# Mechanical Humanoid Robot Hand

How a tendon\-driven humanoid hand moves its 17 joints from a forearm actuator module, with the thumb opposing the fingers in a pinch\.

By chocopoule

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/JIJnCZ2oUE8l)

## Transcript

### 1. Tendon\-driven hand, remote actuators

This representative anthropomorphic hand is 200 mm long and carries no motors in its fingers\. Seventeen independent DOF are driven by servos in a forearm module through braided tendons\. The distal hand weighs about 220 g; the motor mass stays in the module\.

Values from an antagonistic tendon\-driven hand \(arXiv 2609\.36241\)\. Drag empty space to turn the view\.

### 2. Four joints, three actuated DOF per finger

Each finger has a two\-axis MCP \(ab/ad ±35°, flexion ±90°\), a PIP \(±90°\) and a DIP \(±60°\)\. The DIP is not independently driven; it is coupled to the PIP\. Select a DOF to drive it on all four fingers\.

Thumb: CMC flexion ±135° and ab/ad ±90° on two perpendicular axes, MCP ±90°, IP ±90°\. Wrist ±60°\.

### 3. Actuator module in the forearm

Seventeen Robotis XL330\-M288\-T servos sit in a 98\.4 × 93\.8 × 155 mm module\. Each output carries a 3 mm\-radius spool with a 1\.6 mm\-pitch helical groove\. Toggle the housing to inspect the front drive layer\.

Removing motor mass from the hand reduces distal inertia\. The Shadow Dexterous Hand uses the same layout with 20 DC motors in its forearm\.

### 4. Antagonistic tendon pair

A tendon can only pull, so each joint is wrapped by two tendons in opposite directions on one spool: winding the flexor \(amber\) pays out the extensor \(teal\)\. Both directions are actively driven, with no return spring\. Tendons are 0\.6 mm 8\-strand Dyneema, 42 kgf breaking load\.

Joint angle = tendon displacement / pulley radius\. Pre\-stretched tendons give 0\.15 mm flexion repeatability, against 0\.58 mm unstretched\.

### 5. Routing through joint axes

MCP flexion tendons pass through the ab/ad axis, and PIP tendons pass through both MCP axes\. A tendon routed through an axis keeps its length when that joint moves, so the joints stay kinematically decoupled\. Here the MCP flexes and abducts while the PIP tendon marks stay still\.

Spring \(Bowden\) tubes carry every tendon past the wrist axis to the module, limiting friction and length variation\.

### 6. PIP–DIP coupling at 3:2

A coupling cable runs from a 4 mm pulley fixed to the proximal phalanx to a 6 mm pulley on the distal phalanx\. The DIP therefore follows at two thirds of the PIP angle, curling the fingertip without a dedicated actuator\. Set the PIP angle with the slider\.

The Shadow Dexterous Hand couples its distal joint the same way \(middle angle ≥ distal angle\)\.

### 7. Thumb opposition and precision pinch

The thumb CMC combines flexion and ab/ad on two perpendicular axes, swinging the thumb across the palm to oppose the fingers\. With motor position feedback at 200 Hz the hand reaches about 29 N at the fingertip; daily\-living tasks need under 15 N\. Run a tip\-to\-tip pinch\.

Ab/ad repeatability is 0\.02 mm with pre\-stretched tendons\. The thumb CMC pair is crossed before it enters the palm\.