Displaying robot model from URDF in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this ROS launch file display?

Given the following ROS launch snippet that loads a URDF robot model into RViz, what will be the visible output when RViz starts?

ROS

<launch>
  <param name="robot_description" command="$(find xacro)/xacro $(find my_robot_description)/urdf/robot.urdf.xacro"/>
  <node name="rviz" pkg="rviz" type="rviz" args="-d $(find my_robot_description)/rviz/robot.rviz"/>
</launch>

A RViz opens showing the robot model defined in the URDF with all links and joints visualized.

B RViz opens but shows an empty scene because the robot_description parameter is not set.

C RViz crashes due to missing xacro package.

D RViz opens but only displays the robot's base link without any joints or links.

💡 Hint

Check how the robot_description parameter is set and how RViz uses it to display the model.

The launch file sets the robot_description parameter by processing the URDF xacro file, then starts RViz with a config that visualizes the robot. This results in the full robot model being displayed.

📝 Syntax

intermediate

Identify the syntax error in this URDF snippet

What is the syntax error in this URDF snippet that prevents the robot model from loading correctly?

ROS

<robot name="simple_bot">
  <link name="base_link">
    <visual>
      <geometry>
        <box size="1 1 1"/>
      </geometry>
    </visual>
  </link>
  <joint name="joint1" type="revolute">
    <parent link="base_link"/>
    <child link="link1"/>
    <origin xyz="0 0 1" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>
</robot>

A The joint type "revolute" is invalid and should be "rotational".

B The box size attribute should be "1,1,1" with commas instead of spaces.

C The link "link1" is referenced in the joint but not defined anywhere.

D The origin tag is missing the required attribute "frame_id".

💡 Hint

Check if all links referenced in joints are defined.

The joint references a child link named "link1" which is not defined in the URDF. This causes the model to fail loading.

❓ state_output

advanced

What is the effect of changing the tag in a URDF joint?

Consider a revolute joint in a URDF with origin xyz="0 0 1" rpy="0 0 0". If you change the origin to xyz="0 0 0" rpy="0 1.57 0", what will be the effect on the robot model display in RViz?

A The child link will be rotated 90 degrees around the Y-axis relative to the parent link, changing its orientation in RViz.

B The child link will move 1 meter along the Z-axis but keep the same orientation.

C The joint will become fixed and the child link will not move relative to the parent.

D There will be no visible change in RViz because origin only affects simulation, not visualization.

💡 Hint

The rpy attribute defines rotation in roll, pitch, yaw order.

Changing rpy to "0 1.57 0" rotates the child link 90 degrees around the Y-axis, affecting how it appears connected to the parent in RViz.

🔧 Debug

advanced

Why does RViz fail to display the robot model?

You have a launch file that sets the robot_description parameter and starts RViz, but RViz shows no robot model. Which of the following is the most likely cause?

A The ROS master is not running, so RViz cannot connect to the parameter server.

B The robot_description parameter is set after RViz starts, so RViz does not see it.

C The RViz config file is missing, causing RViz to crash on startup.

D The URDF file has no <visual> tags, so RViz cannot render any geometry.

💡 Hint

Consider the order of parameter setting and node startup in ROS launch files.

If RViz starts before the robot_description parameter is set, it will not load the robot model. Parameters must be set before RViz node starts.

🧠 Conceptual

expert

How does the robot_state_publisher use the URDF to update robot visualization?

In ROS, the robot_state_publisher node reads the URDF robot model and publishes TF transforms. What is the main purpose of these transforms for visualization in RViz?

A They send sensor data from the robot to RViz for real-time monitoring.

B They provide collision detection data to RViz to prevent visual overlap of links.

C They convert the URDF into a mesh format that RViz can render efficiently.

D They define the position and orientation of each robot link relative to its parent, enabling RViz to display the robot's current pose.

💡 Hint

Think about how RViz knows where each part of the robot is in space.

The robot_state_publisher publishes TF transforms that describe how each link is positioned and oriented relative to its parent link. RViz uses these transforms to draw the robot in its current pose.

