To visualize the robot in Gazebo, we need to add the <collision> tags, along with the <visual> tags defined in the <link> tag, as follows:

<collision>
<origin
xyz="0 0 0"
rpy="1.5707963267949 0 3.14" />
<geometry>
<mesh filename="package://robot_description/meshes/robot_base.stl" />
</geometry>
</collision>

For the base, add them to the robot_base.urdf.xacro file since we defined base_link there.

For all the wheel links, add them to the robot_essentials.xacro file since we defined the wheel link there:

<collision>
  <origin
   xyz="0 0 0"
   rpy="1.5707963267949 0 0" />
  <geometry>
    <mesh filename="package://robot_description/meshes/wheel.stl" />
  </geometry>
</collision>

Since Gazebo is a physics simulator, we define the physical properties using the <inertial> tags. We can acquire the mass and inertial properties from this third-party software. The inertial properties that are acquired from this software are added inside the <link> tag, along with suitable tags, as indicated here:

• For the base, the following is added:

<inertial>
 <origin
   xyz="0.0030946 4.78250032638821E-11 0.053305"
   rpy="0 0 0" />
 <mass value="47.873" />
 <inertia
   ixx="0.774276574699151"
   ixy="-1.03781944357671E-10"
   ixz="0.00763014265820928"
   iyy="1.64933255189991"
   iyz="1.09578155845563E-12"
   izz="2.1239326987473" />
</inertial>

• For all of the wheels, the following is added:

<inertial>
 <origin
   xyz="-4.1867E-18 0.0068085 -1.65658661799998E-18"
   rpy="0 0 0" />
 <mass value="2.6578" />
 <inertial>
   ixx="0.00856502765719703"
   ixy="1.5074118157338E-19"
   ixz="-4.78150098725052E-19"
   iyy="0.013670640432096"
   iyz="-2.68136447099727E-19"
   izz="0.00856502765719703" />
</inertial>

Now that the Gazebo properties have been created, let's create the mechanisms.