In this project, a-star algorithm is simulated on a differential drive (non-holonomic) mobile robot in a defined static world. We are using Turtlebot in ROS-Gazebo for the simulation. The project generates a text file containing linear and angular velocities needed by the robot to reach its goal position. A ROS node publishes these velocities at regular intervals to simulate Turtlebot's movement in Gazebo. A sample of the simulation from one end of the map to the other is shown below.

Figure 1 - Turtlebot-3 Burger moving in known world using velocities provided by A* planner

The project has been developed over 4 phases:

• Phase-1: Installation of ROS and V-REP
• Phase-2: Implementation of A* on a non-holonomic robot
• Phase-3: Implementation of A* on a differential-drive robot
• Phase-4: Implementation of A* on Turtlebot using ROS

Phases 1 and 2 have been implemented on another repository. A sample output for phase-3 of the project can be found here

Also, this project benefitted from my learnings while working on the Optimal Path Finder project that implements path-finding algorithms such as Breadth-First Search (BFS), Dijkstra, and A* in a 2D-grid environment.

• Umang Rastogi
• Naman Gupta

• Add path smoothing

• Ubuntu 16.04/18.04
• ROS Kinetic/Melodic
• Gazebo
• Turtlebot3 Packages
• Python Packages: Numpy, OpenCV-Python, Math, Queue

• Install Python3, Python3-tk, and the necessary libraries: (if not already installed)

sudo apt install python3 python3-tk
sudo apt install python3-pip
pip3 install numpy opencv-python

• Check if your system successfully installed all the dependencies
• Open terminal using Ctrl+Alt+T and enter python3.
• The terminal should now present a new area represented by >>> to enter python commands
• Now use the following commands to check libraries: (Exit python window using Ctrl+Z if an error pops up while running the below commands)

import tkinter
import numpy
import cv2
import math
import queue

• If you want to work on Ubuntu 16.04, you will need to install ROS Kinetic by following the instructions given on referenced web-page.
• If you want to work on Ubuntu 18.04, you will need to install ROS Melodic by following the instructions given on referenced web-page.
• We recommend installing the full-desktop version of ROS because it automatically installs the latest compatible version of Gazebo on your system.
• If you wish to install Gazebo separately, then follow the instruction on the Gazebo install page.
• Install Turtlebot-3 package and its dependencies:

cd ~/<ROS_Workspace>/src
git clone https://github.com/ROBOTIS-GIT/turtlebot3.git
git clone https://github.com/ROBOTIS-GIT/turtlebot3_msgs.git
git clone https://github.com/ROBOTIS-GIT/turtlebot3_simulations.git
cd  ../ && catkin_make
sudo su
cp -r src/turtlebot3/ /opt/ros/<distro>/share/
cp -r src/turtlebot3_msgs/ /opt/ros/<distro>/share/
cp -r src/turtlebot3_simulations/ /opt/ros/<distro>/share/

• Using the same terminal window, check installation of turtlebot3:

source devel/setup.bash
roslaunch turtlebot3_gazebo turtlebot3_world.launch

• Note that Turtlebot-3 package is supported by ROS Melodic as well as Kinetic.

In phase-3, a video output was generated to show the exploration and final path a differential-drive robot might take using the OpenCV library. The output for one case is presented below.

Figure 2 - Exploration of a differential-drive robot using A* to find optimal path from start to goal

• Clone the repository in your ROS workspace. Type

cd ~/<ROS_Workspace>/src
git clone https://github.com/urastogi885/a-star-turtlebot

• If you have a compressed version of the project, extract it, go into project directory, open the terminal, and type:

python3 a_star_turtlebot.py start_x,start_y,start_orientation goal_x,goal_y rpm1,rpm2,clearance animation
python3 a_star_turtlebot.py -4,-4.5,0 4.25,2.75 30,25,5 0

• This would generate a text file that will be used to run the Turtlebot-3 in Gazebo.
• Note the following to try the code with other inputs:
  • The origin of the map is taken at the center of the map s make sure to take into consideration in which quadrant your points lie.
  • Provide start and goal coordinates in meters while start orientation is to be provided in degrees.
  • In addition to that, clearance is taken in centimeters.
  • Refer documentation of a_star_turtlebot.py to understand further about input arguments.
  • Set animation to 1 to generate an exploration output as shown in Phase3 Output. Note that this drastically increases exploration time.
• Launch the world, spawn turtlebot, navigate it to the desired goal point. Type:

cd ~/<ROS_Workspace>
source devel/setup.bash
catkin_make or catkin build (For ROS Melodic, you might have to use catkin build instead of catkin_make)
roslaunch a-star-turtlebot launcher_1.launch

• Stop the execution, using Ctrl + C. Do not close this terminal.
• Open planner.py from the scripts folder, replace commander_1.txt by commander_2.txt in the main section of the file.
• Essentially, the following line needs to be changed in planner.py:

with open(ros_root.get_path('a-star-turtlebot') + '/output_files/commander_1.txt', 'r') as command:

• From the same terminal, run:

roslaunch a-star-turtlebot launcher_2.launch