ROS2 URDF — Write a Robot Description

Problem Statement

A robot's physical structure is described in a URDF (Unified Robot Description Format) file — an XML file that defines every rigid link and every joint connecting them.

In this problem, you will write a complete URDF for a simple 2-link robot arm by editing robot.urdf directly. solution.py is already written — it loads your URDF, publishes it, and validates it. Your job is to make the URDF correct.

Requirements

Your robot.urdf must define:

Two links:

• base_link — the fixed base of the robot. Use a <box size="0.5 0.5 0.1"/> geometry.
• arm_link — the moving arm segment. Use a <cylinder radius="0.05" length="0.4"/> geometry.

Each link must have a <visual> block containing a <geometry> element.

One joint:

• shoulder_joint — type revolute, connecting base_link (parent) to arm_link (child)
• Must include <axis xyz="0 0 1"/>
• Must include <limit lower="-1.57" upper="1.57" effort="10" velocity="1.0"/>

Input/Output Format

You edit: urdf/robot.urdf Do not edit: solution.py — it loads and validates your URDF automatically

Expected output when testScript runs:

[OK] URDF is well-formed XML
[OK] Robot name: simple_arm
[OK] Links found: ['base_link', 'arm_link']
[OK] All links have <visual> with <geometry>
[OK] Joints found: ['shoulder_joint']
[OK] TF tree has no loops
[PASSED] URDF is valid — 2 links, 1 joints, TF tree is clean

⚠️ Common Pitfalls

• Joint type attribute is case-sensitive — revolute not Revolute
• <parent link="..."/> and <child link="..."/> must exactly match your link names
• Revolute joints require both <axis> and <limit> — missing either will fail
• Every opening XML tag needs a matching closing tag

📚 Helpful Resources

• URDF XML spec
• URDF Tutorials