Planetary Roller Screws
humanityslastmachine.com
humanityslastmachine.com
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.
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?
How to do this in a tight space is a tough mechanical engineering problem. Tesla's Optimus uses a 4-bar linkage as the hinge, and some kind of cylindrical linear actuator as the power drive. Can't tell much about the actuator from the patent for the hinge.
Boston Dynamics used to use hydraulic pistons in their legs, but that did not scale down well from their Big Dog mule-sized machine. They finally went electric, and their machines became far less clunky. Motor power/weight ratios have improved a lot since the early BD days.
Electrical linear motors would be a nice solution. They're rarely used, because they tend to have to be custom for each application. But we might see more of that as humanoid robots approach volume production. The technology has reached 15:1 power/weight ratio.[1] With cooling.
This article is about exotic screw/nut assemblies which have lower friction, longer life, and more strength than ordinary screws. The recirculating ball screw used for automobile steering is the classic of such mechanisms.[1] Both roller and ball screws substitute rolling for sliding, always a win against friction.
Here are some simple online courses that come with basic robotics kits.[1] It's not free, but prices are in the $250 - $600 range, so it's not a big decision like choosing a college or career. This is a way to find out if you want to do this sort of thing.