ROS 2 - NAV 2 - Write URDF (Unified Robot Description Format) for a Robot

 The Robot that we will do write a URDF for look like

 

Define constants using XAcro properties 

  <!-- Define robot constants -->

  <xacro:property name="base_width" value="0.31"/>

  <xacro:property name="base_length" value="0.42"/>

  <xacro:property name="base_height" value="0.18"/>

  <xacro:property name="wheel_radius" value="0.10"/>

  <xacro:property name="wheel_width" value="0.04"/>

  <xacro:property name="wheel_ygap" value="0.025"/>

  <xacro:property name="wheel_zoff" value="0.05"/>

  <xacro:property name="wheel_xoff" value="0.12"/>

  <xacro:property name="caster_xoff" value="0.14"/>

Here is a brief discussion on what these properties will represent in our urdf. 

The base_width, base_length, base_height (base_*) properties all define the size of the robot’s main chassis. 

The wheel_radius and wheel_width define the shape of the robot’s two back wheels. 

The wheel_ygap adjusts the gap between the wheel and the chassis along the y-axis whilst wheel_zoff and wheel_xoff position the back wheels along the z-axis and x-axis appropriately. 

Lastly, the caster_xoff positions the front caster wheel along the x-axis.

  <!-- Robot Base -->

  <link name="base_link">

    <visual>

      <geometry>

        <box size="${base_length} ${base_width} ${base_height}"/>

      </geometry>

      <material name="Cyan">

        <color rgba="0 1.0 1.0 1.0"/>

      </material>

    </visual>

  </link>

Let us then define our base_link - this link will be a large box and will act as the main chassis of our robot. In URDF, a link element describes a rigid part or component of our robot. The robot state publisher then utilizes these definitions to determine coordinate frames for each link and publish the transformations between them.

We will also be defining some of the link’s visual properties which can be used by tools such as Gazebo and Rviz to show us a 3D model of our robot. Amongst these properties are <geometry> which describes the link’s shape and <material> which describes it’s color.

For the code block block below, we access the base properties from the robot constants sections we defined before using the ${property} syntax. In addition, we also set the material color of the main chassis to Cyan. Note that we set these parameters under the <visual> tag so they will only be applied as visual parameters which dont affect any collision or physical properties.

  <!-- Robot Footprint -->

  <link name="base_footprint"/>

  <joint name="base_joint" type="fixed">

    <parent link="base_link"/>

    <child link="base_footprint"/>

    <origin xyz="0.0 0.0 ${-(wheel_radius+wheel_zoff)}" rpy="0 0 0"/>

  </joint>

Next, let us define a base_footprint link. The base_footprint link is a virtual (non-physical) link which has no dimensions or collision areas. Its primary purpose is to enable various packages determine the center of a robot projected to the ground. For example, Navigation2 uses this link to determine the center of a circular footprint used in its obstacle avoidance algorithms. Again, we set this link with no dimensions and to which position the robot’s center is in when it is projected to the ground plane.

After defining our base_link, we then add a joint to connect it to base_link. In URDF, a joint element describes the kinematic and dynamic properties between coordinate frames. For this case, we will be defining a fixed joint with the appropriate offsets to place our base_footprint link in the proper location based on the description above. Remember that we want to set our base_footprint to be at the ground plane when projected from the center of the main chassis, hence we get the sum of the wheel_radius and the wheel_zoff to get the appropriate location along the z-axis.

  <!-- Wheels -->

  <xacro:macro name="wheel" params="prefix x_reflect y_reflect">

    <link name="${prefix}_link">

      <visual>

        <origin xyz="0 0 0" rpy="${pi/2} 0 0"/>

        <geometry>

            <cylinder radius="${wheel_radius}" length="${wheel_width}"/>

        </geometry>

        <material name="Gray">

          <color rgba="0.5 0.5 0.5 1.0"/>

        </material>

      </visual>

    </link>

    <joint name="${prefix}_joint" type="continuous">

      <parent link="base_link"/>

      <child link="${prefix}_link"/>

      <origin xyz="${x_reflect*wheel_xoff} ${y_reflect*(base_width/2+wheel_ygap)} ${-wheel_zoff}" rpy="0 0 0"/>

      <axis xyz="0 1 0"/>

    </joint>

  </xacro:macro>

  <xacro:wheel prefix="drivewhl_l" x_reflect="-1" y_reflect="1" />

  <xacro:wheel prefix="drivewhl_r" x_reflect="-1" y_reflect="-1" />

Now, we will be adding two large drive wheels to our robot. To make our code cleaner and avoid repetition, we will make use of macros to define a block of code that will be repeated with differing parameters. Our macro will have 3 params: prefix which simply adds a prefix to our link and joint names, and x_reflect and y_reflect which allows us to flip the positions of our wheels with respect to the x and y axis respectively. Within this macro, we can also define the visual properties of a single wheel. Lastly, we will also define a continuous joint to allow our wheels to freely rotate about an axis. This joint also connects our wheel to the base_link at the appropriate location.

