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Overview

The quadruped robot features a custom 12-DOF (Degrees of Freedom) design with four identical legs, each implementing a 3-DOF parallel SCARA mechanism based on a 5-bar linkage architecture.
The robot design is fully open-source and fabricated using OpenSCAD. All CAD files can be manufactured for real-world deployment, though the physical prototype has not been tested yet.

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Key Specifications

ParameterValueDescription
Total DOF123 DOF per leg × 4 legs
Leg ConfigurationParallel SCARATwo parallel 2-link arms with 5-bar constraint
Upper Link (L1)0.045 m (45 mm)Shoulder to elbow length
Lower Link (L2)0.06 m (60 mm)Elbow to foot length
Base Distance0.021 m (21 mm)Separation between parallel arms
Default Body Height0.07 m (70 mm)Nominal stance height

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Leg Architecture

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Parallel SCARA Mechanism

Each leg uses a parallel SCARA (Selective Compliance Assembly Robot Arm) design, which provides:
  • High rigidity in the vertical plane for supporting body weight
  • Compliance for terrain adaptation
  • Simplified kinematics with analytical IK solutions
  • Reduced computational cost compared to serial chains

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5-Bar Linkage Structure

The 5-bar linkage consists of:
  1. Fixed base with two parallel mounting points (21 mm apart)
  2. Two shoulder joints (actuated) - controlled independently
  3. Two upper links - connect shoulders to elbows (L1 = 45 mm each)
  4. Two lower links - connect elbows to foot (L2 = 60 mm each)
  5. Foot (end-effector) - shared connection point creating the closure constraint
This configuration creates a constrained parallel mechanism where the foot position uniquely determines the configuration of both arms.
The parallel SCARA design provides better load distribution than serial mechanisms, making it ideal for quadruped locomotion where legs must support dynamic loads during stance phase.

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DOF Breakdown

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Per-Leg DOF (3 total)

  1. Tilt Actuator: Controls lateral leg orientation (y-axis displacement)
  2. Shoulder Left: First parallel arm shoulder angle
  3. Shoulder Right: Second parallel arm shoulder angle
While each leg has 4 servo motors (1 tilt + 2 shoulders + elbow constraint), the parallel SCARA constraint means the elbow angles are kinematically coupled to the shoulder angles. The 5-bar closure constraint reduces independent DOF to 3:
  • Tilt: Independent rotation for lateral positioning
  • Shoulder angles: Two independent angles determine foot position in sagittal plane
  • Elbow angles: Constrained by 5-bar geometry (dependent variables)
This coupling is what enables analytical IK solutions and provides the mechanism’s rigidity.

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CAD Design (OpenSCAD)

The robot is designed entirely in OpenSCAD, a parametric 3D CAD modeling tool ideal for robotic mechanisms.

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Key Components

model/openscad/
├── shoulder_arm.scad          # Upper link (L1 = 45mm)
├── elbow_arm.scad             # Lower link (L2 = 60mm)
├── base_tilt.scad             # Tilt mechanism base
├── two_servo_base.scad        # Dual servo mounting
├── servo.scad                 # Servo motor housing
├── buje.scad                  # Bearing/bushing components
└── viz/                       # Assembly visualizations

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Shoulder Arm Design

From shoulder_arm.scad:4-6: 
module shoulder_arm(){
    height = 3.5;
    length = 45;  // L1 parameter
    $fn = 60;
    // ... link geometry
}

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Elbow Arm Design

From elbow_arm.scad:3-4: 
module elbow_arm(){
    height = 3.5;
    length = 60;  // L2 parameter
    $fn = 60;
    // ... link geometry with bearing mounts
}
All OpenSCAD components include mounting holes for bearings (6.12mm diameter) and servo connections, enabling direct fabrication and assembly.

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Working Modes

The parallel SCARA mechanism supports 4 distinct elbow configurations, providing different workspace regions:
ModeArm AArm BUse Case
1Elbow UpElbow DownExtended reach
2Elbow DownElbow UpDefault mode
3Both UpBoth UpHigh stance
4Both DownBoth DownLow stance
Mode 2 (A down, B up) is used as the default configuration throughout the codebase for optimal balance between workspace and singularity avoidance.

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Workspace Characteristics

The reachable workspace for each leg is determined by:
  • Radial limits: |L1 - L2| ≤ r ≤ L1 + L2 → 0.015 m ≤ r ≤ 0.105 m
  • Height range: Dependent on configuration mode and tilt angle
  • Lateral range: Controlled primarily by tilt actuator
For a 2-link planar arm, the reachable region is an annulus:
  • Minimum radius: |L1 - L2| = |0.045 - 0.06| = 0.015 m (15 mm)
  • Maximum radius: L1 + L2 = 0.045 + 0.06 = 0.105 m (105 mm)
The parallel configuration constrains this further, but maintains good coverage for typical gait step heights (15-20mm) and step lengths (40-50mm).

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MuJoCo Integration

The robot model is exported to MuJoCo XML format with:
  • STL mesh files generated from OpenSCAD
  • Joint definitions for all 12 actuators
  • Collision geometries for contact detection
  • Inertial properties estimated from geometry
Model files:
  • model/robot.xml - Complete robot definition
  • model/assets/ - STL meshes for all components
  • model/world.xml - Flat terrain environment
  • model/world_train.xml - Rough terrain with heightfield

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Design Advantages

✅ Analytical IK: Closed-form solutions enable fast computation ✅ High rigidity: Parallel structure supports dynamic locomotion ✅ Fabrication-friendly: All components designed for 3D printing and assembly ✅ Modular: Identical legs simplify manufacturing and maintenance ✅ Simulation-ready: Direct export to MuJoCo physics engine

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