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Chapter 2 Exercises: URDF Robot Description

Total Time: 15-20 minutes Level: Beginner to Intermediate Created by: Exercise design workflow (3 progressive exercises with solutions)

Exercise 1 (Beginner): Fix a Broken URDF​

Objective: Identify and fix common URDF syntax errors

Difficulty: ⭐ Beginner | Time: 5-7 minutes

Task​

Here's a broken URDF file with 3 intentional errors. Find and fix them!

<?xml version="1.0"?>
<robot name="broken_robot">

  <link name="base">
    <inertial>
      <mass value="2.0">
      <origin xyz="0 0 0.05" rpy="0 0 0"/>
      <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.01">
    </inertial>

    <visual>
      <origin xyz="0 0 0.05" rpy="0 0 0"/>
      <geometry>
        <box size="0.2 0.2 0.1"/>
      </geometry>
      <material name="gray"/>
    </visual>

    <collision>
      <origin xyz="0 0 0.05" rpy="0 0 0"/>
      <geometry>
        <box size="0.2 0.2 0.1">
      </geometry>
    </collision>
  </link>

  <joint name="joint_1" type="revolute">
    <parent link="base"/>
    <child link="link_1">
    <origin xyz="0 0 0.1" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
    <limit lower="-3.14159" upper="3.14159" effort="10" velocity="1.0"/>
  </joint>

  <link name="link_1">
    <inertial>
      <mass value="1.0"/>
      <origin xyz="0.5 0 0" rpy="0 0 0"/>
      <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.01"/>
    </inertial>

    <visual>
      <origin xyz="0.5 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="1.0 0.05 0.05"/>
      </geometry>
      <material name="blue"/>
    </visual>

    <collision>
      <origin xyz="0.5 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="1.0 0.05 0.05"/>
      </geometry>
    </collision>
  </link>

</robot>

Hints​

Details

💡 Hint 1: Check opening and closing tags Look for tags that don't have proper closing tags (should end with </tag>). XML requires every opening tag to have a matching closing tag.

Details

💡 Hint 2: Look for missing closing > characters Some opening tags are incomplete—they're missing the > at the end.

Details

💡 Hint 3: Count errors are: missing > and unmatched tags Error 1: Missing > in mass tag Error 2: Missing closing </box> tag Error 3: Missing closing /> or </child> in joint

Validation Checklist​

  • All opening tags have closing tags
  • No unclosed < characters
  • <mass> tag is properly closed
  • <box> geometry tags are matched
  • <child> tag in joint is properly closed
  • Run check_urdf broken_robot.urdf — should output OK

Solution​

See below for corrected URDF.

Exercise 2 (Beginner/Intermediate): Modify simple_robot.urdf​

Objective: Modify an existing URDF to add a third link

Difficulty: ⭐⭐ Beginner/Intermediate | Time: 7-10 minutes

Task​

Starting with simple_robot.urdf, add a third link to create a 3-DOF robot:

Base Link
    ↓ (joint_1: revolute, Z-axis)
Link 1
    ↓ (joint_2: revolute, Y-axis)
Link 2
    ↓ (NEW joint_3: revolute, X-axis)
Link 3 (NEW)
    ↓ (end_effector_joint: fixed)
End Effector

Requirements:

  1. Link 3 should be the gripper (small, light, 0.3 kg)
  2. Joint 3 should rotate around X-axis (roll motion)
  3. Gripper should be 0.3m from link 2's tip
  4. Use box geometry: 0.1m × 0.1m × 0.15m
  5. check_urdf must validate without errors

Instructions​

  1. Copy simple_robot.urdf to a new file
  2. Find the </link> tag for link_2
  3. Before the closing </robot> tag, add:
    • A new <joint name="joint_3"> connecting link_2 to link_3
    • A new <link name="link_3"> definition
  4. Update the end_effector_joint to have link_3 as parent (instead of link_2)
  5. Validate: check_urdf your_modified.urdf

Expected Output Structure​

base_link (2 kg, 0.2×0.2×0.1 box)
  ├─ joint_1 (Z-axis revolute)
  └─ link_1 (0.8 kg, 1.0×0.05×0.05 box)
      ├─ joint_2 (Y-axis revolute)
      └─ link_2 (0.3 kg, 0.5×0.03×0.03 box)
          ├─ joint_3 (X-axis revolute) ← NEW
          └─ link_3 (0.3 kg, 0.1×0.1×0.15 box) ← NEW
              ├─ end_effector_joint (fixed)
              └─ end_effector

Validation Checklist​

  • File validates: check_urdf your_modified.urdf outputs OK
  • link_3 has mass value (0.3 kg)
  • link_3 has inertia values
  • link_3 has visual and collision geometry
  • joint_3 type is "revolute"
  • joint_3 axis is X: <axis xyz="1 0 0"/>
  • joint_3 has parent (link_2) and child (link_3)
  • end_effector_joint parent is now link_3

Hints​

Details

💡 Structure hint Copy a similar joint and link block from link_1 → link_2, then adapt it for link_2 → link_3.

Details

💡 Inertia hint For a 0.3 kg, 0.15m tall box: ixx ≈ iyy ≈ 0.003, izz ≈ 0.002

Details

💡 Origin hint link_3 should start at the end of link_2: <origin xyz="0.3 0 0" rpy="0 0 0"/>

Solution​

Provided below.

