Hydraulic actuator

A car jack works because a small force applied to a narrow piston gets transmitted through oil to a much wider piston, multiplying the force enormously. With one hand you raise a tonne of car. That force-multiplication trick — using trapped liquid to transmit and amplify mechanical work — is the foundation of every hydraulic system ever built.

A hydraulic actuator is a device that converts pressurised fluid (almost always oil) into mechanical movement. A pump driven by an electric motor or combustion engine pressurises the oil. Control valves direct that pressurised oil into a cylinder or a rotating motor, which moves the robot's joint or limb. The force produced is proportional to pressure multiplied by piston area — and because oil is nearly incompressible, every push at one end appears almost instantly at the other.

Power density: why hydraulics win on brute force

The key advantage is power density — how much force or torque a system produces per kilogram of hardware. An electric motor producing 10,000 N of linear force would be enormous. A hydraulic cylinder producing the same force can be the size of a forearm. This is why excavator arms, aircraft flight control surfaces, and heavy industrial presses are hydraulic rather than electric — there is simply no electric alternative of comparable weight for the forces involved.

Hydraulic actuators also absorb shock loads well. When a robot leg lands hard on uneven ground, the oil acts as a buffer, spreading the impact load across the system. This is much harder to achieve with electric motors.

The disadvantages are significant. Hydraulic systems need a pump, a reservoir, pipes, seals, and control valves — all of which can leak. Oil spills are messy and environmentally problematic. Maintenance is demanding. Efficiency is lower than a direct-drive electric motor because energy is lost to heat in the valves and lines.

Real-world example

The original Atlas robot, built by Boston Dynamics, was hydraulic — a deliberate choice made because no electric actuator in 2013 could match the power density needed for a full-size humanoid to jump, run, and recover from shoves. Engineers described the robot as "a very athletic hydraulic system." In 2024, Boston Dynamics released a fully electric version of Atlas, arguing that electric actuator technology had finally caught up. The original hydraulic Atlas is now retired, but it remains the benchmark for what hydraulic robotics achieved.

Why it matters

Hydraulics defined industrial automation for decades and still dominate wherever mass and force are paramount — construction, mining, aerospace, and heavy manufacturing. Understanding hydraulic actuators explains why early humanoid robots were so bulky, why the shift to electric has taken so long, and what engineering trade-offs underpin every powerful robot ever built.

The transition from hydraulic to electric actuators in humanoid robots mirrors, almost exactly, the earlier shift from hydraulic to electric power steering in cars — each time, the question was whether electronics could finally replace fluid power.