http://www.theconstructsim.com/read-laserscan-data/

We can reproduce the question easily using RDS. If you haven’t had an account yet, you can register one for free here. After registration, you can log in and open the project from Public(click on my projects to switch to public simulations) -> Kobuki. In the project, click simulations -> Turtlebot 2 to launch the simulation.

The simulation is up and running now. You can check all the topic available with the command

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$ rostopic list

The LaserScan topic is called /kobuki/laser/scan. You can type the following command into the terminal to check the topic.

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$ rostopic echo /kobuki/lase/scan -n1

The -n1 flag prints the topic exactly once. You’ll get something like this.

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header:

  seq: 5

  stamp:

    secs: 2829

    nsecs:  69000000

  frame_id: "laser_sensor_link"

angle_min: -1.57079994678

angle_max: 1.57079994678

angle_increment: 0.00436940183863

time_increment: 0.0

scan_time: 0.0

range_min: 0.10000000149

range_max: 30.0

ranges: [inf, inf, inf, inf, inf, ….]

The following command will give you the information for the topic

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rostopic info /kobuki/laser/scan

You’ll see that the topic is using the message type called sensor_msgs/LaserScan. You can further check it with the command

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rosmsg show sensor_msgs/LaserScan

The command gives you the message’s structure.

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std_msgs/Header header

  uint32 seq

  time stamp

  string frame_id

float32 angle_min

float32 angle_max

float32 angle_increment

float32 time_increment

float32 scan_time

float32 range_min

float32 range_max

float32[] ranges

float32[] intensities

The angle_min and angle_max indicate the angle range(from -90 to 90 degree in this case) that the LaserScan is measuring and the ranges is an array which gives you the distance measured for each angle bin.

To explore the range value, let’s create a package.

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$ cd ~/catkin_ws/src

$ catkin_create_pkg laser_values rospy

$ cd laser_values

$ mkdir launch

Add a python script under src with the following code

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#! /usr/bin/env python

import rospy

from sensor_msgs.msg import LaserScan

def callback(msg):

    print len(msg.ranges)

rospy.init_node('scan_values')

sub = rospy.Subscriber('/kobuki/laser/scan', LaserScan, callback)

rospy.spin()

Normally, you’ll need to give the script permission to execute with

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$ chmod +x src/scan.py

Then we create a launch file under lunch directory to launch the script

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Now you can type

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roslaunch laser_values laser.launch

to launch the script. You should also see that the length of the ranges array is 720 in the 180-degree range. So if we want to read the LaserScan data on the left, in front and on the right of the robot, we can change our script a bit.

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…

def callback(msg):

    # values at 0 degree

    print msg.ranges[]

    # values at 90 degree

    print msg.ranges[360]

    # values at 180 degree

    print msg.ranges[719]

…

That’s all now you get the value. You can play with it to navigate the robot. If you want to learn more about this topic, you can go to Robot Ignite Academy. We have tons of courses which teach you how to navigate the robot in a more advanced way.