URDF Links, Joints, and the Robot Tree

Source: Robotics_Book URDF structure chapter

Why This Matters

Many URDF problems happen before collision geometry, inertias, or simulation plugins enter the picture. If the model does not form a clean tree of links and joints with one root robot element, the rest of the stack inherits a bad structural description.

Distilled Takeaways

• URDF is fundamentally a tree of links connected by joints under one robot root element.
• Links describe rigid bodies; joints describe how those bodies connect and move relative to each other.
• The structural tree matters because TF, visualization, and robot-state reasoning all build on it.
• Gazebo-specific additions are useful, but they sit beside the core robot-description model rather than replacing it.

Practical Value

• Start URDF debugging by checking tree structure and parent-child relationships first.
• Make sure every non-root link has exactly one parent joint relationship.
• Separate the core robot description from simulator-specific extras mentally, even when they live in nearby XML.
• Use this view when a robot model feels confusing before you get lost in visuals or inertial data.

Corroborating References

• URDF tutorials
• robot_state_publisher repository

When to Read the Original Source

Go to the original chapter when you want a plain-language walkthrough of the major URDF elements and the structural relationship between the robot, link, joint, material, transmission, and gazebo elements.