Chapter 7: Robot Description with URDF

Overview

What You'll Learn: URDF structure, building humanoid models, links/joints, visual/collision meshes, RViz visualization

Estimated Time: 7-9 hours

Core Concepts

URDF (Unified Robot Description Format)

XML format describing robot kinematics and dynamics:

<?xml version="1.0"?>
<robot name="simple_robot">
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.5 0.3 0.1"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.5 0.3 0.1"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10"/>
      <inertia ixx="0.1" ixy="0" ixz="0" iyy="0.1" iyz="0" izz="0.1"/>
    </inertial>
  </link>

  <link name="wheel_link">
    <visual>
      <geometry>
        <cylinder radius="0.1" length="0.05"/>
      </geometry>
    </visual>
  </link>

  <joint name="base_to_wheel" type="continuous">
    <parent link="base_link"/>
    <child link="wheel_link"/>
    <origin xyz="0.2 0 -0.05" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
  </joint>
</robot>

Joint Types

• fixed: No movement
• revolute: Rotation with limits
• continuous: Unlimited rotation
• prismatic: Linear sliding
• planar: 2D plane movement
• floating: 6 DOF (unconstrained)

Lab: Build Humanoid URDF

Create simple humanoid with torso, legs, arms:

• Base link (torso)
• Hip joints (revolute, 3 DOF)
• Knee joints (revolute, 1 DOF)
• Ankle joints (revolute, 2 DOF)
• Arms similar structure

Validate with: check_urdf my_robot.urdf Visualize in RViz: ros2 launch urdf_tutorial display.launch.py model:=my_robot.urdf

Summary

Key Takeaways:

1. URDF describes robot as tree of links and joints
2. Each link has visual, collision, and inertial properties
3. Joints define kinematic relationships and constraints
4. RViz and Gazebo use URDF for visualization and simulation

Next: Chapter 8: Control Systems