I couldn't get echo values for bumper_vals

edit

this is the rostopic echo output:

$ rostopic echo /bumper_vals

---
header: 
  seq: 822
  stamp: 
    secs: 0
    nsecs:         0
  frame_id: "world"
states: []
---

<gazebo reference="pressure_sensor_l2">     
<sensor name="main_bumper" type="contact">  
<geom>pressure_sensor_l2</geom>
        <selfCollide>true</selfCollide>
        <alwaysOn>true</alwaysOn>
        <updateRate>100.0</updateRate>
        <material>Gazebo/Red</material>
        <contact>
           <collision>pressure_sensor_l2_collision</collision>
        </contact>
        <plugin name="gazebo_ros_bumper_controller" filename="libgazebo_ros_bumper.so">          
                <bumperTopicName>bumper_vals</bumperTopicName>
                <frameName>world</frameName>
        </plugin>   
</sensor> 
</gazebo>

Can you please help me out..