Building a mobile robot URDF in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the robot's base link in URDF.

ROS

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

Drag options to blanks, or click blank then click option'

A0.1 0.1 0.1

B0.05 0.05 0.05

C0.5 0.3 0.2

D1.0 1.0 1.0

Attempts:

3 left

💡 Hint

Common Mistakes

Using too small or too large box sizes that don't match the robot base.

Confusing the order of dimensions.

The base link box size should represent the robot's base dimensions, here 0.5m x 0.3m x 0.2m.

2fill in blank

medium

Complete the code to define a joint connecting the base_link to the wheel_link.

ROS

<joint name="wheel_joint" type="[1]">
  <parent link="base_link"/>
  <child link="wheel_link"/>
  <origin xyz="0 0 0" rpy="0 0 0"/>
</joint>

Drag options to blanks, or click blank then click option'

Afixed

Bfloating

Cprismatic

Dcontinuous

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'fixed' joint type which prevents wheel rotation.

Using 'prismatic' which allows sliding, not rotation.

The wheel joint should be continuous to allow full rotation.

3fill in blank

hard

Fix the error in the joint axis definition to rotate around the correct axis.

ROS

<joint name="wheel_joint" type="continuous">
  <parent link="base_link"/>
  <child link="wheel_link"/>
  <axis xyz="[1]"/>
</joint>

Drag options to blanks, or click blank then click option'

A0 1 0

B1 0 0

C1 1 0

D0 0 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using axis '1 0 0' or '0 0 1' which rotate around forward or vertical axes.

Using non-unit vectors like '1 1 0'.

The wheel rotates around the y-axis (lateral axle), so axis xyz should be '0 1 0'.

4fill in blank

hard

Fill both blanks to define the origin position and rotation of the sensor link relative to the base_link.

ROS

<joint name="sensor_joint" type="fixed">
  <parent link="base_link"/>
  <child link="sensor_link"/>
  <origin xyz="[1]" rpy="[2]"/>
</joint>

Drag options to blanks, or click blank then click option'

A0.2 0 0.1

B0 0 0

D0 1.57 0

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up xyz position values.

Using zero rotation when sensor needs to face sideways.

The sensor is positioned 0.2m forward and 0.1m up, rotated 90 degrees (1.57 radians) around the y-axis.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps each wheel name to its radius if the radius is greater than 0.1.

ROS

wheel_radii = { [1] : [2] for [3] in wheels if wheels[[3]] > 0.1 }

Drag options to blanks, or click blank then click option'

Awheel

Bwheels[wheel]

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Not indexing wheels dictionary correctly.

We use 'wheel' as the variable name, map it to its radius wheels[wheel], and iterate over wheels.

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