Sensor Camera on Gazebo: Could not find parameter robot_description on parameter server

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hi all, i'm trying to create a sensor camera on gazebo, but i have this message on the terminal when i run the launch file:

[ERROR] [1323711066.531080823]: Could not find parameter robot_description on parameter server
[ERROR] [1323711066.531205567]: Could not generate robot model
[ERROR] [1323711066.531238167]: Failed to extract kdl tree from xml robot description
[robot_state_publisher-4] process has died [pid 2161, exit code 255].
log files: /home/mauro/.ros/log/0edc0fb2-24e7-11e1-ba97-0016ea548718/robot_state_publisher-4*.log

i created a launch file:

<launch>

    <node name="gazebo" pkg="gazebo" type="gazebo" 
            args="$(find my_erratic)/worlds/my_world.world" output="screen" respawn="false" />

    <include file="$(find my_erratic)/launch/my_camera.launch" />

</launch>

the file my_camera.lauch is:

<launch> 
    <param name="use_sim_time" value="true" />

    <!-- send the camera XML to param server -->
    <param name="camera_description" command="$(find xacro)/xacro.py '$(find my_erratic)/urdf/my_camera.urdf.xacro'" />

    <!-- push camera_description to factory and spawn camera in gazebo -->
    <node name="spawn_robot" pkg="gazebo" type="spawn_model"
          args="-param camera_description
                -urdf
                -x 2.5
                -z 2
                -P 90
                -model camera_description"
          respawn="false" output="screen" />

    <!-- start robot state publisher -->
    <node pkg="robot_state_publisher" type="state_publisher" name="robot_state_publisher" output="screen" >
        <param name="publish_frequency" type="double" value="50.0" />
        <param name="tf_prefix" type="string" value="" />
    </node>          

</launch>

the file my_camera.urdf .xacro is:

<?xml version="1.0"?>

<robot
    xmlns:sensor="http://playerstage.sourceforge.net/gazebo/xmlschema/#sensor"
    xmlns:controller="http://playerstage.sourceforge.net/gazebo/xmlschema/#controller"
    xmlns:interface="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface">

    <!-- Properties (Constants) -->   
    <property name="M_PI" value="3.14159"/>

    <!--                  Custom Finger Tip Camera                  -->


  <macro name="camera">

    <link name="camera_link">
     <turnGravityOff>true</turnGravityOff>  
        <inertial>
            <mass value="0.001"/>
            <origin xyz="0 0 0"/>
            <inertia ixx="0.0001" ixy="0.0" ixz="0.0" iyy="0.0001" iyz="0.0" izz="0.0001"/>
        </inertial>
        <visual>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
            <box size="0.001 0.001 0.001" />
            </geometry>
        </visual>
        <collision>
            <origin xyz="0 0 -0.0005" rpy="0 0 0" />
            <geometry>
                <box size="0.001 0.001 0.001" />
            </geometry>
        </collision>
    </link>

    <joint name="finger_tip_camera_joint" type="fixed">
      <origin xyz="0 0 0" rpy="0 0 0" />
      <parent link="camera_link" />
      <child link="finger_tip_camera_link"/>
    </joint>            

    <link name="finger_tip_camera_link">        
      <inertial>
        <mass value="0.01" />
        <origin xyz="0 0 0" />
        <inertia ixx="0.001"  ixy="0.0"  ixz="0.0"
                 iyy="0.001"  iyz="0.0"
                 izz="0.001" />
      </inertial>
      <visual>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <geometry>
          <box size="0.001 0.001 0.001" />
        </geometry>
      </visual>
      <collision>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <geometry>
          <box size="0.001 0.001 0.001" />
        </geometry>
      </collision>
    </link>

  <gazebo reference="finger_tip_camera_link">
      <sensor:camera name="finger_tip_camera_sensor">
        <imageSize>640 480</imageSize>
        <imageFormat>R8G8B8</imageFormat>
        <hfov>90</hfov>
        <nearClip>0.01</nearClip>
        <farClip>100</farClip>
        <updateRate>20.0</updateRate>
        <controller:gazebo_ros_camera name="finger_tip_camera_controller" plugin="libgazebo_ros_camera.so">
          <alwaysOn>true</alwaysOn>
          <updateRate>20.0</updateRate>
          <imageTopicName>finger_tip_cam/image</imageTopicName>
          <cameraInfoTopicName>finger_tip_cam/camera_info</cameraInfoTopicName>
          <frameName>finger_tip_camera_link</frameName>
          <interface:camera name="finger_tip_camera_iface" />
        </controller:gazebo_ros_camera>
      </sensor:camera>
      <turnGravityOff>true</turnGravityOff>
      <material>Gazebo/Blue</material>        
    </gazebo>   

    <joint ...