Why robot strangely action in gazebo world?

edit

i wrote URDF.xacro file and showed robot in gazebo world but Big wheels rotate continuously despite i don't any actions.and model is tremble what's the problems? your answer is support gazebo simulations thanks.

here is my launch file

<launch>

  <!-- these are the arguments you can pass this launch file, for example paused:=true -->
  <arg name="paused" default="false"/>
  <arg name="use_sim_time" default="true"/>
  <arg name="gui" default="true"/>
  <arg name="headless" default="false"/>
  <arg name="debug" default="false"/>
  <arg name="model" value="$(find rnd_description)/urdf/test.xacro"/>
 <!-- We resume the logic in empty_world.launch, changing only the name of the world to be launched -->
  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="debug" value="$(arg debug)" />
    <arg name="gui" value="$(arg gui)" />
    <arg name="paused" value="$(arg paused)"/>
    <arg name="use_sim_time" value="$(arg use_sim_time)"/>
    <arg name="headless" value="$(arg headless)"/>
  </include>

<!-- Load the URDF into the ROS Parameter Server -->
  <param name="robot_description" command="$(find xacro)/xacro.py '$(arg model)'" />
  <param name="publish_frequency" type="double" value="50.0"/>

<!-- Run a python script to the send a service call to gazebo_ros to spawn a URDF robot -->
   <node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher"/>
   <node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher">
     <param name="publish_frequency" type="double" value="50.0"/>
   </node>

    <node name="spawn_robot" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen"
    args="-urdf -param robot_description -model robot_model"/>

    <node name="joy_node" pkg="joy" type="joy_node"/>
    <node name="teleop_joy" pkg="teleop_joy" type="teleop_joy"/>

</launch>

here is my urdf file

<?xml version="1.0"?>

 <!-- Used for fixing robot to Gazebo 'base_link' -->
  <link name="world"/>

  <joint name="fixed" type="fixed">
    <parent link="world"/>
    <child link="base_link"/>
  </joint>

<!-- Used for fixing robot to Gazebo 'base_link' -->

<!--*******************base_link********************-->
 <link name="base_link">
     <visual>
       <geometry>
         <box size="0.68 0.9 .28"/>
       </geometry>  
        <origin rpy="0 0 0" xyz="0 0 0"/>
        <material name="black">
        <color rgba="1 1 1 1"/>
        </material>
      </visual>

        <collision>
           <geometry>
        <box size="0.68 0.9 .28"/>
           </geometry>
        </collision>

      <inertial>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <mass value="100"/>
            <inertia
          ixx="7.4" ixy="0.0" ixz="0.0"
          iyx="0.0" iyy="4.51" iyz="0.0"
          izx="0.0" izy="0.0" izz="10.6"/>
          </inertial>
  </link>
<!--*************************************************-->


<!--*******************Black_Deck********************-->
    <link name="Black_Deck">
        <visual>
           <geometry>
             <box size="0.6 0.6 .07"/>
       </geometry>  
             <origin rpy="0 0 0" xyz="0 0 0.17"/>
             <material name="black">
              <color rgba="0 0 0 1"/>
             </material>
         </visual>

      <collision>
        <geometry>
     <box size="0.68 0.9 .28"/>
        </geometry>
       </collision>

    </link>

      <joint name="base_to_Black_Deck" type="fixed">
         <parent link="base_link"/>
         <child link="Black_Deck"/>
         <origin xyz="0 0 0"/>
      </joint>

<!--********************************************************-->


<!--*******************Laser_Sensor*************************-->

      <link name="laser">
         <visual>
            <geometry>
              <mesh filename="package://rnd_description/meshes/hokuyo.dae" />
        </geometry> 
            <origin rpy="0 0 0" xyz="0 0 0"/>
             <material name="black">
               <color rgba="0 0 0 1"/>
             </material>
         </visual>

    </link>

     <joint name="base_to_hokuyo" type="fixed">
         <parent link="base_link"/>
         <child link="laser"/>
         <origin rpy="3.141592 0 1.57" xyz="0 0.46 -0.105"/>
      </joint>

<!--********************************************************-->

<!--*******************Front_Bar*************************-->

    <link name="Front_Bar">
           <visual>
            <geometry ...