The practice of topic communication is mainly through ROS Built in turtlesim Case study , Combined with the introduction of ROS Command to get the node 、 topic of conversation 、 The control of tortoise movement is realized by topic news .

1 Topic Publishing

Requirements describe ： The tortoise motion control is realized by coding , Let the little turtle do circular motion .

Implementation analysis ：

• Realize the control of the tortoise , There are two key nodes (Node)： One is the tortoise motion display node turtlesim_node, The other is the keyboard control node , The two communicate through topic communication .
• Before implementing the control node , You need to know the topics and messages used by node control , have access to ros Relevant orders and rqt Calculate the diagram to obtain .
• After knowing the topic and news , adopt C++ or Python Write node motion control , Through the specified topic , Just publish the message according to a certain logic .

Implementation process ：

• start-up tutlesim node .
• Through the calculation diagram, combined with ros Command to get topic and message information .
• Coding to achieve motion control node .
• start-up roscore、turtlesim_node And custom control nodes , View the run results .

1.1 start-up tutlesim node

• Start three terminals
• terminal 1 type ：roscore
• terminal 2 type ：rosrun turtlesim turtlesim_node
• terminal 3 keyboard ：rosrun turtlesim turtle_teleop_key
  
  Be careful ： The cursor must be focused in the keyboard controlled terminal , Otherwise you can't control the tortoise's movement .

1.2 Topic and message acquisition

1.2.1 Topic acquisition

Check the topic through the calculation chart ：

rqt_graph

Or by rostopic List topics :

rostopic list

1.2.2 Information access

Get message type :

rostopic type /turtle1/cmd_vel
output：
		geometry_msgs/Twist

Get message format :

rosmsg info geometry_msgs/Twist

output：
		geometry_msgs/Vector3 linear         ## xyz Linear velocity in direction 
		  float64 x
		  float64 y
		  float64 z
		geometry_msgs/Vector3 angular		 ## x Roll on the shaft 、y Pitch and on axis z The angular velocity of yaw on the axis 
		  float64 x
		  float64 y
		  float64 z

1.3 Implement the publishing node (C++)

To create a function package, you need to rely on the function package : roscpp rospy std_msgs geometry_msgs

/*  To write  ROS  node , Control the little turtle to draw a circle   preparation : 1. obtain topic( It is known that : /turtle1/cmd_vel) 2. Get message type ( It is known that : geometry_msgs/Twist) 3. Before running , Pay attention to start first  turtlesim_node  node   Implementation process : 1. Include header file  2. initialization  ROS  node  3. Create publisher object  4. Circularly issue motion control messages  */

#include "ros/ros.h"
#include "geometry_msgs/Twist.h"

int main(int argc, char *argv[])
{
    
    setlocale(LC_ALL,"");
    // 2. initialization  ROS  node 
    ros::init(argc,argv,"control");
    ros::NodeHandle nh;
    // 3. Create publisher object 
    ros::Publisher pub = nh.advertise<geometry_msgs::Twist>("/turtle1/cmd_vel",1000);
    // 4. Circularly issue motion control messages 
    //4-1. Organize messages 
    geometry_msgs::Twist msg;
    msg.linear.x = 1.0;
    msg.linear.y = 0.0;
    msg.linear.z = 0.0;

    msg.angular.x = 0.0;
    msg.angular.y = 0.0;
    msg.angular.z = 2.0;

    //4-2. Set the transmission frequency 
    ros::Rate r(10);
    //4-3. Cycle to send 
    while (ros::ok())
    {
    
        pub.publish(msg);

        ros::spinOnce();
    }


    return 0;
}

The configuration file is the same as ROS series ( Four )：ROS Communication mechanism series (1)： Topic communication Publisher profile in .

1.4 function

Run the same as this article 1.1 start-up tutlesim node .

• First , start-up roscore;
• Then start the tortoise display node ;
• Finally, execute the motion control node .

The running results are as follows ：

2 Topic subscription

Requirements describe :
It is known that turtlesim The tortoise display node in , Will release the current turtle's pose ( The coordinates and orientation of the tortoise in the form ), Request control of tortoise movement , And print the current turtle's posture from time to time .
Implementation process :

1. adopt ros Command to get topic and message information .
2. Coding to achieve pose acquisition node .
3. start-up roscore、turtlesim_node 、 Control node and pose subscription node , Control the movement of the tortoise and output the posture of the tortoise .

2.1 Topic and message acquisition

 Get the topic :/turtle1/pose：
	rostopic list
 Get message type :turtlesim/Pose
	rostopic type  /turtle1/pose
 Get message format :
	rosmsg info turtlesim/Pose
	 result ：
	​float32 x
	float32 y
	float32 theta
	float32 linear_velocity
	float32 angular_velocity

2.2 Implement the subscription node (C++)

To create a function package, you need to rely on the function package : roscpp rospy std_msgs turtlesim

/*  Subscribe to the posture of the little turtle :  Get the coordinates of the little turtle in the form from time to time and print   preparation : 1. Get topic name  /turtle1/pose 2. Get message type  turtlesim/Pose 3. Start before operation  turtlesim_node  And  turtle_teleop_key  node   Implementation process : 1. Include header file  2. initialization  ROS  node  3. establish  ROS  Handle  4. Create subscriber object  5. The callback function processes the subscribed data  6.spin */

#include "ros/ros.h"
#include "turtlesim/Pose.h"

void doPose(const turtlesim::Pose::ConstPtr& p){
    
    ROS_INFO(" Turtle pose information :x=%.2f,y=%.2f,theta=%.2f,lv=%.2f,av=%.2f",
        p->x,p->y,p->theta,p->linear_velocity,p->angular_velocity
    );
}

int main(int argc, char *argv[])
{
    
    setlocale(LC_ALL,"");
    // 2. initialization  ROS  node 
    ros::init(argc,argv,"sub_pose");
    // 3. establish  ROS  Handle 
    ros::NodeHandle nh;
    // 4. Create subscriber object 
    ros::Subscriber sub = nh.subscribe<turtlesim::Pose>("/turtle1/pose",1000,doPose);
    // 5. The callback function processes the subscribed data 
    // 6.spin
    ros::spin();
    return 0;
}

The configuration file is the same as ROS series ( Four )：ROS Communication mechanism series (1)： Topic communication Subscriber profile in .

2.3 function

• First , start-up roscore;
• Then start the tortoise display node , Execute motion control node ;
• Finally, start the tortoise pose subscription node .