How can I simulate dynamixel arm in gazebo?

edit

You need to modify your URDF. Add this for each joint you want to control:

 <transmission type="pr2_mechanism_model/SimpleTransmission" name="kinect_trans">
    <actuator name="kinect_motor" />
    <joint name="kinect_joint" />
    <mechanicalReduction>1.0</mechanicalReduction>
    <motorTorqueConstant>1.0</motorTorqueConstant>
 </transmission>

 <gazebo reference="kinect_joint">
    <erp>0.1</erp>
    <stopKd value="100000000.0" />
    <stopKp value="100000000.0" />
    <fudgeFactor value="0.5" />
 </gazebo>

Then you will need one manager:

<gazebo>   
     <controller:gazebo_ros_controller_manager name="gazebo_ros_controller_manager" plugin="libgazebo_ros_controller_manager.so">
          <alwaysOn>true</alwaysOn>
          <updateRate>1000.0</updateRate>
          <robotNamespace></robotNamespace>
          <robotParam>robot_description</robotParam>
          <interface:audio name="gazebo_ros_controller_manager_dummy_iface" />
     </controller:gazebo_ros_controller_manager>
</gazebo>

Then, create configuration (yaml) file like this one:

kinect_controller:
  type: robot_mechanism_controllers/JointPositionController
  joint: kinect_joint
  pid: &kinect_position_gains
    p: 2.0
    i: 0.5
    d: 0.1
    i_clamp: 10.0

And finally, put this into your launch file:

<rosparam file="$(find your_package)/config/conf_file.yaml" command="load" />

<param name="pr2_controller_manager/joint_state_publish_rate" value="100.0" />

<!-- Spawn some controllers stopped/started -->
<node pkg="pr2_controller_manager" type="spawner" args="kinect_controller" name="spawn_dynamix" output="screen"/>

That should be all... You will get command topic for each joint. If you want also follow joint action, you try to write simple script on your own - here is example which I'm using and it works somehow: http://pastebin.com/HaKtegR4. Good luck :-)