My Robotic Manipulator #1: Basic URDF & RViz

Updated: 07 of August 2023

What we are going to learn

How to build a robot in a matter of minutes
How to see the robot in RViz
How to move the parts of the robot using Joint State Publisher

List of resources used in this post

Use the rosject: https://app.theconstructsim.com/l/5a5b6f9f/
The Construct: https://app.theconstructsim.com/
My Robot Manipulator repository: https://bitbucket.org/theconstructcore/my-robotic-manipulator
ROS Courses:
1. URDF for Robot modeling: https://app.theconstructsim.com/Course/13
ROS2 Courses:
1. URDF for Robot Modeling in ROS2 : https://app.theconstructsim.com/courses/83
2. ROS2 Basics in 5 Days Humble (Python): https://app.theconstructsim.com/Course/132
3. ROS2 Basics in 5 Days Humble (C++): https://app.theconstructsim.com/Course/133

Overview

From the URDF model, we’re going to define the first two links and visualize them in RViz. Up to the end of the video, we’ll have a basic model, a launch file to visualize it and a RViz configuration file for this specific project.

ROS Inside!

ROS Inside

Before anything else, if you want to use the logo above on your own robot or computer, feel free to download it and attach it to your robot. It is really free. Find it in the link below:

ROS Inside logo

Opening the rosject

In order to follow this tutorial, we need to have ROS installed in our system, and ideally a ROS Workspace (it can be named simulation_ws). To make your life easier, we have already prepared a rosject with a simulation for that: https://app.theconstructsim.com/l/5a5b6f9f/.

You can download the rosject on your own computer if you want to work locally, but just by copying the rosject (clicking the link), you will have a setup already prepared for you.

After the rosject has been successfully copied to your own area, you should see a Run button. Just click that button to launch the rosject (below you have a rosject example).

My Robotic Manipulator #1: Basic URDF & RViz – Run rosject (example of the RUN button)

After pressing the Run button, you should have the rosject loaded. Now, let’s head to the next section to get some real practice.

Compiling the workspace

As you may already know, instead of using a real robot, we are going to use a simulation. In order to see that simulated robot, we need to have our workspace compiled, and for that, we need a terminal.

Let’s open a terminal by clicking the Open a new terminal button.

Open a new Terminal

Once inside the first terminal, let’s run the commands below, to compile the workspace

cd ~/simulation_ws/

catkin_make

source devel/setup.bash

How the mrm_description package was created

If you check the contents of the ~/simulation_ws/src folder using the Code Editor or the terminal, you will see that it already contains a package named mrm_description there (mrm stands for My Robotic Manipulator). In case you want to know the commands we used to create that package, the commands were the following (you don’t need to run these commands if you already have the rosject).

To create the package that has the urdf package as a dependency, the commands were the following after opening a terminal:

cd ~/simulation_ws/src

catkin_create_pkg mrm_description urdf

We then created a urdf folder inside the mrm_description package with:

cd mrm_description/

mkdir urdf

After the folder was created, we created a file named mrm.xacro using these commands:

cd urdf/

touch mrm.xacro

and we also created the rviz.launch file, used for launching RViz (Robot Visualization tool) with the following commands:

cd ~/simulation_ws/src/mrm_description/

mkdir launch

touch rviz.xacro

You can see the content of these files using the Code Editor, also known as IDE (Integraded Development Environment). In order to open the IDE, just click the Code Editor button, as we can see in the image below:

Open the IDE – Code Editor

Once the Code Editor is open, just navigate to simulation_ws/src/mrm_description/launch/rviz.launhc and simulation_ws/src/mrm_description/urdf/mrm.xacro to see the content of those files.

After opening the IDE, you can browse through folders by double-clicking them. In case you want to create a new folder, right click on the parent folder and choose the option New Folder and type in the name you want to give to the folder.

