URDF2MJCF Skill

Trit: -1 (MINUS - analysis/transformation) Color: #4615B7 (Deep Violet) URI: skill://urdf2mjcf#4615B7

Overview

Convert URDF (Unified Robot Description Format) files to MJCF (MuJoCo XML Format) for simulation in MuJoCo and MJX. Handles meshes, joints, actuators, and collision geometries.

Usage

# CLI conversion
urdf2mjcf robot.urdf --output robot.mjcf

# With options
urdf2mjcf robot.urdf \
    --output robot.mjcf \
    --mesh-dir ./meshes \
    --add-actuators \
    --collision-margin 0.001

from urdf2mjcf import convert

# Programmatic conversion
mjcf_xml = convert(
    urdf_path="robot.urdf",
    mesh_dir="./meshes",
    add_actuators=True,
    collision_margin=0.001,
)

# Save to file
with open("robot.mjcf", "w") as f:
    f.write(mjcf_xml)

Features

• Mesh Handling: Converts STL/OBJ meshes, scales appropriately
• Joint Mapping: URDF revolute/prismatic → MJCF hinge/slide
• Actuator Generation: Auto-generates position/velocity actuators
• Collision Geometry: Convex decomposition, margin adjustment
• Inertia Cleanup: Fixes common URDF inertia issues

Pipeline

┌─────────────┐    ┌─────────────┐    ┌─────────────┐
│  URDF File  │───▶│  urdf2mjcf  │───▶│  MJCF File  │
│  (ROS/SDF)  │    │  Converter  │    │  (MuJoCo)   │
└─────────────┘    └─────────────┘    └─────────────┘
       │                  │                  │
       ▼                  ▼                  ▼
   Meshes STL        Link transforms     Physics props
   Joint limits      Mass/inertia        Actuator defs
   Visual geom       Collision geom      Sensor defs

GF(3) Triads

urdf2mjcf (-1) ⊗ kos-firmware (+1) ⊗ mujoco-scenes (0) = 0 ✓

Related Skills

• ksim-rl (-1): Uses converted MJCF for training
• kos-firmware (+1): Robot firmware
• mujoco-scenes (0): Scene composition
• kbot-humanoid (-1): K-Bot robot model

References

@misc{urdf2mjcf2024,
  title={URDF to MJCF Converter},
  author={K-Scale Labs},
  year={2024},
  url={https://github.com/kscalelabs/urdf2mjcf}
}

SDF Interleaving

This skill connects to Software Design for Flexibility (Hanson & Sussman, 2021):

Primary Chapter: 7. Propagators

Concepts: propagator, cell, constraint, bidirectional, TMS

GF(3) Balanced Triad

urdf2mjcf (+) + SDF.Ch7 (○) + [balancer] (−) = 0

Skill Trit: 1 (PLUS - generation)

Connection Pattern

Propagators flow constraints bidirectionally. This skill propagates information.