Building a mobile robot URDF in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the effect of the element in a URDF?

In a URDF file for a mobile robot, what does the element define?

A It specifies the visual appearance of a robot part.

B It defines the connection and movement relationship between two links.

C It sets the robot's sensor parameters.

D It declares the robot's control algorithms.

💡 Hint

Think about how parts of a robot are connected and move relative to each other.

The element in URDF specifies how two links are connected and how they can move relative to each other, such as rotation or translation.

📝 Syntax

intermediate

Identify the syntax error in this URDF snippet

What is wrong with this URDF snippet for a link element? <link name="base_link"> <visual> <geometry> <box size="1 1 0.5"/> </geometry> </visual> </link

ROS

<link name="base_link">
  <visual>
    <geometry>
      <box size="1 1 0.5"/>
    </geometry>
  </visual>
</link>

A The closing tag for &lt;link&gt; is missing the closing angle bracket '>'.

B The &lt;geometry&gt; tag is not allowed inside &lt;visual&gt;.

C The &lt;visual&gt; tag cannot be inside a &lt;link&gt; tag.

D The size attribute in &lt;box&gt; should use commas instead of spaces.

💡 Hint

Check the last line carefully for missing characters.

The closing <link> tag is missing the '>' character, which causes a syntax error in XML.

❓ state_output

advanced

What is the resulting number of links in this URDF?

Given this URDF snippet, how many links does the robot model have? <robot name="simple_bot"> <link name="base_link"/> <link name="wheel_left"/> <link name="wheel_right"/> <joint name="joint_left" type="continuous"> <parent link="base_link"/> <child link="wheel_left"/> </joint> <joint name="joint_right" type="continuous"> <parent link="base_link"/> <child link="wheel_right"/> </joint> </robot>

ROS

<robot name="simple_bot">
  <link name="base_link"/>
  <link name="wheel_left"/>
  <link name="wheel_right"/>
  <joint name="joint_left" type="continuous">
    <parent link="base_link"/>
    <child link="wheel_left"/>
  </joint>
  <joint name="joint_right" type="continuous">
    <parent link="base_link"/>
    <child link="wheel_right"/>
  </joint>
</robot>

A 4

B 2

C 3

D 5

💡 Hint

Count all the <link> elements defined inside the <robot> tag.

There are three <link> elements: base_link, wheel_left, and wheel_right.

🔧 Debug

advanced

Why does this URDF cause a runtime error in ROS?

This URDF snippet causes a runtime error when loaded in ROS. What is the cause? <joint name="joint1" type="fixed"> <parent link="link1"/> <child link="link2"/> <axis xyz="0 0 1"/> </joint>

ROS

<joint name="joint1" type="fixed">
  <parent link="link1"/>
  <child link="link2"/>
  <axis xyz="0 0 1"/>
</joint>

A The 'xyz' attribute in 'axis' must have four values instead of three.

B The 'parent' and 'child' links are reversed.

C The joint type 'fixed' is not supported in URDF.

D The 'axis' tag is not allowed in a fixed joint and causes a runtime error.

💡 Hint

Think about what a fixed joint means for movement and axis.

Fixed joints do not move, so specifying an axis is invalid and causes errors in ROS.

🧠 Conceptual

expert

What is the purpose of the tag in a mobile robot URDF?

In a mobile robot URDF, what does the <transmission> tag define?

A It links actuators to joints, specifying how motor commands translate to joint movement.

B It defines the robot's communication protocol for sensors.

C It specifies the visual textures for robot parts.

D It sets the robot's battery capacity and power limits.

💡 Hint

Think about how motors control the robot's moving parts.

The tag connects actuators like motors to joints, describing how commands affect joint motion.

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