Hardware

Harmonic Drive

A harmonic drive is a strain-wave gear consisting of an elliptical wave generator, a flexible toothed flexspline, and a rigid circular spline, achieving high reduction ratios (typically 50:1 to 160:1) in a compact, coaxial package with near-zero backlash. Its light weight and precision made it the dominant gearing in collaborative robot and humanoid joints, though it exhibits torsional compliance, friction, and limited backdrivability.

Why it matters for physical AI

Gearbox choice shapes what policies can do: harmonic drives give precise position control but poor torque transparency, pushing designs toward joint torque sensors or alternative transmissions for contact-rich learned skills.