Explainers
Electric vs hydraulic actuators: why humanoid robots moved to electric joints
Short answer: An actuator is the part that makes a robot joint move. Hydraulic actuators push pressurised fluid through cylinders and can deliver very high force for their size. Electric actuators use a motor, usually paired with a gearbox or screw, and are cleaner, quieter and easier to control precisely. Most humanoids being built for factories today use electric joints, and the most famous hydraulic humanoid was retired in 2024.
How hydraulics work. A pump pressurises fluid, valves route it, and pistons convert the pressure into motion. The big advantage is power density: a compact cylinder can produce enough force for jumps, flips and fast recoveries. The costs are a pump, hoses, seals and fluid that can leak, plus noise and heat. Every joint depends on a pressurised system that needs regular maintenance.
How electric actuators work. A motor spins quickly with modest torque, so designers add a reduction stage to trade speed for force. Common choices are planetary gearboxes, harmonic or strain wave drives, and roller screws for linear joints such as knees. Sensors on the motor and output let software control position, speed and torque closely. A joint built this way is sealed and quiet, and it runs from the same battery as the computer.
The Atlas switch. Boston Dynamics spent more than a decade showing what its hydraulic Atlas could do, including parkour and backflips. In April 2024 it announced the retirement of the hydraulic Atlas and unveiled a fully electric Atlas designed for real world applications, with Hyundai's manufacturing operations as the first testing ground. The company framed the change as part of moving from research to a commercial product.
Why electric won for factory work. Factory customers care about uptime, safety around people and cost per unit. Electric joints can be made as repeatable modules, which is why suppliers now sell humanoid joint modules as products. Schaeffler, for example, says its Taicang factory in China assembles joint modules, including actuators, for humanoid robots, as we reported in October. Hydraulic systems are harder to standardise this way.
Where hydraulics still make sense. Very heavy machines, construction equipment and some legged research platforms still use hydraulics because nothing else delivers as much force in a small space. The trade off is complexity, and for a robot meant to work beside people all day, most makers have decided that trade is not worth it.
For another explainer built from company figures, see what humanoid makers have published about runtime and recharge.