Arms Specification

Overview

The robotic arm assembly provides manipulation capabilities essential for interaction with the environment. Each arm consists of three primary joints (shoulder, elbow, wrist) that work together to position the hand within the robot's workspace. The design prioritizes simplicity, use of salvaged components, and sufficient range of motion for common tasks.

Design Requirements

Shoulder Joint

The shoulder is the most complex joint in the arm, requiring multiple degrees of freedom for natural movement:

• Rotation (Yaw): 180° total range (±90° from neutral)
• Flexion/Extension (Pitch): 180° total range (0° to 180° forward)
• Abduction/Adduction (Roll): 120° total range (30° inward to 90° outward)

Elbow Joint

The elbow joint connects the upper arm and forearm, represented as a hinge joint:

• Flexion/Extension: 150° range (0° straight to 150° bent)
• Joint Type: Single-axis hinge
• Hyperextension Prevention: Mechanical stop at 0° prevents backward bending
• Load Capacity: Must support forearm, wrist, hand, and payload (minimum 2 kg)

Wrist Joint

The wrist provides fine positioning of the hand:

• Rotation (Pronation/Supination): 270° total range
• Flexion/Extension: 120° range (60° up, 60° down from neutral)
• Optional: Radial/Ulnar Deviation: 60° range (30° each direction)

Materials and Components

Parts List for Building the Arms

• Rectangular moulding (PVC pipe or wood) - Upper arm and forearm structure
• Hinges - 2-3 per arm for elbow joints
• Windshield wiper motor or high-torque servo - Shoulder actuator (salvaged)
• Plywood (6-12mm thickness) - Joint mounting plates
• Standoffs and spacers - M4 or M5, various lengths
• Rubber bands, surgical tubing, or bungee cord - Counterweight system
• Servo motors - MG996R or similar (15-20 kg-cm torque minimum)
• Ball bearings - 608 skateboard bearings work well (salvaged)
• Chain or timing belt - Power transmission (salvaged from printers)
• Gears or sprockets - Mechanical advantage for shoulder joint

Salvage Sources

• Windshield wiper motors: Junked cars, automotive salvage yards
• Servo motors: Broken RC cars, RC helicopters, old printers
• Hinges: Old furniture, cabinet doors, toolboxes
• Bearings: Skateboards, inline skates, fidget spinners
• Chains/belts: Printers, photocopiers, old bikes
• PVC pipe: Plumbing remnants, broken sprinkler systems

Shoulder Construction

Multi-Axis Design

The shoulder requires careful mechanical design to achieve multiple axes of rotation:

Yaw Axis (Rotation)

• Motor Mounting: Base-mounted servo with output shaft vertical
• Upper Arm Attachment: Servo horn connects to arm assembly
• Bearing Support: Large bearing at top to handle lateral loads

Pitch Axis (Forward/Back)

• Motor Position: Mounted perpendicular to yaw axis
• Drive Method: Direct drive or 2:1 gear reduction for higher torque
• Counterbalance: Elastic cords or springs to reduce motor load

Roll Axis (Up/Down)

• Implementation: Optional third servo for full 3-DOF movement
• Simplified Alternative: Omit for 2-DOF shoulder, reducing complexity

Torque Requirements

Shoulder motors experience the highest loads in the arm:

• Minimum Torque: 20 kg-cm for lightweight arms
• Recommended Torque: 40-60 kg-cm for reliable operation
• Gear Reduction: 2:1 or 3:1 reduction increases effective torque
• Counterbalance: Springs or elastic reduce required holding torque by 30-50%

Elbow Construction

Hinge Mechanism

The elbow is a simpler single-axis joint but must handle significant loads:

Construction Methods

Method 1: Hardware Store Hinge

• Standard door hinge provides strong, reliable pivot
• Mount servo alongside hinge with linkage to hinge arm
• Advantages: Simple, strong, uses common parts
• Disadvantages: Limited range (~120°), bulky appearance

Method 2: Direct Servo Mount

• Large servo becomes the joint, eliminating separate hinge
• Forearm mounts directly to servo horn
• Advantages: Compact, good range of motion (180°+)
• Disadvantages: Requires expensive high-torque servo, more complex mounting

Method 3: Cable/Pulley System

• Servo pulls cable that bends the elbow
• Similar to biological muscle-tendon system
• Advantages: Motor can be located away from joint
• Disadvantages: Cable stretch, complex routing, friction losses

Range Limiting

Prevent hyperextension and protect the servo:

• Mechanical Stops: Physical barriers at 0° and 150°
• Software Limits: Servo control code prevents extreme positions
• Compliance: Slight flexibility prevents damage from impacts

Dimensional Specifications

Electrical Interface

Servo Connections

• Per Arm: 3-5 servos depending on configuration
• Signal: PWM control (50Hz, 1-2ms pulse width)
• Power: Dedicated 6V supply, 2-3A per servo
• Control Board: PCA9685 16-channel PWM driver or similar

Position Feedback (Optional)

• Potentiometers: Analog position sensing
• Encoders: Digital position and velocity
• Current Sensing: Detect obstacles and resistance

Wiring Considerations

• Use flexible wire for moving joints (silicone insulated wire recommended)
• Strain relief at all connection points
• Color coding: Red (6V+), Black (GND), White/Yellow (Signal)
• Twist power pairs to reduce electromagnetic interference

Assembly Tips

Build Order

1. Construct and test elbow joint first (simplest component)
2. Build upper arm and forearm structures
3. Assemble shoulder mechanism (most complex)
4. Integrate all components and test range of motion
5. Add counterbalance system
6. Wire and test electrical systems
7. Attach to torso and calibrate

Common Pitfalls

• Insufficient Torque: Use gear reduction or stronger servos
• Binding Joints: Ensure proper alignment and adequate clearance
• Cable Snagging: Route cables through internal channels
• Overheating Servos: Add cooling or reduce continuous load
• Loose Connections: Use threadlocker on critical fasteners

Testing and Calibration

Range of Motion Test

• Verify each joint reaches specified angles without binding
• Check for smooth movement throughout full range
• Ensure mechanical stops prevent overextension

Load Testing

• Test with hand and expected payload attached
• Verify servos don't overheat during sustained holding
• Check for vibration or oscillation when stationary

Calibration

• Record servo PWM values at key positions (0°, 90°, 180°)
• Map servo commands to actual joint angles
• Account for mechanical slack and backlash
• Document neutral position for repeatability