To visualize the robot in Gazebo, we need to add <collision> tags, along with the <visual> tags we defined in the <link> tag, like we did previously. Hence, just specify the necessary visual and inertial tags on the links:

• For the arm_base link, add the following:

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

<inertial>
      <origin
        xyz="7.7128E-09 -0.063005 -3.01969999961422E-08"
        rpy="0 0 0" />
      <mass
        value="1.6004" />
      <inertia
        ixx="0.00552196561445819"
        ixy="7.9550614501301E-10"
        ixz="-1.34378458924839E-09"
        iyy="0.00352397447953875"
        iyz="-1.10071809773382E-08"
        izz="0.00553739792746489" />
    </inertial>

• For the other links, please have a look at the code for robot_arm.urdf.xacro at https://github.com/PacktPublishing/ROS-Robotics-Projects-SecondEdition/blob/master/chapter_3_ws/src/robot_description/urdf/robot_arm.urdf.xacro.

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