Examples of ros communication methods

ros workspace and feature pack

** All ROS programs, including those developed by ourselves, are organized
into functional packages ** ROS functional packages are stored in the workspace.
** Therefore, before we write the program, the first step is to create a workspace to accommodate our functional package. The ROS workspace is a directory under linux. To create a ROS workspace is to create a linux directory (by default we create a workspace named catkin_ws), but you need to add a src subdirectory to this directory according to the ROS specification. Then execute a ROS command.
For details, please refer to:
https://blog.csdn.net/wwwlyj123321/article/details/83147242

Topic communication

Create a function package hello in the workspace directory

catkin_create_pkg hello roscpp

The function package generates a hello folder in the src directory of the workspace

Create the hello.cpp
content in the src directory of the function package hello :

#include "ros/ros.h"
#include<iostream>
using namespace std;
int main(int argc, char **argv)
{
    
    
    cout<<"hello ros!"<<endl;
    return 0;
 
 
}

Modify the CMakeLists.txt file in the hello package directory.
Open the file and add:

## Declare a cpp executable
add_executable(printf_hello src/hello.cpp)

Compile workspace

catkin_make

Open a new terminal and start the ros core

roscore

After opening a new terminal, run

rosrun hello printf_hello

Will appear

hello ros!

Service communication

Create a new function package hello2, create a server node and a client node in its src directory to
add two integers and output the sum

add_two_ints_server.cpp

#include "ros/ros.h"
#include "learning_communication/AddTwoInts.h"
 
// service回调函数，输入参数req，输出参数res
bool add(learning_communication::AddTwoInts::Request  &req,
         learning_communication::AddTwoInts::Response &res)
{
    
    
  // 将输入参数中的请求数据相加，结果放到应答变量中
  res.sum = req.a + req.b;
  ROS_INFO("request: x=%ld, y=%ld", (long int)req.a, (long int)req.b);
  ROS_INFO("sending back response: [%ld]", (long int)res.sum);
  
  return true;
}
 
int main(int argc, char **argv)
{
    
    
  // ROS节点初始化
  ros::init(argc, argv, "add_two_ints_server");
  
  // 创建节点句柄
  ros::NodeHandle n;
 
  // 创建一个名为add_two_ints的server，注册回调函数add()
  ros::ServiceServer service = n.advertiseService("add_two_ints", add);
  
  // 循环等待回调函数
  ROS_INFO("Ready to add two ints.");
  ros::spin();
 
  return 0;
}

add_two_ints_client.cpp

#include <cstdlib>
#include "ros/ros.h"
#include "learning_communication/AddTwoInts.h"
 
int main(int argc, char **argv)
{
    
    
  // ROS节点初始化
  ros::init(argc, argv, "add_two_ints_client");
  
  // 从终端命令行获取两个加数
  if (argc != 3)
  {
    
    
    ROS_INFO("usage: add_two_ints_client X Y");
    return 1;
  }
 
  // 创建节点句柄
  ros::NodeHandle n;
  
  // 创建一个client，请求add_two_int service，service消息类型是learning_communication::AddTwoInts
  ros::ServiceClient client = n.serviceClient<learning_communication::AddTwoInts>("add_two_ints");
  
  // 创建learning_communication::AddTwoInts类型的service消息
  learning_communication::AddTwoInts srv;
  srv.request.a = atoll(argv[1]);
  srv.request.b = atoll(argv[2]);
  
  // 发布service请求，等待加法运算的应答结果
  if (client.call(srv))
  {
    
    
    ROS_INFO("Sum: %ld", (long int)srv.response.sum);
  }
  else
  {
    
    
    ROS_ERROR("Failed to call service add_two_ints");
    return 1;
  }
 
  return 0;
}

Compile the node and add it to the CMakeLists.txt of the function package

add_executable(add_two_ints_server src/add_two_ints_server.cpp)
target_link_libraries(add_two_ints_server ${
    
    catkin_LIBRARIES})
add_dependencies(add_two_ints_server learning_communication_gencpp)
 
add_executable(add_two_ints_client src/add_two_ints_client.cpp)
target_link_libraries(add_two_ints_client ${
    
    catkin_LIBRARIES})
add_dependencies(add_two_ints_client learning_communication_gencpp)

Function package name: learning_communication
compilation

cd ~/catkin_ws/

catkin_make

Run
terminal one

roscore

Terminal two

rosrun learning_communication add_two_ints_server

Terminal three

rosrun learning_communication add_two_ints_client 53 66

Reference:
https://blog.csdn.net/weixin_42237429/article/details/90301566

Rviz installation and use

I
installed ubantu version 18.04
Terminal command

sudo apt-get install ros-melodic-rviz

rosdep install rviz

rosmake rviz

Use rviz to call the computer camera
1. Set the usb controller compatibility to 3.0
2. Start ros

roscore

3. Create a new terminal and run the camera node

rosrun uvc_camera uvc_camera_node

4. Create a new terminal to view camera and image information

rostopic list

5. Print the camera information, then use ctrl+c to stop printing

rostopic echo /camera_info

6. Open the camera interface

rosrun image_view image_view image:=/image_raw

7. Start rviz

rosrun rviz rviz

Click add, select image

Set image topic properties

Then the camera screen will appear in rviz