[ROS Study Notes] (III): Implementing a Publisher

I. Goal

ROS Master contains two nodes: a Subscriber (turtlesim) and a Publisher. The Publisher is implemented in code to publish a Message containing linear velocity and angle. The Message is transmitted through a Topic channel to the Subscriber, thereby controlling the turtle’s movement.

II. Create a Package

First, create a workspace. For details, see the previous section, [ROS Study Notes] (II): Creating a Workspace and Package.

Then create a package.

cd ~/catkin_ws/src
catkin_create_pkg learning_topic roscpp rospy std_msgs geometry_msgs turtlesim

III. Create the Publisher Code

Go to the src folder of the package and create a cpp file (you can also create it directly in the graphical interface).

cd ~/catkin_ws/src/learning_topic/src
touch velocity_publisher.cpp
sudo gedit velocity_publisher.cpp

Enter the following code.

/**
 * 该例程将发布turtle1/cmd_vel话题，消息类型geometry_msgs::Twist
 */
 
#include <ros/ros.h>
#include <geometry_msgs/Twist.h>

int main(int argc, char **argv)
{
	// ROS节点初始化
	ros::init(argc, argv, "velocity_publisher");

	// 创建节点句柄
	ros::NodeHandle n;

	// 创建一个Publisher，发布名为/turtle1/cmd_vel的topic，消息类型为geometry_msgs::Twist，队列长度10
	ros::Publisher turtle_vel_pub = n.advertise<geometry_msgs::Twist>("/turtle1/cmd_vel", 10);

	// 设置循环的频率
	ros::Rate loop_rate(10);

	int count = 0;
	while (ros::ok())
	{
	    // 初始化geometry_msgs::Twist类型的消息
		geometry_msgs::Twist vel_msg;
		vel_msg.linear.x = 0.5;
		vel_msg.angular.z = 0.2;

	    // 发布消息
		turtle_vel_pub.publish(vel_msg);
		ROS_INFO("Publsh turtle velocity command[%0.2f m/s, %0.2f rad/s]", 
				vel_msg.linear.x, vel_msg.angular.z);

	    // 按照循环频率延时
	    loop_rate.sleep();
	}

	return 0;
}

IV. Configure the Build Rules for the Publisher Code

1. Specify the code to compile and the executable to generate

2. Specify the library to link

In the Build section of Learning_topic/CMakeList.txt (above Install), add the following code:

add_executable(velocity_publisher src/velocity_publisher.cpp)	#描述要把哪个程序文件编译成哪个可执行文件
target_link_libraries(velocity_publisher ${catkin_LIBRARIES})	#把可执行文件和库做链接

V. Compile and Run the Publisher

1. Compile

cd ~/catkin_ws
catkin_make
source devel/setup.bash

You can add the source statement to the end of ~/.bashrc. This way, you do not need to enter the source command in the terminal every time to create the environment variables (replace the username in the path with your own).

source /home/【Username】/catkin_ws/devel/setup.bash

2. Run

roscore
rosrun turtlesim turtlesim_node
rosrun learning_topic velocity_publisher

Reference tutorial: Guyue’s 21-Lecture Introduction to ROS GitHub: https://github.com/guyuehome/ros_21_tutorials Bilibili: https://www.bilibili.com/video/BV1zt411G7Vn