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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.

    Furthermore, we can see that the command interface is of type velocity, which is typical for a differential drive robot.

  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 10 /cmd_vel geometry_msgs/msg/TwistStamped "
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'!

  6. Let’s introspect the ros2_control hardware component. Calling

ros2 control list_hardware_components

should give you

Hardware Component 1
        name: DiffBot
        type: system
        plugin name: ros2_control_demo_example_2/DiffBotSystemHardware
        state: id=3 label=active
        command interfaces
                left_wheel_joint/velocity [available] [claimed]
                right_wheel_joint/velocity [available] [claimed]

This shows that the custom hardware interface plugin is loaded and running. If you work on a real robot and don’t have a simulator running, it is often faster to use the mock_components/GenericSystem hardware component instead of writing a custom one. Stop the launch file and start it again with an additional parameter

ros2 launch ros2_control_demo_example_2 diffbot.launch.py use_mock_hardware:=True

Calling

ros2 control list_hardware_components

now should give you

Hardware Component 1
    name: DiffBot
    type: system
    plugin name: mock_components/GenericSystem
    state: id=3 label=active
    command interfaces
            left_wheel_joint/velocity [available] [claimed]
            right_wheel_joint/velocity [available] [claimed]

You see that a different plugin was loaded. Having a look into the diffbot.ros2_control.xacro, one can find the instructions to load this plugin together with the parameter calculate_dynamics.

<hardware>
  <plugin>mock_components/GenericSystem</plugin>
  <param name="calculate_dynamics">true</param>
</hardware>

This enables the integration of the velocity commands to the position state interface, which can be checked by means of ros2 topic echo /joint_states: The position values are increasing over time if the robot is moving. You now can test the setup with the commands from above, it should work identically as the custom hardware component plugin.

More information on mock_components can be found in the ros2_control documentation.

Files used for this demos

Controllers from this demo