problem with contact sensor. Easy question

edit

hi to everybody, I'm trying to pull out some data from the contact point of an object, so I chose to use the Gazebo Ros Bumper plug in. This is my sdf file

<?xml version="1.0"?>
  <gazebo version="1.0">
    <world name="default">
      <scene>
    <ambient rgba="0.0 0.0 0.0 1"/>
    <background rgba="1 1 1 1"/>
    <shadows enabled="true"/>
      </scene>

      <physics type="ode" update_rate="100">
    <gravity xyz="0 0 -9.8"/>
    <ode>
      <solver type="quick" dt="0.01" iters="100" sor="1.3"/>
    </ode>
      </physics>

    <!-- Ground Plane -->
    <include filename="ground_plane.model"/>

    <model name="cylinder1">
      <link name="link">
    <origin pose="0 0 0.5 0 0 0"/>
    <inertial mass="1.0">
      <inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/>
    </inertial>

    <collision name="cylinder1_geom">
      <geometry>
        <cylinder radius="0.5" length="1.0"/>
      </geometry>
    </collision>

    <visual name="cylinder1_vis">
      <geometry>
        <cylinder radius="0.5" length="1.0"/>
      </geometry>
      <material script="Gazebo/Grey"/>
    </visual>

    <sensor name='contact_sensor' type='contact' always_on='1' update_rate='1.000000' visualize='1'>
      <origin pose='0 0 0.5 0 0 0'/>
      <contact>
        <collision name="cylinder1_geom"/>
      </contact>
      <plugin name="gazebo_ros_bumper" filename="libgazebo_ros_bumper.so"/>
    </sensor>

    </link>
      </model>

      <model name="cylinder">
      <link name="link2">
    <origin pose="0 0 1.5 0 0 0"/>
    <inertial mass="1.0">
      <inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/>
    </inertial>

    <collision name="cylinder2_geom">
      <geometry>
        <cylinder radius="0.3" length="1.0"/>
      </geometry>
    </collision>

    <visual name="cylinder2_vis">
      <geometry>
        <cylinder radius="0.3" length="1.0"/>
      </geometry>
      <material script="Gazebo/Grey"/>
    </visual>


    </link>
    </model>

    <light type="directional" name="sun" cast_shadows="true">
      <origin pose="0 0 10 0 0 0"/>
      <diffuse rgba="1.0 1.0 1.0 1"/>
      <specular rgba=".1 .1 .1 1"/>
      <attenuation range="10" constant="0.8" linear="0.01" quadratic="0.0"/>
      <direction xyz="0 .2 -1.0"/>
    </light>

  </world>
</gazebo>

the problem is that when i type : rostopic echo bumper_states

this error appears on the roslaunch terminal :

Error [ContactSensor.cc:237] Contact Sensor[contact_sensor] has no collision[cylinder1_geom]

one for each clock cycle. I don't really know where i'am wronging.I'm so sorry for the silliness of the question, but I'm new in this world so also simple things are not discounted for me. Thanks a lot in advance.