Displaying robot model from URDF in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the ROS node for displaying the robot model.

ROS

ros::init(argc, argv, [1]);

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

A"robot_display"

B"urdf_viewer"

C"display_robot"

D"robot_model"

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names instead of a string literal.

Omitting the quotes around the node name.

The ROS node name is set by the string argument in ros::init. Here, "robot_display" is the chosen node name.

2fill in blank

medium

Complete the code to load the URDF file into a string variable.

ROS

std::string urdf_string;
if (!ros::param::get("[1]", urdf_string)) {
  ROS_ERROR("Failed to get URDF from parameter server");
}

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

A"urdf_file"

B"robot_description"

C"robot_model"

D"description_param"

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names like "urdf_file".

Forgetting the quotes around the parameter name.

The standard ROS parameter name for the URDF is "robot_description".

3fill in blank

hard

Fix the error in the code to correctly initialize the URDF model from the string.

ROS

urdf::Model model;
if (!model.[1](urdf_string)) {
  ROS_ERROR("Failed to parse URDF model");
}

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

AinitFile

BparseURDF

CloadModel

DinitString

Attempts:

3 left

💡 Hint

Common Mistakes

Using initFile which expects a file path, not a string.

Using incorrect method names like loadModel or parseURDF.

The correct method to initialize a URDF model from a string is initString.

4fill in blank

hard

Fill both blanks to create a ROS publisher for the robot state and advertise it.

ROS

ros::Publisher robot_pub = nh.[1]<sensor_msgs::JointState>("[2]", 10);

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

Aadvertise

Bpublish

Csubscribe

Drobot_state

Attempts:

3 left

💡 Hint

Common Mistakes

Using publish instead of advertise to create the publisher.

Using subscribe which is for subscribers, not publishers.

To create a publisher, use advertise with the topic name, here "robot_state".

5fill in blank

hard

Fill all three blanks to create a loop that publishes the robot state at 10 Hz.

ROS

ros::Rate loop_rate([1]);
while (ros::ok()) {
  sensor_msgs::JointState msg;
  // Fill msg fields here
  [2].publish(msg);
  ros::spinOnce();
  [3].sleep();
}

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

Arobot_pub

B10

Cloop_rate

Dnh

Attempts:

3 left

💡 Hint

Common Mistakes

Using nh instead of the publisher to publish messages.

Calling sleep on nh instead of loop_rate.

The loop rate is set to 10 Hz, the publisher is robot_pub, and sleep is called on loop_rate.

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