Why almost every humanoid joint is a permanent-magnet BLDC servo, and why the magnets matter more than the motor.
Joint motors are almost all permanent-magnet, sensor-commutated brushless DC servos, often in outrunner geometry, with radial-flux designs dominating and coreless motors reserved for dexterous hands. The motor itself has become a commodity; what differentiates a joint is the transmission and the control stack around it.
Electric actuation won the architecture debate. Electric actuators run near 90% efficiency against 30–60% for hydraulics, with simpler wiring, better control and safer failure modes. Boston Dynamics' Atlas stayed hydraulic for 11 years before the fully electric redesign; Tesla's Optimus carries 28 actuators. Hydraulics and pneumatics survive mainly in hands, where Sanctuary AI works with miniaturized hydraulic valves and Clone Robotics runs pneumatic hands with custom hydraulics next.
The supply dependency has moved upstream to materials. Neodymium magnets tie every motor to a Chinese supply chain, which is why the US government has made direct investments and loans into MP Materials. Frames follow the same logic: aluminum for structure, steel for wear parts, titanium for joint carriers, composites for long limb segments, and magnesium, roughly 35% lighter than aluminum, where China produces 85–90% of global supply and is scaling thixomolding capacity.
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