Actuator

The motor-and-gearbox unit that turns electrical power into controlled motion at a robot's joint.

• A fast, weak electric motor is paired with a gearbox that trades speed for force, often 50–100× reduction, so a small motor can lift a limb.
• Heavy gearing adds friction and stiffness, making the joint hard to back-drive when bumped; that's a safety and control problem around people.
• Sustained force is limited by heat: a motor can briefly give several times its rated torque, but long heavy lifting overheats its windings.
• Actuators are a humanoid's biggest cost and weight item, with roughly 20–40 per robot, so cheaper, stronger designs drive the whole industry's economics.