Robot export

URDF, MJCF and SDF straight from your CAD model

Model the robot as parts and joints, the way you would anyway. ProtoJoint writes the robot description from that: links from the parts, joints from the assembly joints, inertia from the exact solids. Drag the joints to check it, then export URDF, MJCF, SDF and a ROS 2 package that opens in RViz.

Open the editor How robot export works

A six-axis desk arm: a base clamped to the desk, six servos on their axes, the upper arm, the forearm, the wrist and a two-finger gripper. — Set each joint on its slider, or drag the picture to turn it.

ANothing drawn twice

How the assembly becomes a robot description

The export reads the assembly you already built: its parts, their placement, the joints between them and the material of each part. Open it with URDF in the editor’s header, or with Export Robot in a part’s right-click menu.

What each export writes for each thing in the model
In ProtoJoint	URDF	MJCF	SDF
A part (component)	`<link>`	`<body>`	`<link>`
Rigid joint	`fixed`	the child body, nested with no joint	`fixed`
Revolute joint with limits	`revolute`	`hinge` with a range	`revolute`
Revolute joint without limits	`continuous`	`hinge`	`continuous`
Slider joint	`prismatic`	`slide`	`prismatic`
Ball joint	`fixed`, with a warning	`ball`	`ball`
Cylindrical, pin-slot, planar joint	`fixed`, with a warning	`slide` and `hinge` joints in one body	`fixed`, with a warning
Motion link (gears, rack, screw)	`<mimic>`	`<equality>`	none: SDF 1.9 has no mimic (a warning)
Mass, centre of mass, inertia	`<inertial>`	`<inertial>`	`<inertial>`

The joints form a tree: one root link, one parent for every other link.

The root link is the part you pinned to the world, or one you pick in the export panel. A part nested in another and moved by no joint of its own rides in its parent’s link. A joint a format cannot express is written as fixed where it stands, with a warning that says what to use instead.

BPhysics you can trust

Inertia that is right, and checked

DETAIL A

From the part, not a box around it

Each link’s mass, centre of mass and inertia come from its exact shape and its part’s material: steel, aluminium 6061, PLA, nylon and more, or a density you type in.

DETAIL B

Checked before the file is written

The joints must form a tree, every link needs an inertia a real object could have, and a limit needs both its ends. A part with no material is written with a warning that says so.

DETAIL C

Moved before you trust it

Select a joint and drag its handle, or type a value. Limits stop it where the real joint stops, and motion links turn geared joints together and are exported as mimic joints.

DETAIL D

A ROS 2 package, not only a file

One zip named after the package, with each link’s meshes and a README. The ROS 2 package opens in RViz with one launch command, and the MuJoCo and Gazebo files come with a scene to drop the robot into.

CWho it is for

Built next to your terminal

ROS 2 developers

Change a bracket, export, rebuild.

ROS 2’s Tier 1 platforms are Ubuntu 24.04 and Windows 10¹. ProtoJoint runs in the browser you already have open next to your terminal.

LeRobot and low-cost arms

Your copy of the arm, your files.

The SO-101’s official robot files were generated from a model in another program. In ProtoJoint you change a part, a gripper finger say, and write URDF and MJCF again from your own copy of the model.

→Your robot

Bring an arm or a gripper.

Import its assembly, a .sldasm or a STEP file split into one component per link. Add the joints, pin the base and export.

Open the editor Tell us about your robot

Sources

Open Robotics, REP 2000: ROS 2 releases and target platforms, accessed October 20, 2026.