Chapter 2 - ROS Fundamentals
IN THIS LESSON
You will learn the basic concepts of URDF.
URDF (Unified Robot Description Format) is an XML format used in robotics to describe the physical configuration of a robot. It is widely used in the Robot Operating System (ROS) to represent the robot's structure, including its links, joints, sensors, and other components.
Here are the key elements of URDF:
1. Links: These represent the rigid bodies of the robot, such as arms, legs, or the body itself. Each link can have physical properties like mass, inertia, and visual representations.
2. Joints: Joints connect two links and define how they move relative to each other. URDF supports various types of joints, such as revolute (rotational), prismatic (linear), fixed, and continuous.
3. Sensors and Actuators: URDF allows for the inclusion of sensors (e.g., cameras, LIDAR) and actuators (e.g., motors) to describe how the robot interacts with its environment.
4. Visual and Collision Properties: URDF can define both the visual appearance of the robot (for rendering in simulation or visualization) and the collision properties (for physics simulation).
5. Transmission: This describes the relationship between the actuators and joints, defining how forces are transmitted to produce movement.
6. Plugins and Extensions: URDF files can be extended with additional elements or plugins to include more complex or custom features, like specifying material properties or integrating with simulation tools.
URDF is essential for simulating and controlling robots in ROS, as it provides a standardized way to represent a robot's physical structure and dynamics, enabling developers to simulate, visualize, and control robots more effectively.
Most of robot manufacturors provide URDF files relevant to the robot models they produce. If you download a URDF file for your robot, there are several steps you can take to utilize it effectively. By following the below steps, you can effectively leverage the URDF file to visualize, simulate, and control your robot in various ROS-based applications.
1. Load and Visualize the Robot
- RViz : You can load the URDF file into RViz to visualize your robot’s 3D model. This helps in understanding the robot's structure, verifying its geometry, and observing how it will look and behave in a simulated environment.
Run the following command in ROS 2:
roslaunch urdf_tutorial display.launch model:=path/to/your_robot.urdf
2. Simulate the Robot
- Gazebo: The URDF file can be used to simulate the robot in Gazebo. You can load the URDF into Gazebo to test the robot’s physical interactions, movements, and sensor data in a simulated environment.
Run the following command in ROS 2:
roslaunch gazebo_ros empty_world.launch
rosparam load path/to/your_robot.urdf robot_description
rosrun gazebo_ros spawn_model -urdf -model my_robot -param robot_description
3. Use with Robot Controllers
- ROS Controllers: If your robot is equipped with actuators (motors, servos), you can use the URDF file to configure controllers in ROS. This allows you to control the robot’s joints and movements using ROS control packages.
Run the following command in ROS 2:
roslaunch your_robot_package robot_control.launch
4. Modify and Customize
- Editing URDF: If the downloaded URDF does not fully match your robot's configuration or if you need to add custom components (like sensors or specific joints), you can edit the URDF file. You can use text editors or tools like xacro to simplify the process.
```xml
<robot name="my_robot">
<!-- Edit or add links, joints, sensors here -->
</robot>
```
5. Integration with MoveIt!
- Motion Planning: If you are planning to use MoveIt! for motion planning and manipulation, you can use the URDF file to define the robot’s kinematic structure. MoveIt! can help with path planning, collision detection, and executing trajectories.
Run the following command in ROS 2:
roslaunch moveit_setup_assistant setup_assistant.launch
6. Test with Sensor Data
- Simulated Sensors: If your robot has sensors, you can simulate sensor data using the URDF and test how the robot perceives its environment. This is useful for testing perception algorithms or sensor fusion.
7. Deploy on Real Hardware
- Real Robot Deployment: After simulation and testing, you can use the same URDF to describe your robot to ROS when deploying on actual hardware. This ensures consistency between simulation and real-world application.
This tutorial assumes you know how to write well-formatted XML code and have the access to download the URDF file of your robot. If you wish to create your own URDF file from scratch, visit the below tutorials for more detailed instructions:
Building a visual robot model from scratch:
Building a movable robot model
Using URDF with robot_state_publisher
If you are not creating your URDF file from scratch or you don’t have an existing URDF file for your robot, you may generate the URDF file from a variety of CAD and 3D modeling software such as the following:
CAD Exporters
• CREO Parametric URDF Exporter
• FreeCAD CAD & ROS Open-Source Synergy (CROSS)
• FusionSDF: Fusion 360 to SDF exporter
• ExportURDF Library (Fusion360, OnShape, Solidworks)
• RoboForge Project (freemium / paid tooling)
Other URDF Export and Conversion Tools
• Gazebo SDFormat to URDF Parser
• SDF to URDF Converter in Python
• URDF to Webots Simulator Format
Viewing URDF & SDF Files
• Examples of Common URDF Launch Files
• Web Viewer for URDF Files:
For more information on how to export and convert URDF files, visit the below tutorial:
https://docs.ros.org/en/humble/Tutorials/Intermediate/URDF/Exporting-an-URDF-File.html
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For more in-depth tutorials and to learn about more advanced aspects of ROS, refer to the documentation at: https://docs.ros.org/en/humble/Tutorials.html