Humanoid robot actuators explained

What is inside one

In almost every modern humanoid: an electric motor, a gearbox, an encoder measuring the output angle, and a small controller — packaged as one sealed unit that bolts into a joint. Hydraulics have largely gone, taking their leaks and compressors with them.

That packaging is why the actuator is the right unit of analysis. It is what gets designed, costed, manufactured, sourced and replaced. We have argued that it is now the physical bottleneck on the whole field: AI capability is moving faster than the hardware that has to execute it.

Rotary versus linear

The first architectural fork. A rotary actuator turns; a linear one pushes and pulls along an axis, typically via a ball screw or roller screw, and is usually paired with a linkage at the knee or hip where the geometry suits a piston.

The debate is unusually public. Figure's CEO publicly refuted a report that the company used planetary roller screws, stating Figure relies on rotary actuators — a correction nobody would bother making if the choice were cosmetic. Engineers have worked through the trade-offs at length: linear units can deliver very high force through a joint's mid-range and package awkwardly; rotary units are simpler to control and reuse across a machine.

How much gearing, and what it costs you

More gear ratio means more torque from a smaller motor, and worse behaviour on contact.

High-ratio geared drives — harmonic (strain wave), cycloidal, planetary — are precise and strong. They are also heavy, expensive and very hard to push backwards.

Quasi-direct drive uses a low ratio, typically under 10:1, keeping most of the responsiveness while getting usable torque. Standard in legged robots for a reason.

Direct-drive removes the gearbox entirely: the smoothest and most force-transparent option, at the price of a large, heavy motor.

The property underneath all of this is backdrivability — whether you can push the limb and the motor turns. A backdrivable joint yields when it hits something unexpected. A heavily geared one resists and keeps pushing, which is a safety question before it is a performance one, and one reason functional safety certification and actuator choice are the same conversation.

The supply chain is the story

Actuators are where humanoid ambitions meet manufacturing reality. Schaeffler has moved in from the industrial side, revealing an all-in-one integrated planetary gear actuator aimed squarely at the supply-chain bottleneck and then claiming a cost breakthrough by forming strain wave gearboxes rather than machining them, targeting mass production in 2027.

That second one matters more than it sounds. Strain wave gears are precise, expensive and slow to machine; a humanoid needs many. Anything that changes their unit cost changes the bill of materials for every machine in the industry.

At the other end, vertical integration: Boston Dynamics has detailed how custom actuators, passive cooling and superhuman range of motion drive the production Atlas design.

Counting them

Actuator count is the honest version of a spec sheet, because each one is cost, mass, wiring, heat and a thing that can fail. It is also where quoted numbers go wrong. Elon Musk said the next Optimus hand would carry 50 actuators, against 17 in the prior prototype — then clarified he had meant 50 across both sides, 25 in each forearm and hand. The headline figure halved on a clarification, which is the whole problem with counting actuators from press remarks.

Where they go is a design signature. The industry has largely converged on a particular hip configuration — flexion-abduction-rotation rather than the older ordering, because the joint ordering changes what the leg can do at the edge of its range. When two robots quote the same DoF count, that is where the difference is hiding.