Hydraulic actuator

A hydraulic actuator uses pressurized fluid (typically oil) to produce mechanical motion. Dominant architecture in heavy industrial robotics, construction equipment, and historically in Boston Dynamics' original Atlas. Distinct from electric actuators (brushless DC motors with harmonic-drive or planetary reduction) and from pneumatic actuators.

The distinction matters because Boston Dynamics' Atlas was the last major humanoid program using hydraulic actuation. The hydraulic Atlas was retired in 2024 and replaced by an all-electric platform. The transition is structural: hydraulics deliver higher peak force per kg but require pumps, reservoirs, hoses, and significant maintenance overhead. The all-electric humanoid wave (Figure, Apollo, Digit, Optimus, NEO) reflects the engineering judgment that hydraulics no longer earn their weight at humanoid scale — and the safety / backdrivability case is much harder with hydraulic stiffness.