Hands Specification

Joint Structure and Movement

The interphalangeal articulations of the hand are represented as hinge joints connecting the phalanges of each digit. The interphalangeal joints connect the proximal, intermediate and distal phalanges and allow for 90 degrees of flexion and extension. Each interphalangeal joint must prevent hyperextension of the phalanges.

The metacarpophalangeal articulations (knuckle joints) connecting each proximal phalanx to the metacarpal should permit movement in two planes, allowing flexion, extension, adduction, abduction, and circumduction.

Finger Length Proportions

Two mathematical approaches provide natural-looking finger proportions:

Symmetric Approach

• Metacarpal Lengths (Symmetry): 80, 89, 95, 89, 80 mm for pinky through thumb
• This creates a symmetric, aesthetically pleasing hand shape with the middle finger longest
• Good for hands that need to look proportional and balanced

Fibonacci Sequence Approach

• Phalanx Ratios (Fibonacci): 1, 1, 2, 3, 5... for distal through metacarpal
• The Fibonacci sequence (each number is sum of previous two) creates natural proportions
• Key Benefit: Allows the hand to make a proper fist with fingers curling naturally
• Example: If distal phalanx = 20mm, then intermediate = 20mm, proximal = 40mm, metacarpal = 60mm

Dimensions of Phalangeal Digits

The following table provides reference dimensions for a medium-sized robotic hand (total length ~18cm):

Note: These dimensions are starting points. Adjust proportionally based on your robot's size and available materials. The thumb is notably shorter as it doesn't align with other fingers but opposes them.

Joint Construction Methods

Method 1: Flexible Material (Simplest)

• Materials: Silicone tubing, rubber, soft plastic, fabric
• Construction: Rigid phalanx segments connected by flexible joint material
• Actuation: Cable/string pulled through center bends finger
• Advantages: Simple, no separate hinge, compliant grip
• Disadvantages: Can't extend beyond neutral, wears over time
• Best For: Quick prototypes, underactuated hands

Method 2: Pin Hinge

• Materials: Small nails, wire, or M2 screws as hinge pins
• Construction: Drill perpendicular holes through adjacent phalanges, insert pin
• Actuation: Cable or linkage pulls segments to bend
• Advantages: Proper hinge motion, can extend to neutral
• Disadvantages: Requires precision drilling, can bind if misaligned
• Best For: Fully actuated fingers with extension control

Method 3: Living Hinge

• Materials: 3D-printed TPU or thin sections in rigid plastic
• Construction: Entire finger printed as one piece with thin joint sections
• Actuation: Cable through finger or external linkage
• Advantages: No assembly required, no wearing pins
• Disadvantages: Requires 3D printer, TPU can be tricky to print
• Best For: 3D-printed hands, integrated designs

Method 4: Elastomer Joints (Advanced)

• Materials: Silicone rubber, urethane casting resin
• Construction: Cast flexible joints with embedded rigid segments
• Actuation: Tendons routed through channels in elastomer
• Advantages: Compliant, natural movement, self-centering
• Disadvantages: Requires mold making, complex fabrication
• Best For: Professional-quality hands, research projects

Thumb Mechanism

The thumb is the most important digit for manipulation, providing opposition to create pinch and grip patterns.

Thumb Requirements

• Opposition: Ability to touch tips of other fingers (especially index)
• Rotation: 90° rotation at base to bring thumb perpendicular to palm
• Flexion: Two joints (IP and MCP) for grasping
• Strength: Higher force capability than other fingers

Simplified Thumb Design

• Base Joint: Single servo provides rotation and some flexion
• Tip Joint: Coupled to base or separately actuated for pinch control
• Mounting Angle: 45-60° from palm plane for natural opposition
• Range: Should reach from side of index finger to pinky

Underactuated Hand (Recommended for Beginners)

Concept

One actuator controls multiple fingers through mechanical coupling. Fingers curl sequentially around objects, providing automatic adaptation to object shape.

