Electric motor scaling laws and inertia in robot actuators

Aaed Musa blew my mind about 18 months ago with his capstan drive video:

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.

	▲	keeda 12 minutes ago \| parent [-]
		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.

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Animats an hour ago | parent | prev | next [-]

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.)

[1] https://news.ycombinator.com/item?id=47184744

	▲	blueblisters 37 minutes ago \| parent [-]
		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.

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pfdietz 2 hours ago | parent | prev | next [-]

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.

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blueblisters an hour ago | parent | next [-]

You would hit electrical steel saturation limits way before you need to pump in enough current to justify super-conductance.

Cooling in general is not a bad idea to allow you dissipate heat as you push motors to their saturation limits.

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Robotbeat 2 hours ago | parent | prev [-]

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.

	▲	hinkley an hour ago \| parent [-]
		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.

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brcmthrowaway an hour ago | parent | prev [-]

The real innovation will be in soft robotics and compliant mechanisms. You read it here first.