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Writing a Simple Subscriber for Odometry
Description: Writing a simple subscriber which get position and speed of the Evarobot over ROS system.Tutorial Level: BEGINNER
Use the catkin_create_pkg script to create a new package called 'evarobot_odom_subs' which depends on nav_msgs, roscpp, and rospy:
> cd ~/catkin_ws/src > catkin_create_pkg evarobot_odom_subs nav_msgs rospy roscpp
Create a src directory in the evarobot_odom_subs package directory.
> mkdir -p ~/catkin_ws/src/evarobot_odom_subs/src
Create the src/odom_listener.cpp file within the evarobot_odom_subs package.
> cd ~/catkin_ws/src/evarobot_odom_subs/src > gedit odom_listener.cpp
And paste the following inside odom_listener.cpp:
1 #include "ros/ros.h"
2 #include "nav_msgs/Odometry.h"
3
4 /**
5 * This tutorial demonstrates simple receipt of position and speed of the Evarobot over the ROS system.
6 */
7
8 /**
9 * Callback function executes when new topic data comes.
10 * Task of the callback function is to print data to screen.
11 */
12 void chatterCallback(const nav_msgs::Odometry::ConstPtr& msg)
13 {
14 ROS_INFO("Seq: [%d]", msg->header.seq);
15 ROS_INFO("Position-> x: [%f], y: [%f], z: [%f]", msg->pose.pose.position.x,msg->pose.pose.position.y, msg->pose.pose.position.z);
16 ROS_INFO("Orientation-> x: [%f], y: [%f], z: [%f], w: [%f]", msg->pose.pose.orientation.x, msg->pose.pose.orientation.y, msg->pose.pose.orientation.z, msg->pose.pose.orientation.w);
17 ROS_INFO("Vel-> Linear: [%f], Angular: [%f]", msg->twist.twist.linear.x,msg->twist.twist.angular.z);
18 }
19
20 int main(int argc, char **argv)
21 {
22 /**
23 * The ros::init() function needs to see argc and argv so that it can perform
24 * any ROS arguments and name remapping that were provided at the command line.
25 * For programmatic remappings you can use a different version of init() which takes
26 * remappings directly, but for most command-line programs, passing argc and argv is
27 * the easiest way to do it. The third argument to init() is the name of the node.
28 *
29 * You must call one of the versions of ros::init() before using any other
30 * part of the ROS system.
31 */
32 ros::init(argc, argv, "odom_listener");
33
34 /**
35 * NodeHandle is the main access point to communications with the ROS system.
36 * The first NodeHandle constructed will fully initialize this node, and the last
37 * NodeHandle destructed will close down the node.
38 */
39 ros::NodeHandle n;
40
41 /**
42 * The subscribe() call is how you tell ROS that you want to receive messages
43 * on a given topic. This invokes a call to the ROS
44 * master node, which keeps a registry of who is publishing and who
45 * is subscribing. Messages are passed to a callback function, here
46 * called chatterCallback. subscribe() returns a Subscriber object that you
47 * must hold on to until you want to unsubscribe. When all copies of the Subscriber
48 * object go out of scope, this callback will automatically be unsubscribed from
49 * this topic.
50 *
51 * The second parameter to the subscribe() function is the size of the message
52 * queue. If messages are arriving faster than they are being processed, this
53 * is the number of messages that will be buffered up before beginning to throw
54 * away the oldest ones.
55 */
56 ros::Subscriber sub = n.subscribe("odom", 1000, chatterCallback);
57
58 /**
59 * ros::spin() will enter a loop, pumping callbacks. With this version, all
60 * callbacks will be called from within this thread (the main one). ros::spin()
61 * will exit when Ctrl-C is pressed, or the node is shutdown by the master.
62 */
63 ros::spin();
64
65 return 0;
66 }
> cd .. > gedit CMakeLists.txt
The generated CMakeLists.txt should look like this
cmake_minimum_required(VERSION 2.8.3) project(evarobot_odom_subs) find_package(catkin REQUIRED COMPONENTS nav_msgs roscpp rospy ) catkin_package() include_directories( ${catkin_INCLUDE_DIRS} ) add_executable(odom_listener src/odom_listener.cpp) add_dependencies(odom_listener nav_msgs_generate_messages_cpp) target_link_libraries(odom_listener ${catkin_LIBRARIES} )
Now run catkin_make
> cd ~/catkin_ws/ > catkin_make
To run odom_listener,
> rosrun evarobot_odom_subs odom_listener