The mrm.xacro file (My Robot Manipulator xacro file)

If we check the content of the mrm.xacro file, be it using the Code Editor or using the terminal (cat ~/simulation_ws/src/mrm_description/urdf/mrm.xacro), we will find the following:

<?xml version="1.0" ?>

<robot name="mrm" xmlns:xacro="http://www.ros.org/wiki/xacro">
    <link name="base_link">
        <visual>
          <origin rpy="0 0 0" xyz="0 0 0" />
          <geometry>
            <box size="1 1 1" />
          </geometry>
        </visual>
    </link>

    <joint name="base_link__link_01" type="revolute">
        <axis xyz="0 0 1"/>
        <limit effort="1000.0" lower="-3.14" upper="3.14" velocity="0.5"/>
        <origin rpy="0 0 0" xyz="0 0 0.5"/>
        <parent link="base_link"/>
        <child link="link_01"/>
      </joint>

    <link name="link_01">
        <visual>
          <origin rpy="0 0 0" xyz="0 0 0.2" />
          <geometry>
            <cylinder radius="0.35" length="0.4" />
          </geometry>
        </visual>
    </link>

</robot>

In the IDE (Code Editor), you can find it under the urdf folder:

Let’s understand the parts of this file.

The xacro file contains the description of our manipulator.

The first line tells us that this is an XML fie:

<?xml version="1.0" ?>

The next line we define a robot using the <robot> tag, that ends in the last line with </robot>:

<robot name="mrm" xmlns:xacro="http://www.ros.org/wiki/xacro">
  ...
</robot>

Then we define a link named base_link , with his visual being a box of width, depth, and height of 1 meter. The origin of the base_link is at the center of the simulated world, at position 0 0 0:

<link name="base_link">
  <visual>
    <origin rpy="0 0 0" xyz="0 0 0" />
    <geometry>
      <box size="1 1 1" />
    </geometry>
  </visual>
</link>

Then we create a joint to connect the base_link we just created, with a link named link_01 that we are about to create. The joint was defined with this code:

<joint name="base_link__link_01" type="revolute">
  <axis xyz="0 0 1"/>
  <limit effort="1000.0" lower="-3.14" upper="3.14" velocity="0.5"/>
  <origin rpy="0 0 0" xyz="0 0 0.5"/>
  <parent link="base_link"/>
  <child link="link_01"/>
</joint>

As we can see in the parent and child tags, we connect base_link to link_01. The link_01 link was defined as follows:

<link name="link_01">
  <visual>
  <origin rpy="0 0 0" xyz="0 0 0.2" />
  <geometry>
    <cylinder radius="0.35" length="0.4" />
  </geometry>
  </visual>
</link>

The link visual is represented by a cylinder.

In summary, this xacro file is a description of our robot in xml language. The robot is composed of various links and joints. In the above xacro we have two links base_link and link1. The base_link is fixed to the ground. It is connected to the link1 with a revolute joint. The joint allows the link1 to rotate about z-axis. We are defining the visual properties in this file.

The geometry property controls the appearance and origin property controls the positioning of the elements.

The joint element has an axis property which defines the degree of freedom of the links connected with joint. In addition to the axis property, the joint element has limit property to control the range of motion, parent and child property to control the relative motion (the child link moves with respect to the parent link).

Now that we have our robot defined, we need a way of launhcing it. For that, we have the rviz.launch file mentioned earlier. Let understand it:

The rviz.launch file

The content of the rviz.launch file that we created in the previous sections can be seen either using the Code Editor or through the terminal using this command:

cat ~/simulation_ws/src/mrm_description/launch/rviz.launch

Its content is as following:

<launch>
  <param name="robot_description" command="$(find xacro)/xacro --inorder '$(find mrm_description)/urdf/mrm.xacro'"/>
  
  <!-- Combine joint values -->
  <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"/>
  
  <!-- Show in Rviz   -->
  <node name="rviz" pkg="rviz" type="rviz" args="-d $(find mrm_description)/launch/config.rviz" />
  
  <!-- send joint values -->
  <node name="joint_state_publisher_gui" pkg="joint_state_publisher_gui" type="joint_state_publisher_gui">
    <param name="use_gui" value="true"/>
  </node>

</launch>

In the fist line we start the definition of the launch file using the <launch> tag.

In the second line, we load a ROS parameter called robot_description using the content of of the urdf/mrm.xacro file that is found on the mrm_description package. It is basically the xacro file explained in the previous section.

