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Example 14: Modular robot with actuators not providing states

The example shows how to implement robot hardware with separate communication to each actuator as well as separate sensors for position feedback:

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

  • Data for all actuators and sensors is exchanged separately from each other

  • Examples: Arduino-based-robots, custom robots

This is implemented with hardware interfaces of type hardware_interface::ActuatorInterface and hardware_interface::SensorInterface.

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_14 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_14 rrbot_modular_actuators_without_feedback_sensors_for_position_feedback.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 four hardware components, one for each actuator and one for each sensor:

    Hardware Component 1
        name: RRBotModularJoint2
        type: actuator
        plugin name: ros2_control_demo_example_14/RRBotActuatorWithoutFeedback
        state: id=3 label=active
        command interfaces
                joint2/velocity [available] [claimed]
Hardware Component 2
        name: RRBotModularJoint1
        type: actuator
        plugin name: ros2_control_demo_example_14/RRBotActuatorWithoutFeedback
        state: id=3 label=active
        command interfaces
                joint1/velocity [available] [claimed]
Hardware Component 3
        name: RRBotModularPositionSensorJoint2
        type: sensor
        plugin name: ros2_control_demo_example_14/RRBotSensorPositionFeedback
        state: id=3 label=active
        command interfaces
Hardware Component 4
        name: RRBotModularPositionSensorJoint1
        type: sensor
        plugin name: ros2_control_demo_example_14/RRBotSensorPositionFeedback
        state: id=3 label=active
        command interfaces

  4. Check if controllers are running

    ros2 control list_controllers

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

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

    ros2 topic pub /forward_velocity_controller/commands std_msgs/msg/Float64MultiArray "data:
- 5
- 5"

    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.

    [RRBotActuatorWithoutFeedback]: Writing command: 5.000000
[RRBotActuatorWithoutFeedback]: Sending data command: 5
[RRBotActuatorWithoutFeedback]: Joints successfully written!
[RRBotActuatorWithoutFeedback]: Writing command: 5.000000
[RRBotActuatorWithoutFeedback]: Sending data command: 5
[RRBotActuatorWithoutFeedback]: Joints successfully written!
[RRBotSensorPositionFeedback]: Reading...
[RRBotSensorPositionFeedback]: Got measured velocity 5.00000
[RRBotSensorPositionFeedback]: Got state 0.25300 for joint 'joint1'!
[RRBotSensorPositionFeedback]: Joints successfully read!
[RRBotSensorPositionFeedback]: Reading...
[RRBotSensorPositionFeedback]: Got measured velocity 5.00000
[RRBotSensorPositionFeedback]: Got state 0.25300 for joint 'joint2'!
[RRBotSensorPositionFeedback]: Joints successfully read!

Files used for this demos

Controllers from this demo