Not able to Publish hokuyo data in ROS

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Hello,

I would like to publish laser scans, that are performed by e.g the Hokuyo Model in Gazebo to ROS. I have tried this with Fuerte and Hydro without success.

I used this materials: Gazebo 1.9 ROS Hydro Ubuntu 12.04

I then started Gazebo with this command

roslaunch gazeboros emptyworld.launch

This then gives me these topics in ROS

rostopic list

/clock

/gazebo/link_states

/gazebo/model_states

/gazebo/parameter_descriptions

/gazebo/parameter_updates

/gazebo/setlinkstate

/gazebo/setmodelstate

/rosout

/rosout_agg

In Gazebo I added the Hokuyo model which is already included. I can see the scanarea indicated in blue. I can'T see any new topic in ROS. I can see in the model.sdf that it uses the libRayPlugin.so Plugin. Furthermore I noticed that a topic tag is missing inside the sensor tag. I added one. Still no publishing.Thas how the sensor section model.sdf of the Hokuyo looks like

    <plugin name="laser" filename="libRayPlugin.so" />
    <always_on>1</always_on>
    <update_rate>30</update_rate>
    <visualize>true</visualize>
  </sensor>

How should I solve it? Thank You