In order to “put the robot pieces together“, we use the robot_state_publisher:

 <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"/>

We also launch RViz in that launch file, and tell it to load the launch/config.rviz file that is also inside our package. Having that config.rviz file, we don’t have to reconfigure the RViz displays each time we open RViz.

Finally, we load the Joint State Publisher that allows us to move/rotate the links of our robot using a Graphical Tool:

<!-- send joint values -->
<node name="joint_state_publisher_gui" pkg="joint_state_publisher_gui" type="joint_state_publisher_gui">
    <param name="use_gui" value="True"/>
</node>

To summarize, this launch file helps in the task of launching different nodes together. The rviz.launch file above creates three nodes. The first node is for publishing the robot state, the second node starts RViz and the last node starts a GUI tool that helps in manipulating the joint angle of the robot. The first line (param) helps in locating and loading the xacro file that contains the robot description.

Compiling the workspace

Now that we have everything in place, we need to compile our workspace. For that, we use the following commands:

cd ~/simulation_ws/

catkin_make

If everything went well, you should have an output similar to the following:

...
-- catkin 0.8.10
-- BUILD_SHARED_LIBS is on
-- BUILD_SHARED_LIBS is on
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- ~~  traversing 1 packages in topological order:
-- ~~  - mrm_description
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- +++ processing catkin package: 'mrm_description'
-- ==> add_subdirectory(mrm_description)
-- Configuring done
-- Generating done
-- Build files have been written to: /home/user/simulation_ws/build
####
#### Running command: "make -j8 -l8" in "/home/user/simulation_ws/build"
####

Lauching RViz to see our robot model

In the previous sections we already learned how to open a terminal.

Now, in order to launch RViz, let’s run the following commands in the first terminal:

source ~/simulation_ws/devel/setup.bash


roslaunch mrm_description rviz.launch

After running this launch file, you should have seen RViz and the Joint State Publisher. If RViz does not open automatically, just click the Graphical Tools button on the bottom bar of The Construct Desktop. If you don’t know which button is, just hover your mouse over the buttons to know which one opens the Graphical Tools.

After having the Graphical Tools open, feel free to move the joining using the Joint State Publisher. It is going to be difficult to see the joint moving because it has the same red color as the base link, but if you look at RViz closely, you will see it moving.

The GUI tool is like a remote desktop, when it starts you can see there are two tabs at the bottom. One is the RViz tool, and the other is the joint_state_publisher GUI as shown below:

In the RViz window, we should have everything in place. We shouldn’t have the red errors like in the image above, because in the rosject we already provided the config.rviz in the launch file, if you remember well.

What we did for creating that config.rviz file was the following, in RViz:

Changed Fixed Frame to base_link
Added an Axis element
Added a RobotModel element

The images below show the operations described above:

Congratulations. Now you know how to create a basic robot with two links and a joint using Xacro files.

We hope this post was really helpful to you. If you want a live version of this post with more details, please check the video in the next section. Although the video was recorded in a previous version of The Construct platform, it still adds a lot of value:

Youtube video

So this is the post for today. Remember that we have the live version of this post on YouTube. If you liked the content, please consider subscribing to our youtube channel. We are publishing new content ~every day.

Keep pushing your ROS Learning.

Related Courses & Training

If you want to learn more about ROS and ROS2, we recommend the following courses:

ROS Courses:
1. URDF for Robot modeling (ROS 1): https://app.theconstructsim.com/Course/13
ROS2 Courses:
1. URDF for Robot Modeling in ROS2 : https://app.theconstructsim.com/courses/83

- ROS2 Basics in 5 Days Humble (Python): https://app.theconstructsim.com/Course/132
- ROS2 Basics in 5 Days Humble (C++): https://app.theconstructsim.com/Course/133
- Open-RMF / Fleet Management Training: https://www.theconstruct.ai/robot-fleet-management-ros2-open-rmf-training/

My Robotic Manipulator #1: Basic URDF & RViz

What we are going to learn

List of resources used in this post

Overview

ROS Inside!

Compiling the workspace

How the mrm_description package was created

The mrm.xacro file (My Robot Manipulator xacro file)

The rviz.launch file

Compiling the workspace

Lauching RViz to see our robot model

Youtube video

Related Courses & Training

Search

RECENT POSTS

Categories

Pin It on Pinterest