Add bumper plugin and read the state [closed]

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Hello,

I'm trying to add a bumper plugin in Gazebo and read the state by the topic in ROS, but I can't ge it. All that I get is add the object and read this:

header: 
  seq: 16
  stamp: 
    secs: 0
    nsecs: 0
  frame_id: my_box
states: []

Time doesn't change and states is empty.

The urdf file is:

<?xml version="1.0"?>
<robot name="simple_box"
      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"
       xmlns:xacro="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface" >

    <link name="my_box">

        <inertial>
            <mass value="1.0" />
            <origin xyz="1 0 0" /> 
            <inertia  ixx="1.0" ixy="0.0"  ixz="0.0"  iyy="100.0"  iyz="0.0"  izz="1.0" />
        </inertial>

        <visual>
            <origin xyz="0 0 0" rpy="0 0 0" />
            <geometry>
                <box size="0.2 0.2 0.2" />
            </geometry>
        </visual>

        <collision>
            <!-- collision origin is defined w.r.t. link local coordinate system -->
            <origin xyz="0 0 0" rpy="0 0 0" />
            <geometry>
                <box size="0.2 0.2 0.2" />
            </geometry>
        </collision>
    </link>


<gazebo reference="my_box">
  <sensor name="gripper_contact_sensor" type="contact">
    <always_on>true</always_on>
    <update_rate>10.0</update_rate>
    <contact>
      <collision>collision</collision>
    </contact>
    <plugin name="gripper_bumper" filename="libgazebo_ros_bumper.so">
      <bumperTopicName>bumper</bumperTopicName>
      <frameName>my_box</frameName>
    </plugin>
  </sensor>
</gazebo>

I saw in some answers that I need to add '<geom>my_box_geom</geom>' [link] , but it doesn't work. I also tried with 'test_bumper' in gazebo_plugins, but neither works.

I use ROS Groovy, Gazebo 1.5 and Ubuntu 12.04 (I can't update, but I tried the same in Ros Hydro and...the same result).

Could someone say how add it correctly? or if you see any mistake in the file... Thank you!