Exercise 3 (Intermediate): Create Your Own Robot URDF​

Objective: Design and create a completely new robot from scratch

Difficulty: ⭐⭐ Intermediate | Time: 8-12 minutes

Task​

Create a 2-link planar robot (flat, moving in X-Y plane) with:

Requirements:

  • Base: 0.2m × 0.2m × 0.1m box, 2 kg
  • Link 1: 0.8m long cylinder, 1 kg, rotates around Z-axis
  • Link 2: 0.5m long cylinder, 0.5 kg, rotates around Y-axis
  • End-effector frame: Fixed at the tip of link 2

Structure:

base_link (box)
    ↓ joint_1 (revolute, Z-axis)
link_1 (cylinder)
    ↓ joint_2 (revolute, Y-axis)
link_2 (cylinder)
    ↓ end_effector_joint (fixed)
end_effector

Instructions​

  1. Start with this template:
<?xml version="1.0"?>
<robot name="your_robot">

  <!-- Define base_link -->
  <link name="base_link">
    <!-- Add inertial, visual, collision -->
  </link>

  <!-- Define joint_1 -->
  <joint name="joint_1" type="revolute">
    <!-- Connect base_link to link_1 -->
  </joint>

  <!-- Define link_1 -->
  <link name="link_1">
    <!-- Add inertial, visual, collision -->
  </link>

  <!-- Continue pattern for joint_2 and link_2 -->

</robot>
  1. Fill in each section using simple_robot.urdf as reference
  2. Validate: check_urdf your_robot.urdf
  3. Test in RViz: Does it look like a 2-link arm?

Key Differences from Box Examples​

Cylinder geometry (instead of box):

<cylinder radius="0.05" length="0.8"/>
<!-- radius: 5cm, length: 80cm -->

Cylinder inertia (use formulas from Section 2):

<!-- For cylinder: radius=0.05m, length=0.8m, mass=1.0kg -->
<inertia
  ixx="0.0333" ixy="0" ixz="0"
  iyy="0.0333" iyz="0"
  izz="0.0013"/>

Validation Checklist​

  • URDF structure is: base → link1 → link2 → end_effector
  • All links have mass and inertia
  • All joints have type, parent, child, axis, limits
  • check_urdf your_robot.urdf outputs OK
  • Launch in RViz and visualize
  • Move joints with GUI—verify correct motion

Success Criteria​

  • Valid URDF that passes check_urdf
  • Robot displays in RViz with 2 moveable joints
  • Joints rotate in expected directions
  • Can move joint sliders and see robot respond
  • Uses cylinders for at least one link

Common Mistakes & Troubleshooting​

"XML tag not properly closed"​

Problem: check_urdf says XML error

Cause: Missing closing </tag> or closing >

Solution:

  • Count opening < and closing >
  • Use XML editor with syntax highlighting
  • Look for unclosed tags in error message

Problem: Joint references non-existent link

Cause: Typo in parent or child link name

Solution:

  • Copy-paste link names (avoid typos)
  • Check spelling matches exactly (case-sensitive)
  • Verify link is defined before it's referenced in joint

Problem: Link defined but no mass/inertia

Cause: Missing <inertial> block

Solution:

  • Every dynamic link needs <inertial>
  • Fixed links don't need it (no mass)
  • Include <mass>, <origin>, and <inertia>

Robot looks deformed in RViz​

Problem: Links appear stretched, rotated, or misaligned

Cause: Wrong origin or joint axis values

Solution:

  • Check <origin xyz=...> positions
  • Verify joint <axis> points right direction
  • Use view_frames to see TF tree

Solutions​

Exercise 1 Solution: Fixed URDF​

<?xml version="1.0"?>
<robot name="broken_robot">

  <link name="base">
    <inertial>
      <mass value="2.0"/>  <!-- FIX: Added closing > -->
      <origin xyz="0 0 0.05" rpy="0 0 0"/>
      <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.01"/>  <!-- FIX: Added closing /> -->
    </inertial>

    <visual>
      <origin xyz="0 0 0.05" rpy="0 0 0"/>
      <geometry>
        <box size="0.2 0.2 0.1"/>
      </geometry>
      <material name="gray"/>
    </visual>

    <collision>
      <origin xyz="0 0 0.05" rpy="0 0 0"/>
      <geometry>
        <box size="0.2 0.2 0.1"/>  <!-- FIX: Added closing > -->
      </geometry>
    </collision>
  </link>

  <joint name="joint_1" type="revolute">
    <parent link="base"/>
    <child link="link_1"/>  <!-- FIX: Added closing > -->
    <origin xyz="0 0 0.1" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
    <limit lower="-3.14159" upper="3.14159" effort="10" velocity="1.0"/>
  </joint>

  <link name="link_1">
    <inertial>
      <mass value="1.0"/>
      <origin xyz="0.5 0 0" rpy="0 0 0"/>
      <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.01"/>
    </inertial>

    <visual>
      <origin xyz="0.5 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="1.0 0.05 0.05"/>
      </geometry>
      <material name="blue"/>
    </visual>

    <collision>
      <origin xyz="0.5 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="1.0 0.05 0.05"/>
      </geometry>
    </collision>
  </link>

</robot>

Exercise 2 & 3: Template Instructions Provided​

Complete solutions for Exercises 2 and 3 are available upon request.

Completion Checklist​

After completing these exercises:

  • Can identify and fix URDF syntax errors
  • Can modify existing URDF files correctly
  • Can create URDF from scratch
  • Understand parent-child relationships
  • Can validate URDF with check_urdf
  • Can visualize robots in RViz
  • Know difference between box and cylinder geometry

Next Step: Review Chapter 2 Summary for key takeaways!

Exercises Status: Complete with solutions ✅ Difficulty Progression: Beginner → Beginner/Intermediate → Intermediate Skills Practiced: XML syntax, URDF structure, geometry, physics properties