Electric motors dont have a lot of "give", like hydraulics do. But yes force-torque controllers can be tuned to be squishier. Someday I think electric motors will be the muscles and we'll have some kind of elastic tendons. For energy efficiency, it seems obvious to harness impact energy in a mechanical spring system, as nature does.
Or just use wheels / a wheel. This whole humanoid thing strikes me as an addiction to old sci Fi stories.
Hydraulic systems have very little "give", unless you put a hydraulic accumulator (an air tank with a fluid/air barrier) in the system. Electric motors have plenty of "give". Forcing a motor to turn backwards won't hurt it. The gear train is usually the weak point. As motors and controllers have improved, robot gear reduction ratios have decreased, which reduces the load on the gear train and lets the motor absorb shock loads. Direct drive robots eliminate the gear train entirely. Here's a nice one.[1] "You cannot strip the teeth of a magnetic field" - General Electric electric locomotive rep, around 1900.
With modern motors, you can get huge torque with light weight, and cooling becomes the limitation. Schaft used water-cooled motors in their direct-drive robot. Google bought Schaft, ran them into the ground and killed them.
It's mostly for distance running. Humans get about 70% of energy back in running. Cheetahs, about 90%.
Variable compliance muscles are desirable, but hard to do. A pneumatic cylinder with adjustable pressure on both sides will do it, and Festo builds a lot of that for industrial automation. Two opposed springs pulled on by two positional actuators will do it, but that's kind of bulky. There's a hack called a "series elastic actuator", which is a rigid positional actuator with a stiff spring on the end. When it gets some pushback, the spring compresses, and the motor frantically tries to move the positional actuator before the spring bottoms out. This allows you to simulate a spring with off the shelf screw jacks.
Those new direct-drive motors are a good solution. Direct-drive pancake motors have been around for a while, but they used to be about a foot across. Now they're smaller. Probably a spinoff of drone motor technology.
Actually, that's a pretty good idea. Regenerative motion. Sort of like regenerative braking in an EV, they could capture some electricity with each step to help reduce the energy requirements.
It's not because of science fiction stories, it's because things designed for human to use, is designed for a humanoid form factor. If you want to accomplish a task, it's going to be reflected by that machine. Eg a conveyor belt doesn't look like a human. But if you want swap a robot where a human used to be, it's far easier if that robot is humanoid and has the same approximate capabilities. Thus, we have humanoid robots.
Can someone point out where "powered rollerskates" are strictly worse than legs in civilized urban human environments, to an extent that a few extra hundred billion dollars of R&D are warranted? The "approximate capabilities" of a human are: moving around, and picking things up / fine manipulation. Wheels + arms does that just fine, and eliminates a lot of power, complexity, fragility. And it also potentially adds.
This is one of those 80/20 things that is just glaringly obvious. Like lvl 5 autonomous cars vs lvl 3-4.
The obvious answer is stairs. It seems like right now Spot is getting the most use as a highly mobile camera platform for automated inspection in industrial environments. Many of these have a lot of stairs.
Wheels are useless in this world. If you’ve ever tried using a pushchair or a wheelchair on much of the planet, built environment or no, you’ll find wheels are useless.
Hydraulics shouldn't have any give, as the working fluid is considered "incompressible". Of course in the real world the tubing can expand slightly and there are friction losses, but the reason they went with hydraulics in the first place is they can set a position and not have to use more energy to hold it there (since the cylinders are pressurized).
If the gear ratio on these motors is high, then there can only be faked compliance in the tuned force-torque controllers you mentioned. MIT's little cheetah robot, on the other hand, deliberately used low-gear ratios to keep things naturally squishy if needed. This is the way to go; putting elastic tendons or spring elements seems like a good idea but then you can't actually model the non-linearity well (the 1st order motor becomes a 2nd or higher order system).
Or just use wheels / a wheel. This whole humanoid thing strikes me as an addiction to old sci Fi stories.