Planetary Gearbox

A gearbox that uses a sun gear, planet gears, and a ring gear to turn a motor's fast spin into high torque.

A planetary gearbox is built from a central sun gear, several planet gears orbiting around it, an outer ring gear with internal teeth, and a carrier that connects the planet gears — named for its resemblance to planets orbiting a sun. A motor spins fast but produces little torque; the gearbox trades that speed for torque so the joint has enough force to move a leg or an arm. Its advantages are a compact structure, load sharing across multiple planet gears, and high efficiency per stage; a single stage also typically has a fairly low reduction ratio (often around 3:1 to 10:1), which makes it easy for an external force to push back through it — well suited to legged robots that need force control and impact tolerance. Its downside is that backlash (play between the gear teeth) is usually larger than in a harmonic drive, giving somewhat lower precision. A quasi-direct-drive joint is generally just a motor paired with a single-stage planetary gearbox.

ExampleMIT's Mini Cheetah uses a single-stage planetary gearbox at roughly a 6:1 ratio in its joints.

Also called

Planetary Gear Reducer, Planetary Gear Set

Related

Speed Reducer / Gearbox · Gear Ratio · Strain Wave Gear (Harmonic Drive) · Quasi-Direct Drive · Backlash · Joint Actuator Module

Sources

Epicyclic gearing - Wikipedia
Mini Cheetah: A Platform for Pushing the Limits of Dynamic Quadruped Control (ICRA 2019)