7.1.1. URDF Import: Turtlebot

Omniverse Isaac Sim have several tools to facilitate integration with the ROS system. We have both ROS and ROS2 bridges in forms of Omnigraph nodes, an URDF importer, as well as connection to Gazebo/Ignition. This tutorial series gives examples of how to use these tools.

7.1.1.1. Learning Objectives

In this example, we will import a Turtlebot3 into Isaac Sim using the URDF importer.

If you already have a robot with rigged joints and properties in USD format, and you wish to jump straight into using our ROS bridges, go to the next tutorial in the series Driving TurtleBot via ROS messages.

7.1.1.2. Getting Started

Prerequisite

  • Completed ROS & ROS 2 Installation so that ROS is available, ROS extension is enabled, and necessary environment variables are set.

  • Basic understanding ROS workspace.

  • Nucleus downloaded to access Isaac Environments

7.1.1.3. Importing TurtleBot URDF

  • Download and build the Turtlebot3 description package if you haven’t done so already. You can build it inside the ROS workspace. You can also use your own ROS workspace. If you choose to use your own workspace, make sure to source that workspace before launching Omniverse Isaac Sim, so that the path to it is part of ROS_PACKAGE_PATH.

git clone -b <distro>-devel https://github.com/ROBOTIS-GIT/turtlebot3.git turtlebot3

  • Navigate to the urdf folder turtlebot3/turtlebot3_description/urdf/. Depending on the distro version, find either the .urdf file, or the .xacro file for turtlebot3_burger, convert the .xacro file to .urdf file by calling

rosrun xacro xacro -o <output_name>.urdf <input_file>.urdf.xacro

  • If your URDF file contains calls to other ROS packages, such as when looking for mesh files of accessories, make sure the path to those ROS packages are also inside the ROS_PACKAGE_PATH before launching the simulator.

  • For the purpose of this tutorial, you only need to build the turtlebot3_description package. Feel free to bypass other packages in the repository.

  1. We will use an Isaac environment for this tutorial, but you can import the robot into any environment of your choosing. Open the environment by going to the Content tab below the viewport, and find Isaac/Environments/Simple_Room/simple_room.usd. If you do not want to use the provided environment, just make sure there is a GroundPlane and a PhysicsScene to your environment. Both can be found in Create -> Physics. You may also need some lighting, play with the various types of lighting in Create -> Light to get the desired effect.

  2. On a new stage, drag the simple_room.usd onto the stage, and place it at the origin by zero out all the Translate components in the Transform Property. You many need to zoom in a bit to see the table inside the room.

  3. Open the URDF importer Isaac Utils > Workflows > URDF Importer.

  4. In the prompt window, inside Import Option section, uncheck clear stage to preserve the existing environment, uncheck Fix Base Link since this is a mobile robot, change Joint Drive Type to Velocity so that wheels can be properly driven later.

  5. Inside Import section, first locate the URDF file you wish to import in the Input File. The Import button will only enable after you’ve selected the file.

  6. Once the asset is imported into Omniverse Kit, a copy of the .usd version of the asset will be automatically saved. Specify the folder you wish to save the asset in Output Directory if it’s different than the folder that the .urdf file is located in. A folder name matching the .urdf file will be created in the specified directory, and the .usd file will be inside the newly created folder.

  7. Make sure to unselect everything on the stage by click on an empty space inside the Stage tab, or select /World on the tree. Otherwise you might be importing the Turtlebot as a child of a random object on the tree.

  8. Click Import.

  9. When the Turtlebot is first imported, it will be on the table. Place it just above the floor of the room using the gizmo.

  10. Press Play and you should see the Turtlebot fall onto the floor.

7.1.1.4. Tune the Robot

The URDF importer automatically imports material, physical, and joint properties whenever it is available and have matching categories in Omniverse Isaac Sim. However, in cases there are no available or matching categories, or if the units are different between the two systems, what gets automatically filled in may not be accurate and changes the robot’s behavior. Here are the steps for tuning some of the common paramters.

Frictional Properties

If your robot’s wheels are slipping, try changing the friction coefficients of the wheels and potentially the ground as well following steps 3.4.2 in Add Simple Objects

Physical Properties

If no explicit mass or inertial properties are given, the physics engine will estimate them from the geometry mesh. To update the mass and inertial properties, find the prim that contains the rigid body for the given link (You can verify this by finding “Physics > Rigid Body” under its property tab). If it already has a “Mass” category under its Physics property tab, modify them accordingly. If there isn’t already a “Mass” category, you can add it by clicking on the +Add button on top of the Propery tab, and select “Physics > Mass”.

Joint Properties

If your robot is oscillating at the joint or moving too slow, take a look at the stiffness and damping parameters for the joints. High stiffness makes the joints snap faster and harder to the desire target, and higher damping smoothes but also slows down the joint’s movement to target. For pure position drives, set relatively high stiffness and low damping. For velocity drives, stiffness must be set to zero with a non-zero damping. We provide two tools to help with inspect the joints and tune the stiffness and damping parameters: Gain Tuner and Articulation Inspector. Please go to the perspective tutorials for details on how to use them.

Note

When the URDF importer finishes, the robot that appears on Stage is usually loaded as a reference. This can be confirmed by an orange arrow on the robot prim on the stage tree eyecon. If you have problem changing the parameters and saving them, you may need to edit the original USD file that the reference is pointing to instead. The path to the original USD file can be found under the property tab References -> Asset Path.

7.1.1.5. Summary

This tutorial covered the following topics:

  1. URDF import

  2. Tuning the robot parameters

7.1.1.5.1. Next Steps

Continue on to the next tutorial in our ROS Tutorials series, Driving TurtleBot via ROS messages, to learn how to add omnigraph nodes to move the robot, and ROS bridge nodes to connect to the ROS network.

7.1.1.5.2. Further Learning