What is MJCF?
MJCF (MuJoCo XML) is the native scene and robot description format for the MuJoCo physics engine — widely used in reinforcement learning research.
Where URDF describes a robot's kinematic structure, MJCF describes a full simulation scene: bodies, joints, actuators, sensors, contact pairs and solver options, all in one XML tree.
MJCF vs URDF
| Aspect | URDF | MJCF |
|---|---|---|
| Native engine | ROS ecosystem (Gazebo, etc.) | MuJoCo |
| Actuators | Minimal (effort/velocity limits) | Rich actuator models (motor, position, velocity, tendon) |
| Contact/solver tuning | Not expressed | Explicit per-geom and global solver parameters |
| Kinematic loops | Tree only | Supports closed loops via equality constraints |
Getting from CAD to MJCF
MuJoCo can import a URDF directly, so the common path is CAD → URDF → MJCF, tuning actuators and contacts in MJCF afterward. Jointly exports MJCF alongside URDF in the same conversion, so you don't need a separate import/convert step. See CAD to MuJoCo and Isaac Sim for the full pipeline.
Skip the manual work
Jointly does everything on this page automatically: drop in your CAD (STEP, mesh, SolidWorks or Onshape), and it infers joints, axes, inertia and collision, then exports a simulation-ready URDF, SDF, MJCF or USD. The first conversions are free.
Try Jointly free →