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)<p>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.<p>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.<p>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.<p>Looks like, when the AI guys get their act together on manipulation, the machinery will be ready.<p>[1] <a href="https://www.youtube.com/watch?v=3pMN3BqGk_o" rel="nofollow">https://www.youtube.com/watch?v=3pMN3BqGk_o</a>