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Example 6: Modular Robots with separate communication to each actuator

The example shows how to implement robot hardware with separate communication to each actuator:

  • The communication is done on actuator level using proprietary or standardized API (e.g., canopen_402, Modbus, RS232, RS485).

  • Data for all actuators is exchanged separately from each other.

  • Examples: Mara, Arduino-based-robots

This is implemented with a hardware interface of type hardware_interface::ActuatorInterface.

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 RRBot descriptions are working properly use following launch commands

    ros2 launch ros2_control_demo_example_6 view_robot.launch.py

    Note

    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. The joint_state_publisher_gui provides a GUI to generate a random configuration for rrbot. It is immediately displayed in RViz.

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

    ros2 launch ros2_control_demo_example_6 rrbot_modular_actuators.launch.py

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

    If you can see two orange and one yellow rectangle in in RViz everything has started properly. Still, to be sure, let’s introspect the control system before moving RRBot.

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

    ros2 control list_hardware_interfaces

    command interfaces
  joint1/position [available] [claimed]
  joint2/position [available] [claimed]
state interfaces
  joint1/position
  joint2/position

    Marker [claimed] by command interfaces means that a controller has access to command RRBot.

    Now, let’s introspect the hardware components with

    ros2 control list_hardware_components

    There are two hardware components, one for each actuator and one for each sensor:

    Hardware Component 1
        name: RRBotModularJoint2
        type: actuator
        plugin name: ros2_control_demo_example_6/RRBotModularJoint
        state: id=3 label=active
        command interfaces
                joint2/position [available] [claimed]
Hardware Component 2
        name: RRBotModularJoint1
        type: actuator
        plugin name: ros2_control_demo_example_6/RRBotModularJoint
        state: id=3 label=active
        command interfaces
                joint1/position [available] [claimed]

  4. Check if controllers are running

    ros2 control list_controllers

    forward_position_controller[forward_command_controller/ForwardCommandController] active
joint_state_broadcaster[joint_state_broadcaster/JointStateBroadcaster] active

  5. If you get output from above you can send commands to Forward Command Controller, either:

    1. Manually using ROS 2 CLI interface.

      ros2 topic pub /forward_position_controller/commands std_msgs/msg/Float64MultiArray "data:
- 0.5
- 0.5"

    2. Or you can start a demo node which sends two goals every 5 seconds in a loop

      ros2 launch ros2_control_demo_example_6 test_forward_position_controller.launch.py

    You should now see orange and yellow blocks moving in RViz. Also, you should see changing states in the terminal where launch file is started, e.g.

    [ros2_control_node-1] [INFO] [1721764663.304187517] [controller_manager.resource_manager.hardware_component.actuator.RRBotModularJoint]: Writing commands:
[ros2_control_node-1]   0.50 for joint 'joint1'
[ros2_control_node-1] [INFO] [1721764663.304196897] [controller_manager.resource_manager.hardware_component.actuator.RRBotModularJoint]: Writing commands:
[ros2_control_node-1]   0.50 for joint 'joint2'

Files used for this demos

Controllers from this demo