Tuesday, May 31, 2011

First exercise with ROS: a node to interface with Player

I am starting to learn ROS. I needed an exercise and I do not have any robot to play with, except Player/Stage. So I decided to write a ROS node for my Player/Stage robot. ROS already has a stack called stageros that allows to interface with Stage. Mine is different: it allows to interface with Player. Hence it can also be used to interface with Stage (via Player), but with any other Player-enabled robot as well. Anyway, it's not very useful, but it's a good experimentation platform and exercise.

- Linux (I use Debian Wheezy)
- ROS (see ROS tutorials on how to install and learn the basics)
- Player and Stage (see respective tutorials as well, mine are install from source)
- A Player configuration file and world to start a robot in Stage

I assume familiarity with ROS and Player/Stage

The final aim is to be able to use the navigation stack with the Player robot. So I followed the guidelines and how to setup a robot for the navigation stack:

This means that we will need to publish laser data on the base_scan topic, odometry data on the odom topic, publish transform configuration too, and accept velocity commands on the cmd_vel topic.

I wrote 2 versions. One version has everything in one node, the other version is made of 3 smaller nodes (one per task aforementioned). We will first check the one node version.

The code is object oriented. I found the solution better than having global variables, because it allowed better control of initialization order: some stuffs can only be created after ros::init has been called.

First of all, let's create a package.

I have created a directory where I can experiment, i.e. ~/ros_playground. I added this directory to the ROS_PACKAGE_PATH environment variable: in ~/.bashrc, after the source xxx/ros/setup.sh line added when installing ROS (see ROS tutorial), I added the following line:

Then from the ros_playground directory:

roscreate-pkg playerros_1node roscpp tf nav_msgs sensor_msgs geometry_msgs

is the name of my package. The roscreate-pkg creates a directory ~/ros_playground/playerros_1node that contains several files, including the CMakeList.txt and manifest.xml

, tf, nav_msgs, sensor_msgs and geometry_msgs are the dependencies we will need. roscpp is needed to write a C++ ROS client, tf is needed to send transform configurations, nav_msgs for the odometry messages, sensor_msgs for the laser messages, and geometry_msgs for the velocity command messages.

Code for the one node version
Create playerros.cpp in the src directory in the playerror_1node directory just created.

First the includes:

Then the class definition:

The constructor creates all the Player objects, connects to Player, sets the callback functions (when new laser or odometry data is available), sets the cmd_vel callback function when a cmd_vel message is received from another ROS node, and starts the Player thread. We will look at all of this in detail, but first the main function:

Up to here, I think everything should be quite obvious. Let's now take a look at the contructor:

Now the odometry callback function:

One note here on frames. This code defines a few frames. One is called base_link, which is the root frame of the robot, i.e. the base. The odom frame is a fixed frame; it's origin is defined by the starting position of the robot. There is also the base_link frame, which is the frame attached to the laser.

Full code: http://pastebin.com/psZyY40E

Building the code
Edit CMakeList.txt:

To build:
rosmake playerros_1node
This will compile all the dependencies and the code we just created.
In case there are errors and you just want to recompile player_node.cpp, simply type make

rosrun playerros_1node playerros_node
(Player must be running first)

The program can be tested by sending commands to the robot. This can be done using rostopic:

rostopic pub cmd_vel geometry_msgs/Twist '[0.2,0,0]' '[0,0,0]' -1

This will set the velocity to 0.2m/s

Or you can use the following program to control the robot from the keyboard.

Add the following line to CMakeList.txt to compile:
rosbuild_add_executable(teleop src/teleop.cpp)


  1. This tutorial is awesome. It helps a lot to bridge the gap between Player and ROS. My robot can previously run in Player, and now also run in ROS with your codes.

  2. This comment has been removed by the author.

  3. Thanks for your tutorial.
    I tried with your code but for the line 88 and 89, my Eclipse shows some errors with method "ConnectReadSignal()". I also tried to find in the C++ library of Player but could not. Could you please show me what I should include more to my C++ file to solve it.
    Thanks and Regards