Glossary · Hardware
Actuator
In brief
An actuator converts energy (electrical, hydraulic, pneumatic) into mechanical motion. In a humanoid, every joint has at least one actuator. Modern bipedal humanoids overwhelmingly use electric actuators — typically a brushless motor plus some form of gearing.
The actuator is the single most cost-driving component in a humanoid robot. A 30-DOF humanoid has 30+ actuators; quality, torque density, back-drivability, and durability per unit set the platform's overall capability. Common types: harmonic-drive (high reduction, very stiff, expensive), planetary-roller-screw (high force, durable, used in Optimus), quasi-direct-drive (low gear ratio, naturally compliant, used in many recent designs).
Actuator choice cascades into control approach: high-gear-ratio rigid actuators force position-control thinking; low-gear-ratio back-drivable actuators enable torque control and compliance.
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Glossary entries are upstream. The catalog is where the implementations live.