8、 Topic communication: topic substitution and monitoring

1、 brief introduction

When we run the little turtle case , We can control its operation through the direction key , How can we control its operation through our own code . The method is actually very simple , Because the case of little turtle is through topic communication , Then we can get the topic of the communication through relevant commands , The message types and details contained in the topic , Then set the corresponding topic through the code .

2、 Publisher implementation steps

1. Topic acquisition

1. By calculating the graph
   rqt_graph
2. adopt rostopic list
   Return results ：

   rostopic list
   /rosout
   /rosout_agg
   /statistics
   /turtle1/cmd_vel
   /turtle1/color_sensor
   /turtle1/pose
   

2. Information access

• rostopic type The node name / Topic name
• Return results ：

  rostopic type /turtle1/cmd_vel 
  geometry_msgs/Twist
  

3. Get message format

• rosmsg info Message name
• Return results ：

  rosmsg info geometry_msgs/Twist
  geometry_msgs/Vector3 linear
  float64 x
  float64 y
  float64 z
  geometry_msgs/Vector3 angular
  float64 x
  float64 y
  float64 z
  

among ,linear Represents linear velocity ,angular For angular velocity , In radians . In this project , Linear speed only X Direction , Angular velocity is only yaw angular velocity .

• 1 radian ： When the arc length is equal to the radius, the corresponding angle is 1 radian
• Three angles ：
  • Roll angle ： Around the X Shaft rotation
  • Pitch angle ： Around the Y Shaft rotation
  • Yaw angle ： Around the Z Shaft rotation

4. Code implementation

1. C++

   /*  The goal is ： Through node replacement , Control the movement of the little turtle   Node replacement refers to the replacement of the topic corresponding to the topic , So you need to know in advance ： 1. Topic name :/turtle1/cmd_vel rqt_graph rostopic list 2. The message type of the topic :geometry_msgs/Twist rostopic type  Topic name  3. Specific information of the topic message : geometry_msgs/Vector3 linear float64 x float64 y float64 z geometry_msgs/Vector3 angular float64 x float64 y float64 z rosmsg info  Message name  */
   //1. Include header file 
   #include"ros/ros.h"
   // Contains header files with corresponding topic message types 
   #include"geometry_msgs/Twist.h"
   
   int main(int argc, char *argv[]){
         
       setlocale(LC_ALL,"");
       //2. initialization ros node 
       ros::init(argc,argv,"turtle_control");
       ros::NodeHandle nh;
       //3. Create publisher object 
       ros::Publisher pub = nh.advertise<geometry_msgs::Twist>("/turtle1/cmd_vel",1000);
       //4. Release control information 
       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 = 1.0;
       ros::Rate r(1);
       while(ros::ok()){
         
           pub.publish(msg);
           ros::spinOnce();
       }
       return 0;
   }
   

2. Python

   #! /usr/bin/env python
   
   from re import T
   from turtle import pu
   import rospy
   from geometry_msgs.msg import Twist
   
   if __name__ == "__main__":
       # initialization ros node 
       rospy.init_node("turtle_control_p")
       # Create publisher object 
       pub = rospy.Publisher("/turtle1/cmd_vel",Twist,queue_size=1000)
       # Publish the information 
       rate = rospy.Rate(10)
       msg = Twist()
       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 = 1.0
   
       # Circular Publishing  rospy.is_shutdown(): Judge whether the node is closed 
       while not rospy.is_shutdown():
           pub.publish(msg)
           rate.sleep()
   

3、 Subscriber implementation steps

1. Get topics and news

• rostopic list
• Return results ：

  rostopic list
  /rosout
  /rosout_agg
  /turtle1/cmd_vel
  /turtle1/color_sensor
  /turtle1/pose
  
  
  [email protected]:/media/d102/EPAN/Desktop/code_study_ubuntu/rosdemo_05$ rostopic type /turtle1/pose 
  turtlesim/Pose
  
  
  [email protected]:/media/d102/EPAN/Desktop/code_study_ubuntu/rosdemo_05$ rosmsg info turtlesim/Pose 
  float32 x
  float32 y
  float32 theta
  float32 linear_velocity
  float32 angular_velocity
  

2. Changing configuration files

In this project , Due to the use of turtlesim Function pack , Therefore, you need to add the function package dependency , modify CMakeLists.txt file ：

find_package(catkin REQUIRED COMPONENTS
  roscpp
  rospy
  std_msgs
  turtlesim
)

3. Code implementation

1. C++

   /*  topic of conversation ：/turtle1/pose  Message type ：turtlesim/Pose */
   #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,"");
       ros::init(argc,argv,"turtle_pose_sub");
       ros::NodeHandle nh;
       ros::Subscriber sub = nh.subscribe<turtlesim::Pose>("/turtle1/pose",1000,doPose);
       ros::spin();
       return 0;
   }
   

2. Python

#! /usr/bin/env python
"""  The goal is ： Get the location of the little turtle , And print it out   topic of conversation ：/turtle1/pose  Message type :turtlesim/Pose  Preparation before operation :turtle_start.launch """
import rospy
from turtlesim.msg import Pose
def doPose(data1):#data1 Is a formal parameter 
    rospy.loginfo("x=%.2f,y=%.2f,theta=%.2f",data1.x,data1.y,data1.theta)
if __name__ == "__main__":
    rospy.init_node("turtle_pose_p")
    sub = rospy.Subscriber("/turtle1/pose",Pose,doPose,queue_size=1000)
    rospy.spin()