Comment by vintagedave
13 days 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?
My interpretation is that for joints, these are like the muscle, and there still needs to be a tendon.
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/
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