<transmission> element is used to link actuators to joints

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For the purposes of gazebo_ros_control in its current implementation, the only important information in these transmission tags are:

<joint name=""> - the name must correspond to a joint else where in your URDF
<type> - the type of transmission. Currently only "transmission_interface/SimpleTransmission" is implemented. (feel free to add more)
<hardwareInterface> - within the <actuator> tag, this tells the gazebo_ros_control plugin what hardware interface to load (position, velocity or effort interfaces). Currently only effort interfaces are implemented. (feel free to add more)

The rest of the names and elements are currently ignored.

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In addition to the transmission tags, a Gazebo plugin needs to be added to your URDF that actually parses the transmission tags and loads the appropriate hardware interfaces and controller manager.By default the gazebo_ros_control plugin is very simple, though it is also extensible via an additional plugin architecture to allow power users to create their own custom robot hardware interfaces between ros_control and Gazebo.

The default plugin XML should be added to your URDF:

<gazebo>  <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">    <robotNamespace>/MYROBOT</robotNamespace>  </plugin></gazebo>

The gazebo_ros_control <plugin> tag also has the following optional child elements:

<robotNamespace>: The ROS namespace to be used for this instance of the plugin, defaults to robot name in URDF/SDF
<controlPeriod>: The period of the controller update (in seconds), defaults to Gazebo's period
<robotParam>: The location of the robot_description (URDF) on the parameter server, defaults to '/robot_description'
<robotSimType>: The pluginlib name of a custom robot sim interface to be used (see below for more details), defaults to 'DefaultRobotHWSim'

RRBot Example

rrbot.xacro

  <transmission name="tran1">    <type>transmission_interface/SimpleTransmission</type>    <joint name="joint1"/>    <actuator name="motor1">      <hardwareInterface>EffortJointInterface</hardwareInterface>      <mechanicalReduction>1</mechanicalReduction>    </actuator>  </transmission>  <transmission name="tran2">    <type>transmission_interface/SimpleTransmission</type>    <joint name="joint2"/>    <actuator name="motor2">      <hardwareInterface>EffortJointInterface</hardwareInterface>      <mechanicalReduction>1</mechanicalReduction>    </actuator>  </transmission>

rrbot.gazebo

<gazebo>  <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">    <robotNamespace>/rrbot</robotNamespace>  </plugin></gazebo>

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