Practice

1. What is the main purpose of a URDF file in ROS?

easy

A. To describe the robot's parts and how they connect

B. To write control algorithms for the robot

C. To visualize sensor data in RViz

D. To manage robot communication protocols

Solution

1. Step 1: Understand URDF role and differentiate

   A URDF file defines the robot's physical structure and joint connections. Control algorithms and communication are handled elsewhere, not in URDF.

2. Final Answer:

   To describe the robot's parts and how they connect -> Option A

3. Quick Check:

   URDF = Robot structure description [OK]

Hint: URDF = robot shape and joints description [OK]

Common Mistakes:

• Confusing URDF with control code
• Thinking URDF manages communication
• Assuming URDF handles sensor visualization

2. Which command correctly launches the display of a robot model from a URDF file using ROS?

easy

A. rosrun rviz rviz_display

B. rosrun urdf display_model

C. roslaunch display.launch

D. roslaunch robot_model.launch

Solution

1. Step 1: Identify the launch file for display and check options

   The standard way to show a robot model is using roslaunch display.launch. Other commands either don't exist or are incorrect for displaying URDF models.

2. Final Answer:

   roslaunch display.launch -> Option C

3. Quick Check:

   Use roslaunch with display.launch to show URDF [OK]

Hint: Use roslaunch with display.launch to show robot [OK]

Common Mistakes:

• Using rosrun instead of roslaunch
• Wrong launch file name
• Trying to run rviz directly without launch

3. Given the following command:
roslaunch display.launch robot:=my_robot.urdf
What will happen when this command runs?

medium

A. The robot starts moving automatically

B. RViz opens and shows the robot model defined in my_robot.urdf

C. An error occurs because the file extension is wrong

D. The robot model is saved but not displayed

Solution

1. Step 1: Understand the command and what display.launch does

   The command launches display.launch and loads the robot model from my_robot.urdf. It opens RViz to visualize the robot model from the URDF file.

2. Final Answer:

   RViz opens and shows the robot model defined in my_robot.urdf -> Option B

3. Quick Check:

   roslaunch display.launch + URDF = RViz shows robot [OK]

Hint: roslaunch display.launch loads URDF into RViz [OK]

Common Mistakes:

• Assuming robot moves automatically
• Thinking file extension causes error
• Believing model is saved but not shown

4. You tried to display your robot model using roslaunch display.launch but RViz shows no robot. What is a likely cause?

medium

A. The URDF file path is incorrect or missing

B. RViz is not installed on your system

C. You forgot to start roscore

D. The robot model is too complex to display

Solution

1. Step 1: Check URDF availability and other causes

   If the URDF file path is wrong or missing, RViz cannot load the robot model. While roscore is needed, usually the launch file starts it; RViz installation issues cause errors, not empty display; complexity doesn't prevent display.

2. Final Answer:

   The URDF file path is incorrect or missing -> Option A

3. Quick Check:

   Missing URDF = no robot shown in RViz [OK]

Hint: Check URDF file path if robot not visible [OK]

Common Mistakes:

• Ignoring URDF file path errors
• Assuming RViz is not installed without checking
• Forgetting roscore is usually auto-started

5. You want to display a robot model from a URDF file but also highlight a specific joint in RViz. Which approach is best?

hard

A. Change the URDF to remove all other joints except the one to highlight

B. Edit the robot's control code to move the joint visibly

C. Use rosrun to start RViz and manually select the joint

D. Modify the URDF to add a visual marker on the joint and launch display.launch

Solution

1. Step 1: Understand highlighting and evaluate options

   Adding a visual marker in the URDF on the joint allows RViz to show it distinctly. Control code changes don't affect visualization markers; manual selection in RViz doesn't highlight; removing joints loses model context.

2. Final Answer:

   Modify the URDF to add a visual marker on the joint and launch display.launch -> Option D

3. Quick Check:

   Highlight joint by adding marker in URDF [OK]

Hint: Add visual marker in URDF to highlight joint [OK]

Common Mistakes:

• Trying to highlight by control code only
• Removing other joints instead of marking
• Relying on manual RViz selection for highlight