What is planetary joint module?
A planetary joint module is a rotary actuator whose reduction comes from one or more planetary gear stages. It trades a little backlash and length for shock tolerance, efficiency and cost — the usual choice for legs, ankles and wheeled bases.
A planetary joint module is the same idea as a harmonic module — motor, gearbox, encoder and often a brake in one bolt-in housing — but the reduction comes from planet gears running between a sun and a ring gear. Each stage gives roughly 3:1 to 10:1, so higher ratios are built by stacking stages, which makes the module longer and slightly less efficient than a single harmonic stage.
What you gain is tolerance. Load is shared across several planet gears at once, so impact and sustained duty are handled better, and the gear set is cheaper to make. That is why planetary joints dominate the axes that hit the ground: knees, ankles, hips and wheeled bases, plus the waist on robots that carry load through it.
Where planetary gearing is the better answer
- Impact and shock loads — landing, stepping onto an unexpected edge, or a stop under load.
- Sustained duty near the thermal limit — higher efficiency means less heat to reject.
- Cost-sensitive axes — several axes of the same robot rarely all need harmonic precision.
- Backdrivability needs — some planetary sets are easier to drive backwards, which matters for compliant control and for teaching by hand.
Compare both types on one torque scale in the cross-brand torque reference, or start from planetary joint modules.
FAQ
How much backlash is acceptable on a planetary joint?
It depends on the axis. A wrist or a hand axis notices every arcminute; a knee that cycles under gravity does not. Ask for the figure measured at the output, under load, and bidirectional — a unidirectional number hides the backlash.
Can I mix planetary and harmonic joints on one robot?
Yes, and most humanoids do. Harmonic on the axes that place the end effector, planetary on the axes that take impact. The practical constraint is keeping one control protocol and one mounting envelope across the build.