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Comment by margalabargala | Hacker News Reader

The motor you linked, and what they are using, is not direct drive. That motor has a 1:6.33 integrated gearbox.

Too bad. It looks a lot like these real direct drive motors.[1] Those are in the same price range, and could get much cheaper if produced in quantity. The nice thing about direct drive motor robots is that you can't strip the teeth of a magnetic field. Impact forces are not a big problem. Robot gear trains are usually the weak point.

[1] https://shop.directdrive.com/collections/m15-series

The 8Nm/kg is impressive for the direct driver servo. It's still however a long way off from the 43.3Nm/kg gained by adding a gear box. Direct drive motors have other advantages such as better torque control and no backlash but out side of very novel motor design or making motor diameters impactical it's hard to beat the torque density of a gearbox. Maybe one day.

Watching those direct-drive robots jump and land smoothly is impressive. No problem with shock loads. They look like they're on springs, but there are no springs; it's all magnetic fields in the motor.

With good temperature monitoring and over-speced MOSFETS, those direct drive motors can probably be way overdriven for a few hundred milliseconds. That's probably the future. You don't need that extra power all that often, but sometimes you do.

Gear trains are the killer robots' weak point, eh?

Good to know.

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