MATLAB Examples

Explore Basic Behavior of the TurtleBot

Contents

Introduction

This example helps you to explore basic autonomy with the TurtleBot®. The described behavior drives the robot forward and changes its direction when there is an obstacle. You will subscribe to the laser scan topic and publish the velocity topic to control the TurtleBot.

Prerequisites: docid:robotics_examples.example-TurtleBotCommunicationExample

Hardware Support Package for TurtleBot

This example gives an overview of working with a TurtleBot using its native ROS interface. The Robotics System Toolbox™ Support Package for TurtleBot®-Based Robots provides a more streamlined interface to TurtleBot. It allows you to:

  • Acquire sensor data and send control commands without explicitly calling ROS commands
  • Communicate transparently with a simulated robot in Gazebo or with a physical TurtleBot

To install the support package, open Add-Ons > Get Hardware Support Packages on the MATLAB Home tab and select "TurtleBot-Based Robots". Alternatively, use the roboticsAddons command.

Connect to the TurtleBot

Make sure you have a TurtleBot running either in simulation through Gazebo® or on real hardware. Refer to docid:robotics_examples.example-GettingStartedWithGazeboExample or docid:robotics_examples.example-GettingStartedWithRealTurtleBotExample for the startup procedure. Any Gazebo world works while running in simulation, however, Gazebo TurtleBot World is the most interesting for the purposes of this example.

  • Initialize ROS. Connect to the TurtleBot by replacing the sample IP address (192.168.1.1) with the IP address of the TurtleBot.
ipaddress = '192.168.1.1'
rosinit(ipaddress)
  • Create a publisher for the robot's velocity and create a message for that topic.
  robot = rospublisher('/mobile_base/commands/velocity');
  velmsg = rosmessage(robot);

Receive Scan Data

Make sure that you start the Kinect® camera if you are working with real TurtleBot hardware. That command is: roslaunch turtlebot_bringup 3dsensor.launch. You must execute the command in a terminal on the TurtleBot. The TurtleBot uses the Kinect data to simulate a laser scan that is published on the /scan topic. For the remainder of this example, the term "laser scan" refers to data published on this topic.

  • Subscribe to the topic /scan.
  laser = rossubscriber('/scan');
  • Wait for one laser scan message to arrive and then display it.
  scan = receive(laser,3)
  figure
  plot(scan);

If you see an error, it is possible that the laser scan topic is not receiving any data. If you are running in simulation, try restarting Gazebo. If you are using hardware, make sure that you started the Kinect camera properly.

  • In Gazebo, the scan is similar to this scan:

  • Run the following lines of code, which plot a live laser scan feed for twenty seconds. Move an object in front of the TurtleBot and bring it close enough until it no longer shows up in the plot window. The laser scan has a limited range because of hardware limitations of the Kinect camera. The Kinect has a minimum sensing range of 0.8 meters and a maximum range of 4 meters. Any objects outside these limits will not be detected by the sensor.
  tic;
  while toc < 10
      scan = receive(laser,3);
      plot(scan);
  end

Simple Obstacle Avoidance

Based on the distance readings from the laser scan, you can implement a simple obstacle avoidance algorithm. You can use a simple while loop to implement this behavior.

  • Set some parameters that will be used in the processing loop. You can modify these values for different behavior.
  spinVelocity = 0.6;       % Angular velocity (rad/s)
  forwardVelocity = 0.1;    % Linear velocity (m/s)
  backwardVelocity = -0.02; % Linear velocity (reverse) (m/s)
  distanceThreshold = 0.6;  % Distance threshold (m) for turning
  • Run a loop to move the robot forward and compute the closest obstacles to the robot. When an obstacle is within the limits of the distanceThreshold, the robot turns. This loop stops after 20 seconds of run time. CTRL+C (or Control+C on the Mac) also stops this loop.
  tic;
  while toc < 20
      % Collect information from laser scan
      scan = receive(laser);
      plot(scan);
      data = readCartesian(scan);
      x = data(:,1);
      y = data(:,2);
      % Compute distance of the closest obstacle
      dist = sqrt(x.^2 + y.^2);
      minDist = min(dist);
      % Command robot action
      if minDist < distanceThreshold
          % If close to obstacle, back up slightly and spin
          velmsg.Angular.Z = spinVelocity;
          velmsg.Linear.X = backwardVelocity;
      else
          % Continue on forward path
          velmsg.Linear.X = forwardVelocity;
          velmsg.Angular.Z = 0;
      end
      send(robot,velmsg);
  end

Disconnect from the Robot

  • It is good practice to clear the workspace of publishers, subscribers, and other ROS related objects when you are finished with them.
clear
  • It is recommended to use rosshutdown once you are done working with the ROS network. Shut down the global node and disconnect from the TurtleBot.
  rosshutdown

More Information

The laser scan has a minimum range at which it no longer sees objects in its way. That minimum is somewhere around 0.5 meters from the Kinect camera.

The laser scan cannot detect glass walls. Following is an image from the Kinect camera:

Here is the corresponding laser scan:

The trash can is visible, but you cannot see the glass wall. When you use the TurtleBot in areas with windows or walls that the TurtleBot might not be able to detect, be aware of the limitations of the laser scan.

Next Steps