DiffBot

DiffBot, or ‘’Differential Mobile Robot’’, is a simple mobile base with differential drive. The robot is basically a box moving according to differential drive kinematics.

For example_2, the hardware interface plugin is implemented having only one interface.

  • The communication is done using proprietary API to communicate with the robot control box.

  • Data for all joints is exchanged at once.

The DiffBot URDF files can be found in description/urdf folder.

Note

The commands below are given for a local installation of this repository and its dependencies as well as for running them from a docker container. For more information on the docker usage see Using Docker.

Tutorial steps

  1. To check that DiffBot description is working properly use following launch commands

    ros2 launch ros2_control_demo_example_2 view_robot.launch.py

    Warning

    Getting the following output in terminal is OK: Warning: Invalid frame ID "odom" passed to canTransform argument target_frame - frame does not exist. This happens because joint_state_publisher_gui node need some time to start.

    Differential Mobile Robot

  2. To start DiffBot example open a terminal, source your ROS2-workspace and execute its launch file with

    ros2 launch ros2_control_demo_example_2 diffbot.launch.py

    The launch file loads and starts the robot hardware, controllers and opens RViz. In the starting terminal you will see a lot of output from the hardware implementation showing its internal states. This excessive printing is only added for demonstration. In general, printing to the terminal should be avoided as much as possible in a hardware interface implementation.

    If you can see an orange box in RViz everything has started properly. Still, to be sure, let’s introspect the control system before moving DiffBot.

  3. Check if the hardware interface loaded properly, by opening another terminal and executing

    ros2 control list_hardware_interfaces

    You should get

    command interfaces
      left_wheel_joint/velocity [available] [claimed]
      right_wheel_joint/velocity [available] [claimed]
state interfaces
      left_wheel_joint/position
      left_wheel_joint/velocity
      right_wheel_joint/position
      right_wheel_joint/velocity

    The [claimed] marker on command interfaces means that a controller has access to command DiffBot.

  4. Check if controllers are running

    ros2 control list_controllers

    You should get

    diffbot_base_controller[diff_drive_controller/DiffDriveController] active
joint_state_broadcaster[joint_state_broadcaster/JointStateBroadcaster] active

  5. If everything is fine, now you can send a command to Diff Drive Controller using ROS 2 CLI interface:

    ros2 topic pub --rate 30 /diffbot_base_controller/cmd_vel_unstamped geometry_msgs/msg/Twist "linear:
  x: 0.7
  y: 0.0
  z: 0.0
angular:
  x: 0.0
  y: 0.0
  z: 1.0"

    You should now see an orange box circling in RViz. Also, you should see changing states in the terminal where launch file is started.

    [DiffBotSystemHardware]: Got command 43.33333 for 'left_wheel_joint'!
[DiffBotSystemHardware]: Got command 50.00000 for 'right_wheel_joint'!

Files used for this demos

Controllers from this demo