Hello,my name is Bernardo,I've got a problem and I don't know how to solve it. I want to create a very simple program: the robot advances in a straight line and as it approaches an obstacle, it must decelerate until it stops. The problem is that while I'm subscribing to the laser, the robot (when I start the run) moves at the speed I told him, but when it reaches the minimum distance, the robot doesn't stop and I don't know how to make him do that. Please, I need help. Here's the code:

#include <ros/ros.h>
#include <sensor_msgs/LaserScan.h>
#include <geometry_msgs/Twist.h>


void scanCallback (const sensor_msgs::LaserScan::ConstPtr& scan_msg)
 {
 geometry_msgs::Twist vel; //Declaración de la variable global vel

 ros::NodeHandle n;
 ros::Publisher vel_pub_=n.advertise<geometry_msgs::Twist>("cmd_vel", 1);
 ros::Rate loop_rate(50);// Frecuencia de realializacion del bucle
 ros::Time ahora; 


    for (unsigned int x=0;x< scan_msg->ranges.size();x++){   


             if(scan_msg->ranges[x]>=0.8) {

                vel.linear.x = 0.3;//velocidad de avance
                vel.angular.z = 0.0;//velocidad de giro
                vel_pub_.publish(vel);

                }


              ahora = ros::Time::now();
              if(scan_msg->ranges[x]<=0.7){   

                 while (ros::Time::now() < (ahora + ros::Duration(0.5))){

                    vel.linear.x = 0.05;//velocidad de avance
                    vel.angular.z = 0.0;//velocidad de giro
                    vel_pub_.publish(vel);
                    loop_rate.sleep();

}
               }

               ahora = ros::Time::now(); 
               if(scan_msg->ranges[x]<= 0.6){

                    while (ros::Time::now() < (ahora + ros::Duration(0.5))){ 
                    vel.linear.x = 0.0;//velocidad de avance
                    vel.angular.z = 0.0;//velocidad de giro
                    vel_pub_.publish(vel);}
                    loop_rate.sleep();

                }


     }
 }
int main(int argc, char** argv){
  ros::init(argc, argv, "DistanceObstacle");

  ros::NodeHandle n;
  ros::Publisher vel_pub_=n.advertise<geometry_msgs::Twist>("cmd_vel", 1);
  ros::Subscriber scan_sub = n.subscribe("scan", 1, scanCallback);

  ros::spin(); //Mantiene la suscripción al topic hasta que se reciba "Ctrl+C"
  return 0;

}

I've just managed to make my robot to stop when it's at a minimum distance,But, as you told me, I want to reduce the velocity with Vel=vel*0.5, but instead the robot always keeps moving at a constant velocity.What can I do? Thank you for your help

include <ros ros.h="">

include <sensor_msgs laserscan.h="">

include <geometry_msgs twist.h="">

void scanCallback (const sensor_msgs::LaserScan::ConstPtr& scan_msg) { geometry_msgs::Twist vel; //Declaración de la variable global vel

ros::NodeHandle n; ros::Publisher vel_pub_=n.advertise<geometry_msgs::twist>("cmd_vel", 1); ros::Time ahora; double minDist=10;
double angle;
double speed=0.5;

for (unsigned int i=0;i< scan_msg->ranges.size();i++){   

  angle = ((double)i*scan_msg->angle_increment)+scan_msg->angle_min;

   if (scan_msg->ranges[i]< minDist){
       minDist=scan_msg->ranges[i];      
       std::cout << minDist << std::endl;          
   }
 } 

if( minDist<=0.6){

            vel.linear.x = 0.0;//velocidad de avance
            vel.angular.z = 0.0;//velocidad de giro
            vel_pub_.publish(vel);
 }
else {


            speed=0.5*speed;
            vel.linear.x = speed;//velocidad de avance
            vel.angular.z = 0.0;//velocidad de giro
            vel_pub_.publish(vel);
            //std::cout << speed ...