In this video we are going to explain about a problem trying to build a ROS C++ node using ros::Time and ros::Duration.

It is because the order of the terms in sum operation matters. You can check in the video it is simple to solve but it can be a trap if you do not pay attention to that during the development.

Step 1. Create a project in ROS Development Studio(ROSDS)

ROSDS helps you follow our tutorial in a fast pace without dealing without setting up an environment locally. If you haven’t had an account yet, you can create a free account here. Let’s call the project ros time error.

Step 2. Create a package

Let’s start by creating a ROS package for our code

cd ~/catkin_ws/src
catkin_create_pkg mypkg roscpp geometry_msgs 

Then we call the source file mypkg_node.cpp and put it under the mypkg/src folder with the content from the question.

// This program publishes randomly-generated velocity

// messages for turtlesim.
#include <ros/ros.h>
#include <geometry_msgs/Twist.h>  // For geometry_msgs::Twist
#include <stdlib.h> // For rand() and RAND_MAX

int main(int argc, char **argv) {



  ros::init(argc, argv, "publish_velocity");

  ros::NodeHandle nh; // handles node


  ros::Publisher pub = nh.advertise<geometry_msgs::Twist>(
    "turtle1/cmd_vel", 1000); //links publisher to the node


  while(ros::ok()) {

    geometry_msgs::Twist msg; //Object msgs

    msg.linear.x = 0.5; 
    msg.angular.z = 0;
    msg.linear.z = 0.5;

    // Time tracking
    ros::Time beginTime = ros::Time::now();
    ros::Duration MessageTime = ros::Duration(3);
    ros::Time endTime = MessageTime + beginTime; 

    while(ros::Time::now() < endTime){
        pub.publish(msg);


        ROS_INFO_STREAM("Sending velocity command:"
          << " linear=" << msg.linear.x
          << " angular=" << msg.angular.z
           <<"linearZ="<<msg.linear.z);


        ros::Duration(0.3).sleep();

    }


  }
}

To compile the code, we need to at first uncomment the add_executable and target_link_libraries in the CMakeLists.txt. After that, we can compile it with

cd ~/catkin_ws
catkin_make

However, some error related to ROS time occurs. After checking the documentation ROS C++ Time and Duration, we found out that the order is not quite right. The following part should be changed.

...
    ros::Time endTime = beginTime +  MessageTime; 
...

If we compile again, the problem is solved.

Want to learn more?

If you are interested in using ROS with C++, please check our ROS Basics in 5 days(C++) course for more information.

 

Edit by: Tony Huang

RELATED LINKS

▸ The original question on ROS_Answers
▸ ROS Development Studio (ROSDS)
▸ Robot Ignite Academy
▸ ROS C++ Time and Duration

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