Actuators convert electrical (or hydraulic/pneumatic) energy into mechanical motion — the core hardware of robot joints. Humanoid robots have approximately 20–30 joints with 1–2 actuators per joint; actuator performance (torque density, power density, backdrivability, response speed) directly determines robot movement speed, payload capacity, and safe interaction characteristics.
## Actuator Technology Routes
**Hydraulic**: high torque density, suited for high-load heavy tasks. Boston Dynamics’ early Atlas used hydraulic drive with impressive running and jumping. Disadvantages: complex systems, seal wear, noise, hydraulic oil maintenance, difficult precise force control.
**Pneumatic**: flexible and safe for human-robot collaboration scenarios. Difficult precision control; mainly used for gripper end-effectors, not suited for joint primary drives.
**Electric drive (mainstream)**: simple structure, high control precision — current humanoid robot mainstream. Two types: position-controlled motors (traditional industrial robot standard — high precision but rigid, easily damaging personnel or equipment in unexpected collisions); torque/force-controlled motors (implementing “compliant control” — robots sense and control output torque, actively adapting to the environment like human muscles, with better safety and adaptability — humanoid robot mainstream).
**Key parameters**: torque density (Nm/kg — torque output per kilogram of weight); power density (W/kg); backdrivability (natural compliance when external force is applied, affecting collision safety); bandwidth (response speed).
## Domestic Supply Chain Competition
Core bottleneck: high-performance force control motors have long depended on US/European/Japanese suppliers (Maxon, Harmonic Drive, Nabtesco, etc.). As domestic demand explodes, Unitree Robotics (proprietary motors), Maijie Electric, Innfos, and Han’s Laser subsidiaries are continuously advancing in core motors and harmonic reducers, with some specifications approaching international advanced levels.
See [Humanoid Robot Overview](https://sunqi.org/humanoid-robot-overview-en/).