Typical Configuration

• Actuators: 1-2 servos total
• Design: All four fingers connected by single cable/tendon
• Thumb: Separate servo for opposition
• Springs: Return fingers to open position

Advantages

• Very simple control (just open/close command)
• Self-adapting to object shapes
• Minimal actuators = low cost and weight
• Robust and reliable

Disadvantages

• Cannot perform complex gestures
• Limited precision in grip force
• All fingers move together

Construction Steps

1. Build four fingers with flexible joints
2. Route single cable through all four fingers
3. Attach cable to servo or winch mechanism
4. Add springs or elastic to return fingers to open
5. Build separately-actuated thumb
6. Test grip patterns with various objects

Fully Actuated Hand

Configuration Options

5-Actuator Hand

• One servo per finger (all segments coupled per finger)
• Allows independent finger control
• Can perform basic gestures (pointing, counting)
• Moderate complexity and cost

10+ Actuator Hand

• Multiple servos per finger for phalanx control
• Highly dexterous manipulation
• Complex control algorithms required
• Expensive and mechanically challenging

Actuation Methods

Direct Servo Drive

• Micro servos (SG90 size) mounted in palm
• Linkages connect servo to finger joints
• Simple mechanically but requires space
• Heavy and bulky

Cable/Tendon Drive

• Servos in forearm pull cables through fingers
• Biomimetic design (similar to human tendons)
• Lightweight hand but complex cable routing
• Friction and cable stretch are challenges

Linkage Drive

• Mechanical linkages transfer motion from palm to fingers
• Precise, minimal slack
• Complex geometry, difficult to design
• Can be heavy and space-inefficient

Sensing and Feedback

Position Sensing

• Servo Feedback: Some servos provide position feedback
• Flex Sensors: Resistive sensors measure finger bend angle
• Hall Effect: Magnetic sensors detect joint angles
• Optical Encoders: High precision but expensive

Force/Touch Sensing

• FSR (Force Sensitive Resistor): Simple pressure sensors on fingertips
• Capacitive: Detect proximity and touch
• Current Sensing: Monitor servo current for grip force
• Strain Gauges: Precise force measurement but complex

Tactile Feedback Benefits

• Prevents crushing delicate objects
• Enables force-controlled manipulation
• Provides data for machine learning
• Allows blind grasping (without visual feedback)

Beginner: Simple Gripper

• Design: Two "fingers" (pincer style) or three-finger underactuated
• Actuators: 1 servo
• Construction: Popsicle sticks, flexible tubing joints, fishing line
• Features: Basic open/close grip
• Build Time: 4-8 hours
• Cost: $10-20

Intermediate: Underactuated Hand

• Design: Four fingers + opposing thumb
• Actuators: 2 servos (one for fingers, one for thumb)
• Construction: 3D printed or plywood palm, flexible finger joints
• Features: Power grip and pinch grip, self-adapting
• Build Time: 20-40 hours
• Cost: $40-80

Advanced: Fully Articulated Hand

• Design: Five fingers with independent control
• Actuators: 5-10 servos depending on DOF per finger
• Construction: 3D printed components, tendon-driven from forearm
• Features: Individual finger control, force sensing, multiple grip patterns
• Build Time: 60-120 hours
• Cost: $150-400

Functional Tests

1. Range of Motion: Verify all joints reach specified angles
2. Grip Strength: Test with objects of various weights
3. Precision: Ability to grasp small objects (coin, pen)
4. Power: Ability to hold heavy objects (1-2 kg)
5. Durability: Repeated open/close cycles (100+)

Standard Test Objects

• Tennis ball (sphere, compressible)
• Water bottle (cylinder, smooth)
• Book (rectangular, rigid)
• Coin (small, flat, precise grip)
• Marker/pen (cylindrical, precision grip)
• Egg (fragile, force control)

Common Issues and Solutions