Harmonic Drive

A compact gear mechanism (also called strain-wave gear) that achieves very high reduction ratios (50:1–200:1) in a small package with near-zero backlash. Widely used in robot arm joints (UR, Franka, KUKA) for its compactness and precision. Disadvantages include limited backdrivability, nonlinear friction, and a characteristic flexibility that must be modeled in high-performance controllers.