Control

Underactuation

Underactuation is the condition in which a mechanical system has fewer independent actuators than degrees of freedom, so some directions of motion cannot be commanded directly and the dynamics must be exploited rather than overridden. Walking robots in flight or single-support phases, quadrotors, the Acrobot and cart-pole, and objects manipulated through contact are all underactuated. Control relies on trajectory optimization, partial feedback linearization, and limit-cycle stability rather than exact tracking.

Why it matters for physical AI

Legged locomotion and dynamic manipulation are irreducibly underactuated, so both classical controllers and learned policies for these systems must reason about momentum and contact timing instead of simply servoing every degree of freedom.