMechs. Meaning that future weapon systems, e.g. fighters or tanks (not the upcoming generation but those after that, F-35's successor' successors) will feature LRUs (line-replaceable units) that carry different mission capabilities. That way, a fighter can land as a ground support fighter or drone controller and lift of as air superiority fighter by swapping a hand full of modules out.

I do wonder if it can walk backwards or handle uneven terrain. The bird likely has a muscle to extend the foot when needed, the bot does not.

Well yes, and they say why in the article. It doesn't yet have the degree of freedom in its hips to balance properly. That's not especially hard though - a pivot around the main spring axis would do the trick, and the dynamics of that part are well-understood at this point.

Our gait isn't the reason humans can travel long-distance. It's that humans have more effective cooling and don't overheat in high temperatures.

In cold climates, gray wolves have advantage in ultramarathon distances, provided they don't overheat. They can maintain 5~6mph on rough surfaces. Humans maintain 4~5 mph on prepared surfaces. (Wolves are lighter and their four clawed feet have significantly better traction than our two feet.)

Any surprise we domesticated dogs? They can keep up with us. :)

This fact about the energy efficiency of human locomotion is frequently mentioned.

However, what is not mentioned is that this high efficiency is attained only when humans walk or when they run slowly, much more slowly than almost any other animal of a similar size.

This ability would not have been of any use, had it not developed simultaneously with the ability of throwing sticks and stones much faster than any prey animal could accelerate.

High endurance, even at slow speeds, was perfect for following the track of an animal wounded by a throwing weapon, until it became exhausted and it could be caught easily.

Otherwise, walking randomly across the plains, with the hope of encountering by chance an animal which was exhausted enough to not be able to escape from your very slow, but efficient, running, would have been a losing strategy.

maybe a smaller contact point means a simpler model that doesn't have to take the orientation of a larger foot into account?

Only until something falls down on the floor or the robot needs to use stairs etc. Loading into smaller vehicles is also not done with wheels but with hands and feet.

And kangaroos? Okay, so their knees are possibly somewhat more inconspicuous, but looking at a kangaroo skeleton, what to my eyes looks like their ankles also actually is their ankles.

Whereas with birds the confusion comes in because

a) they're walking on their toes, so what to us looks like an ankle is in fact a toe joint, and then moving further up what we think looks like a "reverse knee" is in fact the actual ankle, and

b) their true knees are usually hidden somewhere beneath their feathers

And even we humans switch to "heel up" for high intensity bipedalism. Are we are secret members of the reverse knees club?

And safety for bystanders.

Birds had a lot longer to optimize for bipedal walking than humans did. If we had stayed bipedal without developing advanced technology it’s likely we would have gotten a lot more efficient at it over 10’s of millions of years.

We really are not optimal in any particular way. We started out as monkeys, and have done the best we could from a bad starting point. Turning the spine vertical really did not work out well at all, and knees are unfortunate at best.

Probably worse than all that is how we go all to hell when we don't get exercise. Cats can sleep 18 hours a day without ever getting out of shape. Bears can can get super fat and then sleep for months, and be fine. A lot of birds can go a week without drinking or eating, working out the whole time with no sleep or even rest breaks.

Hum... We are a bit optimized for flexibility, and a lot the way we are by accident. As many people already said in other threads, we are probably the least optimized walking animal of all, because our body is recent and walking efficiency was never as relevant as other characteristics (like flexibility).

The birds body, by their turn, appeared in a time closer to the first animals than to today, took over the lands of the world in large part because of their ability to run, and kept them for more time than any other kind of animal.

There are some freedom in robot bodies that animals don't have. So it's not immediately obvious that an animal inspired body is perfect. But birds have had hundreds of millions of years to perfect their shape, it's not clear that we can do any better either.