contact sensor no data output

edit

Hello! I am using ros hydro and gazebo-1.9.5 (my os is Ubuntu precise 32bit). I encounter a problem when testing the contact sensor. Part of my urdf file regarding the contact sensor is listed below:

 <joint name="base_link_simple_collision_joint" type="fixed">
      <origin xyz="0 0 0.1"/>
      <parent link="base_link"/>
      <child link="simple_collision_link" />
    </joint>

    <link name="simple_collision_link">
      <inertial>
        <mass value="0.1" />
        <origin xyz="0 0 0" />
        <inertia ixx="0.1" ixy="0.0" ixz="0.0"
          iyy="0.1" iyz="0.0" 
          izz="0.1" />
      </inertial>
      <visual>
        <origin xyz="0 0 0"/>
        <geometry>
          <box size="0.001 0.001 0.001" />
        </geometry>
      </visual>
      <collision name="segbot_simple_collision">
        <origin xyz="0 0 0.1"/>
        <geometry>
          <cylinder length="0.2" radius="0.8" />
        </geometry>
      </collision>
    </link>

    <gazebo reference="simple_collision_link">
      <selfCollide>false</selfCollide>
      <sensor type="contact" name="simple_collision_sensor">
        <always_on>true</always_on>
        <update_rate>100.0</update_rate>
        <contact>
          <collision>segbot_simple_collision</collision>
          <topic>simple_contact</topic>
        </contact>
        <plugin name="simple_collision_gazebo_ros_bumper_controller" filename="libgazebo_ros_bumper.so">
          <alwaysOn>true</alwaysOn>
          <updateRate>100</updateRate>
          <frameName>base_link</frameName>
          <bumperTopicName>simple_collision_bumper</bumperTopicName>
        </plugin>
      </sensor>
    </gazebo>

The problem is that neither the gazebo topic nor ros topic outputs any collision information, when I control the robot to "crash" into a wall in the environment, even though the robot can stop against the obstacle within the distance which I set in the collision tag ("radius"). Thanks for any suggestions and helps in advance!