r2d2 differential drive and hokuyo laser scan plugin not working

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I have a urdf file for r2d2 robot in which I have added hokuyo laser plugin for gazebo The code for hokuyo lasercan plugin which I have added in my urdf is:link text

I have also initialized a differential skid plugin for the 4 wheels of the robot, the code for which is link text

The complete urdf is : link text

My launch file looks like this:link text

I am able to have the launch file open the robot in gazebo for me.

But no /odom, /cmd_vel or /base_scan topic shows so that I can drive the robot up when I type rostopic list

/clock
/gazebo/link_states
/gazebo/model_states
/gazebo/parameter_descriptions
/gazebo/parameter_updates
/gazebo/set_link_state
/gazebo/set_model_state
/gazebo_gui/parameter_descriptions
/gazebo_gui/parameter_updates
/joint_states
/rosout
/rosout_agg
/tf
/tf_static