An underactuated hand has more finger joints than motors — so its fingers passively conform around whatever they touch, grabbing many shapes securely with simple, cheap, robust hardware.
An underactuated hand has fewer motors than joints, so when it closes, the fingers automatically wrap and mold around the object's shape by themselves. Simple, cheap, and it grabs a huge range of objects without careful control.
A human hand has dozens of muscles for dozens of joints. Copying that in a robot is expensive and hard to control. The underactuated hand takes a clever shortcut: fewer motors than joints, and let physics do the rest.
The idea
An underactuated hand has more joint degrees of freedom than actuators. When it closes, a single motor drives a finger through tendons or linkages with springs at the joints. The finger's segments bend one after another as they meet resistance, so the finger automatically wraps and conforms to whatever it's touching — no per-joint control needed.
One motor, self-shaping fingers
Passive springs/linkages let the finger mold to the object automatically — one input produces a shape-adapted grasp.
Why it's so effective
Shape adaptability. One command grasps a ball, a pen, a bottle, or an odd part — the fingers find the shape themselves. This gives robust enveloping grasps that approach form closure.
Simple and cheap. Far fewer motors, sensors, and control complexity than a fully actuated dexterous hand — light, low-cost, and reliable.
Forgiving of uncertainty. Because the fingers conform, the hand tolerates imperfect perception and positioning — it doesn't need to know the object's exact shape to grip it well.
Robust to impact. Compliant joints absorb bumps instead of jamming.
The trade-off
Giving up independent joint control means giving up fine dexterity. An underactuated hand grasps beautifully but can't precisely position each fingertip or perform delicate in-hand manipulation — for that you need more actuators and full control. So underactuation is the sweet spot for robust general-purpose grabbing, not for fine finger gymnastics.
Where you'll see it
Prosthetic hands (secure everyday grasping on a budget), adaptive industrial grippers, humanoid hands where robustness matters more than fingertip precision, and research grippers designed to handle unknown objects gracefully.
Why it matters
The underactuated hand is a beautiful example of mechanical intelligence — letting the hardware's passive dynamics solve the grasping problem, so a simple, cheap hand grabs a wide variety of objects reliably. It's a dominant design philosophy for practical robot hands.
