Planetary Roller Screws

https://www.humanityslastmachine.com/#planetary-roller-screws

by everlier • 6 months ago

49 10 6 months ago

Animats4• 6 months ago

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

vintagedave• 6 months ago

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?

convolvatron• 6 months ago

these translate rotary motion into linear motion. if you hold the screw fixed (with bearings), and let the nut float, then turning the screw moves the nut back and forth along the screw

analog31• 6 months ago

My interpretation is that for joints, these are like the muscle, and there still needs to be a tendon.

Animats4• 6 months ago

Yes, this is just the power for the joint, not the hinge.

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.

[1] https://irisdynamics.com/

detourdog• 6 months ago

I enjoy these types of documents that layout the issue. I was surprised by the description of the shortcomings of composites. I understand carbon fiber may be prone to the dielectric corrosion but the other fabrics he mentioned should be immune. The repairability should be straight forward and joining metals is pretty common. Fabrics in composites use different weaves and are laid up with different orientations depending on anticipated stressing. My experience with composites is in boat building and it’s limited but growing. I’m currently a novice but plan on pushing composite construction as far as I can. I’m not saying he is wrong but I’m surprised at the criticism of composites.

golden-face• 6 months ago

What is the mechanism which turns these assemblies to do work and move/pull/push something? The inner screw is rotated by a motor, which causes the planetary roller system assembly to move, where the thing you want to move is attached to the moving roller assembly?

Animats4• 6 months ago

Right. It's like pushing a nut along a screw by turning the screw.

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.

[1] https://en.wikipedia.org/wiki/Recirculating_ball

vivzkestrel• 6 months ago

- stupid question, if you have absolutely no knowledge about electronics and robotics, what is the roadmap you think would teach you how to build a robot like this from scratch?

Animats4• 6 months ago

A degree in mechatronics gets you up to speed on the electromechanical side.

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.

[1] https://www.skyfilabs.com/mechatronics-online-courses

Planetary Roller Screws

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