Nice. Here's a video by a maker of planetary roller screws showing how they work.[1] The pitch shown is high enough that those don't look back-driveable. So if those drive a leg joint, they have to be able to absorb impacts directly. They can't pass them back to the motor, which can absorb them in a magnetic field. ("You cannot strip the teeth of a magnetic field." - GE electric locomotive salesman, circa 1900)

There's a basic conflict. Small electric motors want to turn fast, so they're usually followed by gear reduction. But that loses feedback precision and back-driveability. A pure direct drive motor works great, but they're large diameter devices. Some SCARA robots use them, but the motor is a foot across. Washing machines have gone direct drive, since there's enough space for a large diameter motor. There's a direct drive electric motorcycle with a hollow rear wheel. There are "pancake" motors, with large diameter but little thickness. None of those devices have a good form factor for humanoid robots.

That leads to tradeoffs such as quasi-direct drive, where there's some gear reduction, but not too much. The article suggests that 20:1 is an upper limit for back driveability. That's pushing it for a leadscrew-type device, but maybe it's possible now.

It's neat seeing all this progress in robotic components. Historically, robotics has been a small niche, and had to use components developed for other purposes. This made for clunky robots. Now we're seeing more purpose-designed components made in volume. Drones made 3-phase synchronous motors and their controllers small, light, and cheap. Now the same thing is happening for other needed components.

Looks like, when the AI guys get their act together on manipulation, the machinery will be ready.

[1] https://www.youtube.com/watch?v=3pMN3BqGk_o

> Planetary roller screws are the gold standard for high-performance joints such as knees, ankles, and hips.

It's hard to understand how these are used for joints. I think of a screw as something that rotates many times. Are these used for things that rotate only a few degrees, as a knee might?