[Gazebo in 5 minutes] 011 – How to use the joint_state_publisher

In this post, you will learn how to use the joint_state_publisher node. At the end of the post, you will be able to debug your robot model joints using a graphical interface. Let’s go!

Step 1: Fire up a system with ROS installation

“Hey, do you mean I have to install ROS first?” Absolutely not! Just log in to The Construct to get access to virtual machines pre-installed with ROS.

Once logged in, click here to copy the ROS project (rosject) we’ll use for this post. Once copied, click the Run button to open the rosject.

You might also want to try this on a local PC if you have ROS installed. However, please note that we don’t support local PCs and you will have to fix any errors you run into on your own. The rest of the instruction assumes that you are working on The Construct; please adapt them to your local PC and ROS installation.

PS: we are using ROS Kinetic. You should be able to replicate the same on any ROS 1 distro, with minor changes.

Step 2: Identify the joints in your robot

For this post, we are using the following robot model, which you’ll find by opening the IDE and locating the file as shown in the image below.

<?xml version="1.0" ?>
<robot name="my_robot" xmlns:xacro="http://www.ros.org/wiki/xacro">
    
    <!-- 1st link -->
  <link name="link_chassis">
    <pose>0 0 0.1 0 0 0</pose>
    
    <inertial>
      <mass value="5"/>
      <origin xyz="0 0 0.1" rpy="0 0 0"/>
      <inertia ixx="0.0395416666667" ixy="0" ixz="0" iyy="0.106208333333" iyz="0" izz="0.106208333333"/>
    </inertial>
    
    <collision name="collision_chassis">
      <geometry>
        <box size="1 1 2"/>
      </geometry>
    </collision>
    
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <box size="1 1 2"/>
      </geometry>
    </visual>
  </link>
  
  <joint name="joint1" type="continuous">
    <origin xyz="0.6 0 1" rpy="0 0 0"/>
    <parent link="link_chassis"/>
    <child link="link_arm"/>
  </joint>
  
  <!-- 2nd link -->
  <link name="link_arm">
    <pose>0 0 0 0 0 0</pose>
    
    <inertial>
      <mass value="0.5"/>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <inertia ixx="0.0395416666667" ixy="0" ixz="0" iyy="0.106208333333" iyz="0" izz="0.106208333333"/>
    </inertial>
    
    <collision name="collision_chassis">
      <origin xyz="0 0 0.5" rpy="0 0 0"/>
      <geometry>
        <box size="0.2 0.2 1"/>
      </geometry>
    </collision>
    
    <visual>
      <origin xyz="0 0 0.5" rpy="0 0 0"/>
      <geometry>
        <box size="0.2 0.2 1"/>
      </geometry>
    </visual>
  </link>
 
</robot>

The joint in question here is highlighted in the code above.

Step 3: Use the joint_state_publisher node with the rviz node

The launch file is in simulation_ws/src/my_robot_description/launch/rviz.launch. The lines where the joint_state_publisher node is started is highlighted below.

<?xml version="1.0"?>
<launch>

    <param name="robot_description" command="$(find xacro)/xacro --inorder '$(find my_robot_description)/urdf/robot.urdf'" />    

    <!-- send fake joint values -->
    <node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher">
        <param name="use_gui" value="True"/>
    </node>
    
    <!-- Combine joint values -->
    <node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher"/>
    
    <!-- Show in Rviz   -->
    <node name="rviz" pkg="rviz" type="rviz" args="-d $(find my_robot_description)/rviz/robotmodel.rviz" />

</launch>

Step 4: Use the joint_state_publisher to move the joint

Open a web shell and launch rviz with the command shown below.

roslaunch my_robot_description rviz.launch

You should now see two displays: rviz and the joint_state_publisher gui. You should be able to move the joint as shown in the image below.

Step 5: Check your learning

Do you now understand how to use the joint_state_published node? If not, please go over the post again, more carefully this time.

Step 6 (Optional): watch the video to learn more

Feedback

Did you like this video? Whatever the case, please leave a comment in the comments section below, so we can interact and learn from each other.

If you want to learn about other ROS topics, please let us know on the comments area, and we will do a video about it.

Thank you!

[Gazebo in 5 minutes] 004 – How to create a gazebo model using SDF

What you will learn

• Learn how to  create a gazebo model using the SDF Format

List of resources used in this post:

• Robot Ignite Academy, the place to learn to program robots using only a web browser
• ROS Development Studio (the environment used in the video), another powerful online tool for pushing your ROS learning in a practical way
• SDFormat website: http://sdformat.org/
• URDF website: http://wiki.ros.org/urdf

Preparing the environment

In order to load gazebo using ROS, you need to have Gazebo and ROS installed. If you don’t want to install everything, we highly recommend using ROSDS (ROS Development Studio) which gives you access to an online environment with ROS already installed. Indeed, we are going to use that tool for easiness. You can follow the same steps on your computer as well if you don’t want to use ROSDS.

To use ROSDS, you can just create an account and start using it.

Once you have your account created, you have to create a ROSject by clicking on the New ROSject button:

Creating a new ROSject in ROSDS

Once you have the ROSject, you can open it by clicking Open:

Opening a ROSject in ROSDS

Creating the ROS Package

In order to run anything using ROS, we need a ROS package, so, let’s create one. For that, you are going to need a terminal/shell. In ROSDS, you can have a terminal by clicking Tools -> Shell.

