Exploring ROS2 utilizing wheeled Robotic – #3 – Shifting the robotic

Robotics

2 min read

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By Marco Arruda

On this publish you’ll discover ways to publish to a ROS2 matter utilizing ROS2 C++. As much as the tip of the video, we’re shifting the robotic Dolly robotic, simulated utilizing Gazebo 11.

You’ll study:

The best way to create a node with ROS2 and C++
The best way to public to a subject with ROS2 and C++

1 – Setup atmosphere – Launch simulation

Earlier than anything, be sure to have the rosject from the earlier publish, you possibly can copy it from right here.

Launch the simulation in a single webshell and in a special tab, checkout the matters we now have obtainable. You will need to get one thing much like the picture beneath:

2 – Create a subject writer

Create a brand new file to container the writer node: moving_robot.cpp and paste the next content material:

#embody <chrono>
#embody <practical>
#embody <reminiscence>

#embody "rclcpp/rclcpp.hpp"
#embody "geometry_msgs/msg/twist.hpp"

utilizing namespace std::chrono_literals;

/* This instance creates a subclass of Node and makes use of std::bind() to register a
 * member perform as a callback from the timer. */

class MovingRobot : public rclcpp::Node {
public:
  MovingRobot() : Node("moving_robot"), count_(0) {
    publisher_ =
        this->create_publisher("/dolly/cmd_vel", 10);
    timer_ = this->create_wall_timer(
        500ms, std::bind(&MovingRobot::timer_callback, this));
  }

personal:
  void timer_callback() {
    auto message = geometry_msgs::msg::Twist();
    message.linear.x = 0.5;
    message.angular.z = 0.3;
    RCLCPP_INFO(this->get_logger(), "Publishing: '%f.2' and %f.2",
                message.linear.x, message.angular.z);
    publisher_->publish(message);
  }
  rclcpp::TimerBase::SharedPtr timer_;
  rclcpp::Writer::SharedPtr publisher_;
  size_t count_;
};

int most important(int argc, char *argv[]) {
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared());
  rclcpp::shutdown();
  return 0;
}QoS (High quality of Service)

Just like the subscriber it’s created a category that inherits Node. A publisher_ is setup and in addition a callback, though this time will not be a callback that receives messages, however a timer_callback referred to as in a frequency outlined by the timer_ variable. This callback is used to publish messages to the robotic.

The create_publisher methodology wants two arguments:

matter identify
QoS (High quality of Service) – That is the coverage of knowledge saved within the queue. You may make use of various middlewares and even use some offered by default. We’re simply establishing a queue of 10. By default, it retains the final 10 messages despatched to the subject.

The message revealed have to be created utilizing the category imported:

message = geometry_msgs::msg::Twist();

We make sure the callback strategies on the subscribers facet will all the time acknowledge the message. That is the best way it must be revealed by utilizing the writer methodology publish.

3 – Compile and run the node

To be able to compile we have to modify some issues within the ~/ros2_ws/src/my_package/CMakeLists.txt. So add the next to the file:

Add the geometry_msgs dependency
Append the executable moving_robot
Add set up instruction for moving_robot

find_package(geometry_msgs REQUIRED)
...
# shifting robotic
add_executable(moving_robot src/moving_robot.cpp)
ament_target_dependencies(moving_robot rclcpp geometry_msgs)
...
set up(TARGETS
  moving_robot
  reading_laser
  DESTINATION lib/${PROJECT_NAME}/
)

We will run the node like beneath:

supply ~/ros2_ws/set up/setup.bash ros2 run my_package