Underactuated Hands: Fewer Motors, Surprising Dexterity

You don't need one motor per joint to get a hand that grasps a wide range of objects well.

An underactuated hand has fewer motors than degrees of freedom — a single motor might drive several finger joints through a mechanical linkage (often tendons or a differential mechanism) rather than each joint getting its own dedicated actuator.

Why this works

The mechanical linkage is designed so the fingers passively conform to whatever object they contact, automatically distributing contact across the object's actual shape without needing a controller to explicitly calculate that shape first — a bit like how your fingers naturally wrap around a doorknob without consciously planning each joint angle.

The tradeoff

What you gain in mechanical simplicity and grasp robustness, you lose in fine control — an underactuated hand generally can't perform precise, independent finger movements the way a fully actuated hand can, making it well suited to general grasping but less suited to tasks needing individual finger dexterity, like typing or manipulating small tools.