ROS-Publisher

Study note for ROS publisher.

Model of Topic

• Publisher(turtle Velocity)
• • –[Message(geometry_msgs::Twist)]–>
• Topic (/turtle1/cmd_vel)
• • –[Message(geometry_msgs::Twist)]–>
• Subscriber(turtlesim)

Programming realization

Take turtlesim as the example.
We write a program acting like a publisher and sending controlling messages.

Create package

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

can see:

$ ls
CMakeLists.txt  include  package.xml  src

How to realize a publisher?

• Initialize ROS node
• register node info from ROS Master, including name of the topic and msg type
• create message data
• publish message at a specific frequency

Code

create velocity_publisher.cpp file in learning_topic/src

#include <ros/ros.h>
#include <geometry_msgs/Twist.h>

int main(int argc, char *argv[])
{
    //init the node
    ros::init(argc,argv,"velocitypublisher");

    //create node handle
    ros::NodeHandle n;

    //create a publisher, who publish topic names '/turtle1/cmd_vel', message type 'geometry_msgs:Twist', queue length 10
    ros::Publisher turtle_vel_pub = n.advertise<geometry_msgs::Twist>("/turtle1/cmd_vel",10);

    //set loop rate
    ros::Rate loop_rate(10);
    while (ros::ok()) {
        //initialize geometry_msgs:Twist msg
        geometry_msgs::Twist vel_msg;
        vel_msg.linear.x = 0.5;
        vel_msg.angular.z = 0.2;

        //publish msgs
        turtle_vel_pub.publish(vel_msg);
        ROS_INFO("Publish turtle velocity command at %0.2f m/s, %0.2f rad/s",
                vel_msg.linear.x,vel_msg.angular.z);

        //delay
        loop_rate.sleep();
    }
    return 0;
}

What is queue length?

• msgs can be published very fast while the data transmission rate is limited, so the queue act as a buffer
• first in, first thrown, keeping the remained msgs in the queue relatively new

Configure the compile rule of publisher

Steps:

• set the code need to be complied and the generated excutable file
• set the link library

Add two lines to learning\_topic/CMakeLists.txt (in build part):

add_executable(velocity_publisher src/velocity_publisher.cpp)
target_link_libraries(velocity_publisher ${catkin_LIBRARIES})

Compile

$ cd ~/ROS_study/catkin_ws
$ catkin_make
$ source ./devel/setup.zsh
$ roscore
$ rosrun turtlesim turtlesim_node
$ rosrun learning_topic velocity_publisher

Then, you can see turtle:

And outputs:

...
[ INFO] [1650088100.376299177]: Publish turtle velocity command at 0.50 m/s, 0.20 rad/s
[ INFO] [1650088100.476312726]: Publish turtle velocity command at 0.50 m/s, 0.20 rad/s
[ INFO] [1650088100.576304995]: Publish turtle velocity command at 0.50 m/s, 0.20 rad/s
...