Writing a Hardware Component

In ros2_control hardware system components are libraries, dynamically loaded by the controller manager using the pluginlib interface. The following is a step-by-step guide to create source files, basic tests, and compile rules for a new hardware interface.

  1. Preparing package

    If the package for the hardware interface does not exist, then create it first. The package should have ament_cmake as a build type. The easiest way is to search online for the most recent manual. A helpful command to support this process is ros2 pkg create. Use the --help flag for more information on how to use it. There is also an option to create library source files and compile rules to help you in the following steps.

  2. Preparing source files

    After creating the package, you should have at least CMakeLists.txt and package.xml files in it. Create also include/<PACKAGE_NAME>/ and src folders if they do not already exist. In include/<PACKAGE_NAME>/ folder add <robot_hardware_interface_name>.hpp and <robot_hardware_interface_name>.cpp in the src folder. Optionally add visibility_control.h with the definition of export rules for Windows. You can copy this file from an existing controller package and change the name prefix to the <PACKAGE_NAME>.

  3. Adding declarations into header file (.hpp)

    1. Take care that you use header guards. ROS2-style is using #ifndef and #define preprocessor directives. (For more information on this, a search engine is your friend :) ).

    2. Include "hardware_interface/$interface_type$_interface.hpp" and visibility_control.h if you are using one. $interface_type$ can be Actuator, Sensor or System depending on the type of hardware you are using. for more details about each type check Hardware Components description.

    3. Define a unique namespace for your hardware_interface. This is usually the package name written in snake_case.

    4. Define the class of the hardware_interface, extending $InterfaceType$Interface, e.g., .. code:: c++ class HardwareInterfaceName : public hardware_interface::$InterfaceType$Interface

    5. Add a constructor without parameters and the following public methods implementing LifecycleNodeInterface: on_configure, on_cleanup, on_shutdown, on_activate, on_deactivate, on_error; and overriding $InterfaceType$Interface definition: on_init, export_state_interfaces, export_command_interfaces, prepare_command_mode_switch (optional), perform_command_mode_switch (optional), read, write. For further explanation of hardware-lifecycle check the pull request and for exact definitions of methods check the "hardware_interface/$interface_type$_interface.hpp" header or doxygen documentation for Actuator, Sensor or System.

  4. Adding definitions into source file (.cpp)

    1. Include the header file of your hardware interface and add a namespace definition to simplify further development.

    2. Implement on_init method. Here, you should initialize all member variables and process the parameters from the info argument. In the first line usually the parents on_init is called to process standard values, like name. This is done using: hardware_interface::(Actuator|Sensor|System)Interface::on_init(info). If all required parameters are set and valid and everything works fine return CallbackReturn::SUCCESS or return CallbackReturn::ERROR otherwise.

    1. Write the on_configure method where you usually setup the communication to the hardware and set everything up so that the hardware can be activated.

    2. Implement on_cleanup method, which does the opposite of on_configure.

    3. Implement export_state_interfaces and export_command_interfaces methods where interfaces that hardware offers are defined. For the Sensor-type hardware interface there is no export_command_interfaces method. As a reminder, the full interface names have structure <joint_name>/<interface_type>.

    4. (optional) For Actuator and System types of hardware interface implement prepare_command_mode_switch and perform_command_mode_switch if your hardware accepts multiple control modes.

    5. Implement the on_activate method where hardware “power” is enabled.

    6. Implement the on_deactivate method, which does the opposite of on_activate.

    7. Implement on_shutdown method where hardware is shutdown gracefully.

    8. Implement on_error method where different errors from all states are handled.

    9. Implement the read method getting the states from the hardware and storing them to internal variables defined in export_state_interfaces.

    10. Implement write method that commands the hardware based on the values stored in internal variables defined in export_command_interfaces.

    11. IMPORTANT: At the end of your file after the namespace is closed, add the PLUGINLIB_EXPORT_CLASS macro.

    For this you will need to include the "pluginlib/class_list_macros.hpp" header. As first parameters you should provide exact hardware interface class, e.g., <my_hardware_interface_package>::<RobotHardwareInterfaceName>, and as second the base class, i.e., hardware_interface::(Actuator|Sensor|System)Interface.

  5. Writing export definition for pluginlib

    1. Create the <my_hardware_interface_package>.xml file in the package and add a definition of the library and hardware interface’s class which has to be visible for the pluginlib. The easiest way to do that is to check definition for mock components in the hardware_interface mock_components section.

    2. Usually, the plugin name is defined by the package (namespace) and the class name, e.g., <my_hardware_interface_package>/<RobotHardwareInterfaceName>. This name defines the hardware interface’s type when the resource manager searches for it. The other two parameters have to correspond to the definition done in the macro at the bottom of the <robot_hardware_interface_name>.cpp file.

  6. Writing a simple test to check if the controller can be found and loaded

    1. Create the folder test in your package, if it does not exist already, and add a file named test_load_<robot_hardware_interface_name>.cpp.

    2. You can copy the load_generic_system_2dof content defined in the test_generic_system.cpp package.

    3. Change the name of the copied test and in the last line, where hardware interface type is specified put the name defined in <my_hardware_interface_package>.xml file, e.g., <my_hardware_interface_package>/<RobotHardwareInterfaceName>.

  7. Add compile directives into ``CMakeLists.txt`` file

    1. Under the line find_package(ament_cmake REQUIRED) add further dependencies. Those are at least: hardware_interface, pluginlib, rclcpp and rclcpp_lifecycle.

    2. Add a compile directive for a shared library providing the <robot_hardware_interface_name>.cpp file as the source.

    3. Add targeted include directories for the library. This is usually only include.

    4. Add ament dependencies needed by the library. You should add at least those listed under 1.

    5. Export for pluginlib description file using the following command: .. code:: cmake

      pluginlib_export_plugin_description_file(hardware_interface <my_hardware_interface_package>.xml)

    6. Add install directives for targets and include directory.

    7. In the test section add the following dependencies: ament_cmake_gmock, hardware_interface.

    8. Add compile definitions for the tests using the ament_add_gmock directive. For details, see how it is done for mock hardware in the ros2_control package.

    9. (optional) Add your hardware interface`s library into ament_export_libraries before ament_package().

  8. Add dependencies into ``package.xml`` file

    1. Add at least the following packages into <depend> tag: hardware_interface, pluginlib, rclcpp, and rclcpp_lifecycle.

    2. Add at least the following package into <test_depend> tag: ament_add_gmock and hardware_interface.

  9. Compiling and testing the hardware component

    1. Now everything is ready to compile the hardware component using the colcon build <my_hardware_interface_package> command. Remember to go into the root of your workspace before executing this command.

    2. If compilation was successful, source the setup.bash file from the install folder and execute colcon test <my_hardware_interface_package> to check if the new controller can be found through pluginlib library and be loaded by the controller manager.

That’s it! Enjoy writing great controllers!

Useful External References