ros2_control hardware interfaces

Worked example

Read it before you run it

class MockArm : public hardware_interface::SystemInterface {
// A hardware plugin for a simulated arm.
public:
// The functions the controller manager calls.
  std::vector<hardware_interface::StateInterface> export_state_interfaces() override {
// Tell ROS which values the hardware can report.
    return {hardware_interface::StateInterface("shoulder", "position", &pos_)};
// The shoulder's position is readable from pos_.
  }
// End of export_state_interfaces.
  std::vector<hardware_interface::CommandInterface> export_command_interfaces() override {
// Tell ROS which values controllers can command.
    return {hardware_interface::CommandInterface("shoulder", "position", &cmd_)};
// A controller writes the target into cmd_.
  }
// End of export_command_interfaces.
  hardware_interface::return_type write(const rclcpp::Time&, const rclcpp::Duration&) override { pos_ = cmd_; return hardware_interface::return_type::OK; }
// A mock: pretend the joint reaches its command at once.
private:
// State and command storage.
  double pos_{0.0}, cmd_{0.0};
// Current position and commanded position.
};
// End of the class.

Practice code

Now type it yourself

Copy the starter into your ROS 2 package, replace every TODO, then build with colcon build and run it from a sourced terminal.

<ros2_control name="mock_arm" type="system">
  <hardware>
    <!-- TODO: use the plugin mock_components/GenericSystem -->
  </hardware>
  <joint name="shoulder">
    <command_interface name="position"/>
    <state_interface name="position"/>
  </joint>
</ros2_control>

ros2 control list_hardware_interfaces shows shoulder/position as command and state

<ros2_control name="mock_arm" type="system">
  <hardware>
    <plugin>mock_components/GenericSystem</plugin>
  </hardware>
  <joint name="shoulder">
    <command_interface name="position"/>
    <state_interface name="position"/>
  </joint>
</ros2_control>