Let’s first create the workspace. In this case, let’s call it ~/simulation_ws

mkdir ~/simulation_ws/src -p

Now let’s then compile our empty workspace

source /opt/ros/kinetic/setup.bash 
source /usr/share/gazebo/setup.sh

cd ~/simulation_ws/

catkin_make

Now let’s create our ROS package. Let’s call it my_simulations:

source ~/simulation_ws/devel/setup.bash

cd ~/simulation_ws/src

catkin_create_pkg my_simulations

Now, let’s create a launch and a world folder inside the my_simulations package.

cd my_simulations

mkdir launch world

Now, inside the launch folder, let’s create a file named my_world.launch:

touch launch/my_world.launch

In that file, let’s put the following content:

<?xml version="1.0" encoding="UTF-8" ?>
<launch>
        <!-- overwriting these args -->
        <arg name="debug" default="false" />
        <arg name="gui" default="true" />
        <arg name="pause" default="false" />
        <arg name="world" default="$(find my_simulations)/world/empty_world.world" />

        <!-- include gazebo_ros launcher -->
        <include file="$(find gazebo_ros)/launch/empty_world.launch">
                <arg name="world_name" value="$(arg world)" />
                <arg name="debug" value="$(arg debug)" />
                <arg name="gui" value="$(arg gui)" />
                <arg name="paused" value="$(arg pause)" />
                <arg name="use_sim_time" value="true" />
        </include>
</launch>

In order to put the content on that file, you can do it using the code editor. For that, click on Tools -> IDE

Now, let’s create a file named empty_world.world in the world folder:

touch world/empty_world.world

In that file, let’s add the following content:

<?xml version="1.0" ?>

<sdf version="1.5">
	<world name="default">
		<!-- A global light source -->
		<include>
			<uri>model://sun</uri>
		</include>

		<!-- A ground plane -->
		<include>
			<uri>model://ground_plane</uri>
		</include>
	</world>
</sdf>

If you followed the instructions correctly, you should have the following structure:

user:~/simulation_ws/src$ tree .
.
|-- CMakeLists.txt -> /opt/ros/kinetic/share/catkin/cmake/toplevel.cmake
`-- my_simulations
    |-- CMakeLists.txt
    |-- launch
    |   `-- my_world.launch
    |-- package.xml
    `-- world
        `-- empty_world.world

3 directories, 5 files

Creating our model

Let’s create a model named my1stmodel on the my_simulations package:

~/simulation_ws/src/my_simulations/
mkdir -p models/my1stmodel

Inside the my1stmodel folder, let’s create two files, one named model.config and another named model.sdf.

cd models/my1stmodel/
touch model.config model.sdf

These two files, model.config and model.sdf, are required for every gazebo model. You can find those files in the ground_plane model provided by gazebo, for example.

Let’s paste the following content on our my1stmodel/model.config

<?xml version="1.0"?>

<model>
  <name>My first model</name>
  <version>1.0</version>
  <sdf version="1.5">model.sdf</sdf>

  <author>
    <name>Your name</name>
    <email>your@email.com</email>
  </author>

  <description>
	My first model for gazebo
  </description>
</model>

And in our models/my1stmodel/model.sdf, let’s add the following:

<?xml version="1.0" ?>
<sdf version="1.5">
  <model name="my1stmodel">
    <static>false</static>
    <link name="link">
      <collision name="collision">
        <geometry>
          <box>
            <size>3 2 5</size>
          </gox>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>100</mu>
              <mu2>50</mu2>
            </ode>
          </friction>
        </surface>
      </collision>
      <visual name="visual">
        <geometry>
          <box>
            <size>3 2 5</size>
          </gox>
        </geometry>
        <material>
          <script>
            <uri>file://media/materials/scripts/gazebo.material</uri>
            <name>Gazebo/Grey</name>
          </script>
        </material>
      </visual>
    </link>
  </model>
</sdf>

Launching our Gazebo model

Let’s modify our world/empty_world.world file created earlier to add the following:

<!-- A ground plane -->
<include>
 <uri>model://my1stmodel</uri>
</include>

In the end, the final content of the file would be:

<?xml version="1.0" ?>

<sdf version="1.5">
	<world name="default">
		<!-- A global light source -->
		<include>
			<uri>model://sun</uri>
		</include>

		<!-- A ground plane -->
		<include>
			<uri>model://ground_plane</uri>
		</include>

        <!-- Our custom model -->
		<include>
			<uri>model://my1stmodel</uri>
		</include>
        
	</world>
</sdf>

Now that we have everything in place, we can run our package in two ways:

Option one: Click Simulation -> Choose File, then select my_world.launch. This should automatically load the web version of gazebo, called gzweb.

Custom gazebo robot model running in ROSDS

Option two: If you are running the tests on your computer, or you want to manually run the simulation, you can just:

source ~/simulation_ws/devel/setup.bash
roslaunch my_simulations my_world.launch --screen

If you are in ROSDS and chose to run the simulation manually, then you have to manually open the Gazebo Web (gzweb) by clicking on Tools -> Gazebo.

Congratulations. You have successfully launched your first custom Gazebo World using ROS.

Youtube video

If you didn’t understand well all the steps explained here or need more understanding of the files we created, remember that we have the live version of this post on YouTube. Also, if you liked the content, please consider subscribing to our youtube channel. We are publishing new content ~every day.