[ROS Q&A] 124 – Range sensor does not detect obstacles in Gazebo

22/05/2018

In this video, we are answering a question about why the range sensor plugin for Gazebo is not detecting obstacles. The answer to that problem will surprise you !!! (ha, ha, ha).

In this posting, we will go through a demo of laser data visualization

NOTE: This article assumes that the audience is familiar with the following:

Running scripts on the terminal
Ros filesystem layout (what goes where launch dir, src dir etc)
RDS Environment (How to launch a project, how to navigate the directories.

Step – 1. Creating a new project

Head to https://rds.theconstructsim.com/ (if not there already)

Create a new project. Choose an informative name and add some description in the description field.

Load the project. (This can take a few moments to a minute depending on your internet)

Run the IDE from the Tools menu and verify the filesystem layout it should have a similar structure

root ├── ai_ws ├── catkin_ws │ ├── build │ ├── devel │ └── src ├── notebook_ws │ ├── default.ipynb │ └── images └── simulation_ws ├── build ├── devel └── src

Step – 2. Cloning the existing source

Start the shell, browse to catkin_ws/src
Clone the git repository located at https://github.com/erenaud3/fakeLaserScan (more information on cloning a git-repo is available here

$ git clone https://github.com/erenaud3/fakeLaserScan
The filesystem tree should look like the following

root ├── ai_ws ├── catkin_ws │ ├── build │ ├── devel │ └── src │ ├── CMakeLists.txt │ └── fakeLaserScan │ ├── CMakeLists.txt │ ├── launch │ │ └── fakeLaserScan.launch │ ├── package.xml │ ├── README.md │ └── src │ ├── fakeLaserScan.cpp │ ├── listener.cpp │ └── talker.cpp ├── notebook_ws │ ├── default.ipynb │ └── images └── simulation_ws ├── build ├── devel └── src

Step – 3. Briefly skim through the code and run it

First, we go through the contents of package.xml (located inside the fakeLaserScan directory) which has the following content
(NOTE comments are removed in this reproduced code snippet for brevity)

<?xml version="1.0"?>
<package>
  <name>beginner_tutorials</name>
  <version>0.0.0</version>
  <description>The beginner_tutorials package</description>

  <maintainer email="user@todo.todo">user</maintainer>

  <license>TODO</license>

  <buildtool_depend>catkin</buildtool_depend>
  <build_depend>roscpp</build_depend> 
  <build_depend>rospy</build_depend> 
  <build_depend>sensor_msgs</build_depend> 
  <build_depend>std_msgs</build_depend> 
  <build_export_depend>roscpp</build_export_depend> 
  <build_export_depend>rospy</build_export_depend> 
  <build_export_depend>sensor_msgs</build_export_depend> 
  <build_export_depend>std_msgs</build_export_depend> 
  <exec_depend>roscpp</exec_depend> 
  <exec_depend>rospy</exec_depend> 
  <exec_depend>sensor_msgs</exec_depend> 
  <exec_depend>std_msgs</exec_depend>

  <export>
  </export>
</package>

This file contains various pieces of information about the project such as the package dependencies (roscpp, rospy, sensor_msgs, std_msgs) project name, version, license, maintainer etc. Some of this information is vital for building the project (like the dependencies). While other information is important from the perspective of sharing one’s work with the community.

In the given contents of the package.xml file, we notice that the project name specified in the file (beginner_tutorials) is different from the directory name (fakeLaserScanner). While this will not cause the project to crash but it is inconsistent with the project naming guideline.

Next, we take a look at the code inside fakeLaserScan.cpp

#include <ros/ros.h>
#include <sensor_msgs/LaserScan.h>
​
int main(int argc, char** argv){
  ros::init(argc, argv, "fake_laser_scan_publisher");
​
  ros::NodeHandle n;
  ros::Publisher scan_pub = n.advertise<sensor_msgs::LaserScan>("fakeScan", 50);
​
  unsigned int num_readings = 100;
  double laser_frequency = 40;
  double ranges[num_readings];
  double intensities[num_readings];
​
  int count = 0;
  ros::Rate r(1.0);
  while(n.ok()){
    //generate some fake data for our laser scan
    for(unsigned int i = 0; i < num_readings; ++i){
      ranges[i] = 5;
      intensities[i] = 100 + count;
    }
    ros::Time scan_time = ros::Time::now();
​
    //populate the LaserScan message
    sensor_msgs::LaserScan scan;
    scan.header.stamp = scan_time;
    scan.header.frame_id = "fake_laser_frame";
    scan.angle_min = -1.57;
    scan.angle_max = 1.57;
    scan.angle_increment = 3.14 / num_readings;
    scan.time_increment = (1 / laser_frequency) / (num_readings);
    scan.range_min = 0.0;
    scan.range_max = 100.0;
​
    scan.scan_time = (1 / laser_frequency);
​
    scan.ranges.resize(num_readings);
    scan.intensities.resize(num_readings);
    for(unsigned int i = 0; i < num_readings; ++i){
      scan.ranges[i] = ranges[i];
      scan.intensities[i] = intensities[i];
    }
​
    scan_pub.publish(scan);
    ++count;
    r.sleep();
  }
}

This code creates a publisher with the name fakeScan

The scan values are being generated inside a for loop (line 15-18 (excluding the blank lines))

These scan values are then copied to a LaserScan message (line 20, 32-35)

The message is being published at line 36. Also, note that the publish rate is 1 Hz (line 12)

As all seems okay we will go ahead and build the project. On the console browse to the catkin_ws directory and then run

$ catkin_make

This should proceed without any errors.

Next, we launch the project with the following command

$ roslaunch beginner_project fakeLaserScan.launch

This also should proceed without any errors.

Now we will open another console from the Tools menu and verify the topics with following commands

$ rostopic list

Outputs :

/fakeScan
/rosout
/rosout_agg

In the result, we see out topic /fakeScan appears. Further, we can check if the topic is publishing the correct information or not with the following command