At the end of this code block, we will be instantiating two wheels using the macro we just made through the xacro:wheel tags. Note that we also define the parameters to have one wheel on both sides at the back of our robot.

  <!-- Caster Wheel -->

  <link name="front_caster">

    <visual>

      <geometry>

        <sphere radius="${(wheel_radius+wheel_zoff-(base_height/2))}"/>

      </geometry>

      <material name="Cyan">

        <color rgba="0 1.0 1.0 1.0"/>

      </material>

    </visual>

  </link>

  <joint name="caster_joint" type="fixed">

    <parent link="base_link"/>

    <child link="front_caster"/>

    <origin xyz="${caster_xoff} 0.0 ${-(base_height/2)}" rpy="0 0 0"/>

  </joint>

Next, we will be adding a caster wheel at the front of our robot. We will be modelling this wheel as a sphere to keep things simple. Again, we define the wheel’s geometry, material and the joint to connect it to base_link at the appropriate location.

The full URDF

<?xml version="1.0"?>
<robot name="sam_bot" xmlns:xacro="http://ros.org/wiki/xacro">

  <!-- Define robot constants -->
  <xacro:property name="base_width" value="0.31"/>
  <xacro:property name="base_length" value="0.42"/>
  <xacro:property name="base_height" value="0.18"/>

  <xacro:property name="wheel_radius" value="0.10"/>
  <xacro:property name="wheel_width" value="0.04"/>
  <xacro:property name="wheel_ygap" value="0.025"/>
  <xacro:property name="wheel_zoff" value="0.05"/>
  <xacro:property name="wheel_xoff" value="0.12"/>

  <xacro:property name="caster_xoff" value="0.14"/>

  <!-- Robot Base -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="${base_length} ${base_width} ${base_height}"/>
      </geometry>
      <material name="Cyan">
        <color rgba="0 1.0 1.0 1.0"/>
      </material>
    </visual>
  </link>
 
  <!-- Robot Footprint -->
  <link name="base_footprint"/>

  <joint name="base_joint" type="fixed">
    <parent link="base_link"/>
    <child link="base_footprint"/>
    <origin xyz="0.0 0.0 ${-(wheel_radius+wheel_zoff)}" rpy="0 0 0"/>
  </joint>
 
    <!-- Wheels -->
  <xacro:macro name="wheel" params="prefix x_reflect y_reflect">
    <link name="${prefix}_link">
      <visual>
        <origin xyz="0 0 0" rpy="${pi/2} 0 0"/>
        <geometry>
            <cylinder radius="${wheel_radius}" length="${wheel_width}"/>
        </geometry>
        <material name="Gray">
          <color rgba="0.5 0.5 0.5 1.0"/>
        </material>
      </visual>
    </link>

    <joint name="${prefix}_joint" type="continuous">
      <parent link="base_link"/>
      <child link="${prefix}_link"/>
      <origin xyz="${x_reflect*wheel_xoff} ${y_reflect*(base_width/2+wheel_ygap)} ${-wheel_zoff}" rpy="0 0 0"/>
      <axis xyz="0 1 0"/>
    </joint>
  </xacro:macro>

  <xacro:wheel prefix="drivewhl_l" x_reflect="-1" y_reflect="1" />
  <xacro:wheel prefix="drivewhl_r" x_reflect="-1" y_reflect="-1" />
 
  <!-- Caster Wheel -->
  <link name="front_caster">
    <visual>
      <geometry>
        <sphere radius="${(wheel_radius+wheel_zoff-(base_height/2))}"/>
      </geometry>
      <material name="Cyan">
        <color rgba="0 1.0 1.0 1.0"/>
      </material>
    </visual>
  </link>

  <joint name="caster_joint" type="fixed">
    <parent link="base_link"/>
    <child link="front_caster"/>
    <origin xyz="${caster_xoff} 0.0 ${-(base_height/2)}" rpy="0 0 0"/>
  </joint>

</robot>

And that’s it! We have built a URDF for a simple differential drive robot. In the next section, we will focus on building the ROS Package containing our URDF, launching the robot state publisher, and visualizing the robot in RVIz.



https://docs.nav2.org/setup_guides/urdf/setup_urdf.html#urdf-and-the-robot-state-publisher

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