Building a mobile robot URDF in ROS - Mini Project: Build & Apply

Building a mobile robot URDF

📖 Scenario: You are creating a simple mobile robot model using URDF (Unified Robot Description Format) for ROS. This robot has a base and two wheels. You will build the URDF step-by-step to describe the robot's parts and their connections.

🎯 Goal: Build a basic URDF file describing a mobile robot with a base link and two wheel links connected by joints.

📋 What You'll Learn

Create the base link element with a box geometry

Define a parameter for wheel radius

Add two wheel links with cylinder geometry using the wheel radius parameter

Add joints connecting the wheels to the base link

💡 Why This Matters

🌍 Real World

Robots need URDF files to describe their physical shape and joints so simulators and controllers can understand their structure.

💼 Career

Robotics engineers use URDF to model robots for simulation, visualization, and control in ROS-based projects.

1

Create the base link element

Create a <link> element named base_link with a <visual> child containing a <geometry> element. Inside <geometry>, add a <box> with size 0.5 0.3 0.1.

ROS

Hint

Use the <link> tag with the attribute name="base_link". Inside it, add <visual> and then <geometry> with a <box> element specifying the size.

2

Define a wheel radius parameter

Add a <xacro:property> named wheel_radius with value 0.1 inside the <robot> element, before the <link> elements.

ROS

Hint

Use <xacro:property> to define a variable named wheel_radius with value 0.1. Remember to add the xmlns:xacro namespace to the <robot> tag.

3

Add two wheel links using the wheel radius

Add two <link> elements named left_wheel and right_wheel. Each should have a <visual> with a <geometry> containing a <cylinder> with radius="${wheel_radius}" and length="0.05".

ROS

Hint

Use two <link> tags named left_wheel and right_wheel. Inside each, add <visual> and <geometry> with a <cylinder> using the ${wheel_radius} property for radius and length 0.05.

4

Add joints connecting wheels to the base

Add two <joint> elements named left_wheel_joint and right_wheel_joint. Both should be of type continuous. Set parent to base_link and child to the respective wheel link. Position left_wheel_joint at 0 0.15 0 and right_wheel_joint at 0 -0.15 0 using <origin>.

ROS

Hint

Use two <joint> elements with names left_wheel_joint and right_wheel_joint. Set type to continuous. Use <parent> and <child> tags to connect links. Use <origin> to position joints.

Practice

1. What does a URDF file primarily describe in ROS for a mobile robot?

easy

A. The robot's parts (links) and how they connect (joints)

B. The robot's sensor data processing algorithms

C. The robot's network communication protocols

D. The robot's battery charging schedule

Solution

1. Step 1: Understand URDF purpose

   A URDF (Unified Robot Description Format) file describes the physical structure of a robot, including its parts and connections.

2. Step 2: Identify mobile robot components

   Mobile robots have links (like base and wheels) connected by joints, which URDF models.

3. Final Answer:

   The robot's parts (links) and how they connect (joints) -> Option A

4. Quick Check:

   URDF = robot parts and joints [OK]

Hint: URDF = robot structure, not software or data [OK]

Common Mistakes:

• Confusing URDF with software algorithms
• Thinking URDF handles communication
• Assuming URDF manages battery or sensors

2. Which of the following is the correct syntax to define a link named base_link in a URDF file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recognize URDF link syntax

   In URDF, links are defined with the <link> tag and a name attribute.

2. Step 2: Check option correctness

   <link name="base_link"/> uses <link name="base_link"/>, which is correct syntax. Other options use wrong tags or attributes.

3. Final Answer:

   <link name="base_link"/> -> Option A

4. Quick Check:

   Link tag uses name attribute [OK]

Hint: Links use syntax in URDF [OK]

Common Mistakes:

• Using <joint> instead of <link> for links
• Using id instead of name attribute
• Using non-existent tags like <base>

3. Given this URDF snippet for a wheel joint:

<joint name="wheel_joint" type="continuous">
  <parent link="base_link"/>
  <child link="wheel_link"/>
</joint>

What does the type="continuous" mean for this joint?

medium

A. The joint rotates but only up to 90 degrees

B. The joint is fixed and cannot move

C. The joint moves only in a straight line

D. The joint can rotate infinitely without limits

Solution

1. Step 1: Understand joint types in URDF

   URDF joint types include fixed, revolute, continuous, prismatic, etc. Continuous means unlimited rotation.

2. Step 2: Interpret continuous joint meaning

   Continuous joints rotate endlessly, suitable for wheels that spin freely.

3. Final Answer:

   The joint can rotate infinitely without limits -> Option D

4. Quick Check:

   continuous joint = infinite rotation [OK]

Hint: Continuous joint means unlimited rotation [OK]

Common Mistakes:

• Confusing continuous with fixed joint
• Thinking continuous means linear movement
• Assuming rotation limits apply

4. You wrote this URDF joint definition but your robot's wheel does not move:

<joint name="wheel_joint" type="revolute">
  <parent link="base_link"/>
  <child link="wheel_link"/>
</joint>

What is the likely problem?

medium

A. The joint type should be fixed for wheels

B. Missing <limit> tag specifying joint rotation limits

C. The parent and child links are reversed

D. The joint name cannot contain underscores

Solution

1. Step 1: Check joint type and limits

   Revolute joints require <limit> tags to define rotation range; missing limits can cause no movement.

2. Step 2: Verify other options

   Fixed joints do not move, so B is wrong. Parent/child order is correct. Underscores are allowed in names.

3. Final Answer:

   Missing <limit> tag specifying joint rotation limits -> Option B

4. Quick Check:

   Revolute joints need limits to move [OK]

Hint: Revolute joints need <limit> tags to move [OK]

Common Mistakes:

• Omitting <limit> tag for revolute joints
• Using fixed joint for moving parts
• Swapping parent and child links
• Thinking joint names can't have underscores

5. You want to build a mobile robot URDF with a base and two wheels. Which joint types and connections correctly model the wheels that spin freely?

hard

A. Use connecting wheel_link to base_link for both wheels

B. Use connecting base_link to wheel_link for both wheels

C. Use connecting base_link to wheel_link for both wheels

D. Use without <limit> tags connecting wheel_link to base_link

Solution

1. Step 1: Identify wheel joint requirements

   Wheels spin freely, so joints must allow infinite rotation, which is continuous type.

2. Step 2: Check connection direction and joint type

   Parent link is base_link, child is wheel_link. Continuous joint type is correct for free spinning wheels.

3. Step 3: Eliminate incorrect options

   Fixed joints don't move. Prismatic joints slide linearly, not rotate. Revolute without limits won't move properly.

4. Final Answer:

   Use <joint type="continuous"> connecting base_link to wheel_link for both wheels -> Option C

5. Quick Check:

   Continuous joints + correct parent-child = wheels spin [OK]

Hint: Wheels need continuous joints from base to wheel [OK]

Common Mistakes:

• Using fixed joints for wheels
• Using prismatic joints for rotation
• Omitting limits on revolute joints
• Reversing parent and child links