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Writing a new controller

In this framework controllers 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 controller.

Preparing package

If the package for the controller 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.
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 <controller_name>.hpp and <controller_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>.
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 "controller_interface/controller_interface.hpp" and visibility_control.h if you are using one.
3. Define a unique namespace for your controller. This is usually a package name written in snake_case.
4. Define the class of the controller, extending ControllerInterface, e.g., .. code:: c++ class ControllerName : public controller_interface::ControllerInterface
5. Add a constructor without parameters and the following public methods overriding the ControllerInterface definition: init, command_interface_configuration, state_interface_configuration, on_configure, on_activate, on_deactivate, update. For exact definitions check the controller_interface/controller_interface.hpp header or one of the controllers from ros2_controllers.
6. (optional) Often, controllers accept lists of joint names and interface names as parameters. If so, you can add two protected string vectors to store those values.
Adding definitions into source file (.cpp)
1. Include the header file of your controller and add a namespace definition to simplify further development.
2. (optional) Implement a constructor if needed. There, you could initialize member variables. This could also be done in the init method.
3. Implement the init method. The first line usually calls the parent init method. Here is the best place to initialize the variables, reserve memory, and most importantly, declare node parameters used by the controller. If everything works fine return controller_interface::return_type::OK or controller_interface::return_type::ERROR otherwise.
4. Write the on_configure method. Parameters are usually read here, and everything is prepared so that the controller can be started.
5. Implement command_interface_configuration and state_interface_configuration where required interfaces are defined. There are three options of the interface configuration ALL, INDIVIDUAL, and NONE defined in controller_interface/controller_interface.hpp". ALL and NONE option will ask for access to all available interfaces or none of them. The INDIVIDUAL configuration needs a detailed list of required interface names. Those are usually provided as parameters. The full interface names have structure <joint_name>/<interface_type>.
6. Implement the on_activate method with checking, and potentially sorting, the interfaces and assigning members’ initial values. This method is part of the real-time loop, therefore avoid any reservation of memory and, in general, keep it as short as possible.
7. Implement the on_deactivate method, which does the opposite of on_activate. In many cases, this method is empty. This method should also be real-time safe as much as possible.
8. Implement the update method as the main entry point. The method should be implemented with real-time constraints in mind. When this method is called, the state interfaces have the most recent values from the hardware, and new commands for the hardware should be written into command interfaces.
9. 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 controller class, e.g., <controller_name_namespace>::<ControllerName>, and as second the base class, i.e., controller_interface::ControllerInterface.
Writing export definition for pluginlib
1. Create the <controller_name>.xml file in the package and add a definition of the library and controller’s class which has to be visible for the pluginlib. The easiest way to do that is to check other controllers in the ros2_controllers package.
2. Usually, the plugin name is defined by the package (namespace) and the class name, e.g., <controller_name_package>/<ControllerName>. This name defines the controller’s type when the controller manager searches for it. The other two files have to correspond to the definition done in macro at the bottom of the <controller_name>.cpp file.
Writing 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_<controller_name>.cpp.
2. You can safely copy the file’s content for any controller defined in the ros2_controllers package.
3. Change the name of the copied test and in the last line, where controller type is specified put the name defined in <controller_name>.xml file, e.g., <controller_name_package>/<ControllerName>.
Add compile directives into ``CMakeLists.txt`` file
1. Under the line find_package(ament_cmake REQUIRED) add further dependencies. Those are at least: controller_interface, hardware_interface, pluginlib, rclcpp and rclcpp_lifecycle.
2. Add a compile directive for a shared library providing the <controller_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(controller_interface <controller_name>.xml)
6. Add install directives for targets and include directory.
7. In the test section add the following dependencies: ament_cmake_gmock, controller_manager, hardware_interface, ros2_control_test_assets.
8. Add compile definitions for the tests using the ament_add_gmock directive. For details, see how it is done for controllers in the ros2_controllers package.
9. (optional) Add your controller`s library into ament_export_libraries before ament_package().
Add dependencies into ``package.xml`` file
1. Add at least the following packages into <depend> tag: controller_interface, hardware_interface, pluginlib, rclcpp and rclcpp_lifecycle.
2. Add at least the following package into <test_depend> tag: ament_add_gmock, controller_manager, hardware_interface, and ros2_control_test_assets.
Compiling and testing the controller
1. Now everything is ready to compile the controller using the colcon build <controller_name_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 <controller_name_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

Templates and scripts for generating controllers shell