Displaying robot model from URDF in ROS - Mini Project: Build & Apply

Displaying robot model from URDF

📖 Scenario: You are working on a robot simulation project using ROS. You want to display your robot model visually in RViz by loading its URDF description.

🎯 Goal: Build a ROS node that loads a URDF robot description and publishes it so RViz can display the robot model.

📋 What You'll Learn

Create a ROS node in Python

Load the URDF robot description from a file

Publish the robot description on the /robot_description parameter

Launch RViz to visualize the robot model

💡 Why This Matters

🌍 Real World

Robots often need their physical structure visualized in simulation or debugging tools. Loading URDF models into RViz helps developers see the robot's shape and joint states.

💼 Career

Robotics engineers and developers use ROS and URDF to build and test robot models. Knowing how to display robot models in RViz is essential for simulation and diagnostics.

1

Create ROS node and load URDF file

Create a Python ROS node called robot_model_publisher that reads the URDF file located at src/robot_description/urdf/my_robot.urdf into a variable called robot_urdf.

ROS

# Your code here to create ROS node and load URDF file

Hint

Use open() to read the URDF file and store its contents in self.robot_urdf.

2

Set robot description parameter

Inside the RobotModelPublisher class constructor, add a parameter called robot_description and set its value to the robot_urdf string you loaded.

ROS

import rclpy
from rclpy.node import Node

class RobotModelPublisher(Node):
    def __init__(self):
        super().__init__('robot_model_publisher')
        with open('src/robot_description/urdf/my_robot.urdf', 'r') as urdf_file:
            self.robot_urdf = urdf_file.read()
        # Set robot_description parameter here


def main(args=None):
    rclpy.init(args=args)
    node = RobotModelPublisher()
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Hint

Use self.declare_parameter('robot_description', self.robot_urdf) to set the parameter.

3

Publish robot description parameter

Add a publisher in the RobotModelPublisher class that publishes the robot_description parameter on the /robot_description topic as a std_msgs/msg/String message. Publish the message once after node initialization.

ROS

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class RobotModelPublisher(Node):
    def __init__(self):
        super().__init__('robot_model_publisher')
        with open('src/robot_description/urdf/my_robot.urdf', 'r') as urdf_file:
            self.robot_urdf = urdf_file.read()
        self.declare_parameter('robot_description', self.robot_urdf)
        # Create publisher and publish robot_description here


def main(args=None):
    rclpy.init(args=args)
    node = RobotModelPublisher()
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Hint

Create a publisher with self.create_publisher(String, '/robot_description', 10) and publish the URDF string once.

4

Launch RViz to visualize robot

Add a ROS 2 launch file named display_robot.launch.py that starts RViz with the default configuration and loads the robot_description parameter so the robot model is visible.

ROS

# Your code here to create a ROS 2 launch file that launches RViz with robot_description parameter

Hint

Use launch_ros.actions.Node to start RViz and pass the robot_description parameter.

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