The sensing choices that decide whether a learned policy transfers to the plant floor, and what they cost.
Magnetic absolute rotary encoders are emerging as the standard at the joint. The design decision with the biggest commercial consequence is whether to add a second, output-side encoder: it doubles sensing cost per joint but measures what the joint actually did after the reducer, which shrinks the gap between simulation and the real robot. Optical encoders from Heidenhain, Broadcom, Renishaw, Baumer, Dynapar, Omron and Kübler and magnetic parts from Heidenhain, Renishaw, SICK, Baumer, ZeroErr and Melexis are the named suppliers.
Bearings went from a design constraint to a commodity. Cross-roller bearings now handle radial, axial and moment loads in one part, which simplified joint design across the industry. Force and torque sensing at the joint, an IMU and a small number of cameras (Apollo uses two) complete a typical stack; tactile sensing sits mostly in the hands.
Batteries remain the unsolved constraint. Lithium-ion is standard and nickel-rich NMC cathodes are preferred, but no current chemistry delivers both the power density and the energy density a humanoid needs. Runtimes of about four hours on a swappable pack, as on Apollo, are representative.
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