Eight months ago he built a quadrupedal robot that could step sideways using three of them per leg. I’m not going to link that, you’ll have to find it from his YouTube page because you should look around.
Sorry for going off topic. "Electric Motor Scaling Laws and Inertia in Robot Actuators" by Ben Katz who designed the MIT Mini Cheetah in 2018 is very well known in the legged robotics community. His master’s thesis on actuator design is also widely referenced.
During the COVID period, some Chinese companies even sold variants of actuators inspired by the Mini Cheetah design.
Aaed Musa has also mentioned in some of his videos that his actuator designs were inspired by the Mini Cheetah actuator. Yes, His capstan drive video is especially impressive.
For example, in Aaed Musa’s video "I Built a Rubik's Cube Solving Robot" (https://www.youtube.com/watch?v=m0bMMALYMYk), he states in the description that the design was inspired by Ben Katz’s work.
Thanks for this. I almost never have the patience for any videos, but this one hooked me and kept me engaged throughout. Worth it for that random "yo mama" joke alone.
This channel about retrofitting industrial robotic arm control systems is quite practical. Could always run the playback at 1.25 speed for slow talkers/edits =3
Same issue covered on HN a few weeks ago.[1] This one has more motor theory but less machine learning theory.
Too much gear reduction, and you can't back-drive or sense forces from the motor end. Too little gear reduction, and your motors are too bulky or too weak. Reflected inertia goes up as the square of the gear ratio, as the article points out, because the gear ratio gets you both coming and going. So high gear ratios really hurt.
Robots, like drones, need custom motors sized for the specific requirements of the joint. For a long time, the robotics industry was too tiny to get such custom motors engineered, and had to use motors designed for other purposes. This will become a non-problem as volume increases. Especially since 3-phase servomotor controllers, which drones need, are now small and cheap. They used to be the size of a paperback book or larger.
(I've been out of this for years. I've used hydraulic robots and R/C servo powered robots.
The newer machinery sucks a lot less.)
Reflected inertia does scale as the square of the gear ratio but it's a bit misleading unless you also consider the change in rotor inertia, which scales as a cube of the rotor radius (as the article points out).
The other side of the scaling laws say that motor torque scales as a square of air gap radius (roughly rotor radius), and output torque scales as linearly with gearing ratio.
When you balance these out, the reflected inertia depends on the inverse of power dissipated for a fixed output torque.
In an ideal world, your total reflected inertia is independent of the gearbox and largely depends on the motor fill factor and how hot you can run it.
I wonder if robots could be made to work better at cryogenic temperature, so superconductors could be used. The figure of merit would be much higher if resistance was zero. Or maybe this is another reason to want room temperature superconductors.
Even copper has vastly lower resistance when cryogenically cooled. It's not a bad idea for some applications, and water cooling is already a good way to increase power density.
I learned recently that the inductive heating coils used for metallurgy (smithing) are copper tubing with coolant flushing through them. The copper tries to heat up along with the bar you’re heating in the coil. Both from resistance and from radiative heating.
https://youtube.com/watch?v=MwIBTbumd1Q
Eight months ago he built a quadrupedal robot that could step sideways using three of them per leg. I’m not going to link that, you’ll have to find it from his YouTube page because you should look around.
During the COVID period, some Chinese companies even sold variants of actuators inspired by the Mini Cheetah design.
Aaed Musa has also mentioned in some of his videos that his actuator designs were inspired by the Mini Cheetah actuator. Yes, His capstan drive video is especially impressive.
For example, in Aaed Musa’s video "I Built a Rubik's Cube Solving Robot" (https://www.youtube.com/watch?v=m0bMMALYMYk), he states in the description that the design was inspired by Ben Katz’s work.
Ben Katz master thesis, is worth reading: "A low cost modular actuator for dynamic robots" 2018 https://dspace.mit.edu/handle/1721.1/118671 and also has a good post https://robot-daycare.com/posts/2019-12-16-the-mini-cheetah-...
And also, The Rubik's Contraption (2018), 297 points, the work done by same author https://news.ycombinator.com/item?id=16561049
https://www.youtube.com/@ExcessiveOverkill/videos
Too much gear reduction, and you can't back-drive or sense forces from the motor end. Too little gear reduction, and your motors are too bulky or too weak. Reflected inertia goes up as the square of the gear ratio, as the article points out, because the gear ratio gets you both coming and going. So high gear ratios really hurt.
Robots, like drones, need custom motors sized for the specific requirements of the joint. For a long time, the robotics industry was too tiny to get such custom motors engineered, and had to use motors designed for other purposes. This will become a non-problem as volume increases. Especially since 3-phase servomotor controllers, which drones need, are now small and cheap. They used to be the size of a paperback book or larger.
(I've been out of this for years. I've used hydraulic robots and R/C servo powered robots. The newer machinery sucks a lot less.)
[1] https://news.ycombinator.com/item?id=47184744
The other side of the scaling laws say that motor torque scales as a square of air gap radius (roughly rotor radius), and output torque scales as linearly with gearing ratio.
When you balance these out, the reflected inertia depends on the inverse of power dissipated for a fixed output torque.
In an ideal world, your total reflected inertia is independent of the gearbox and largely depends on the motor fill factor and how hot you can run it.
Cooling in general is not a bad idea to allow you dissipate heat as you push motors to their saturation limits.