Mobile Robot in gazebo using URDF files instead of SDF, the wheels are disappearing

edit

Hello,

I am trying to create a mobile robot using urdf and following the tutorial "Gazebo : Tutorial : Make a Mobile Robot" that creates SDF file. In my case I changed the size of the robot and I am creating it using URDF. I did the following: 1- I created a file called mobileRobot.urdf.xacro

 <?xml version="1.0"?>
 <robot name="my_robot" xmlns:xacro="http://www.ros.org/wiki/xacro"> 
 <xacro:include filename="$(find my_robot_description)/urdf/my_robot.gazebo" /> 
 <xacro:include filename="$(find my_robot_description)/urdf/my_robot.xacro" />
 </robot>

2- I created another file called my_robot.xacro

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro"> 
<property name="pi" value="3.1415926535897931" />
<link name="base_link" >
<inertial>  <mass value="0.5"/>
<inertia ixx="0.005395" ixy="0.0" ixz="0.0" iyy="0.005395" iyz="0.0" izz ="0.007470"/>
</inertial>
<collision name="collision">
    <geometry>
    <box size="0.3 0.3 0.2" />
    </geometry>
  </collision>
  <visual name="visual">
     <geometry>
              <box size="0.3 0.3 0.2"/>
              <material name="blue"> </material>
    </geometry>
    </visual>
    </link>

   <link name="left_wheel">
      <collision>
  <origin xyz="0.0 0.18 -0.1" rpy="0 1.5707 1.5707" /> 
      <geometry>
            <cylinder radius=".1" length=".05"/>     
      </geometry>
    </collision>
    <visual>
<origin xyz="0.0 0.18 -0.1" rpy="0 1.5707 1.5707" /> 
      <geometry>
        <cylinder radius=".1" length=".05"/>
      </geometry>
    </visual>
  </link>


<link name="right_wheel">
    <collision>
<origin xyz="0.0 -0.18 -0.1" rpy="0 1.5707 1.5707" /> 
      <geometry>
        <cylinder radius=".1" length=".05"/>     
      </geometry>
    </collision>
    <visual>
<origin xyz="0.0 -0.18 -0.1" rpy="0 1.5707 1.5707" />
      <geometry>
        <cylinder radius=".1" length=".05"/>
      </geometry>
    </visual>
  </link>

<joint type="revolute" name="right_wheel_hinge">
    <limit effort="30" velocity="1.0" lower="-2.792526" upper="2.792526"/>
<origin xyz="0 0 0.03" rpy="0 0 0" /> 
    <child link="right_wheel"/>
    <parent link="base_link"/>
    <axis xyz="0 1 0"/>
    </joint>

<joint type="revolute" name="left_wheel_hinge">
    <!--<limit effort="30" velocity="1.0" lower="-2.792526" upper="2.792526"/>-->
<origin xyz="0 0 0.03" rpy="0 0 0" />
    <child link="left_wheel"/>
    <parent link="base_link"/>
    <dynamics damping="0.7"/>
    <axis xyz="0 1 0"/>
    </joint>

  </robot>

3- File my_robot.gazebo

  <?xml version="1.0"?>
  <robot>
  <!-- ros_control plugin -->
   <gazebo>
        <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
        <robotNamespace>/My_ROBOT</robotNamespace>
        </plugin>
   </gazebo>
   <!-- base_link -->
   <gazebo reference="base_link">
               <selfCollide>true</selfCollide>
               <gravity>true</gravity>
               <material>Gazebo/Blue</material>
     </gazebo>
   </robot>

Finally I created a launch file

    <?xml version='1.0'?>
    <launch>
<arg name="model" default="$(find my_robot_description)/robots/mobileRobot.urdf.xacro"/>
<param name="robot_description" command="$(find xacro)/xacro.py $(arg model)" />
<include file="$(find gazebo_ros)/launch/empty_world.launch"> <arg name="use_sim_time" value="true"/></include>

<node name="Robot1" pkg="gazebo_ros" type="spawn_model"
    args="-x 2.0 -unpause -urdf -model robot1 -param robot_description" respawn="false" output="screen" />

<!--<node name="Robot2" pkg="gazebo_ros" type="spawn_model"
    args="-x 3.0 -unpause -urdf -model robot2 -param robot_description" respawn="false" output="screen" />
-->
<node pkg="robot_state_publisher" type="robot_state_publisher"  name="robot_state_publisher">
        <param name="publish_frequency" type="double" value="30.0" />
</node>
<node ...