## title = transmission
## description = Transmissions link actuators to joints and represents their mechanical coupling
## XMLCategory
<<TableOfContents(4)>>

== URDF Transmissions ==
The transmission element is an extension to the [[urdf/XML|URDF]] robot description model that is used to describe the relationship between an actuator and a joint. This allows one to model concepts such as gear ratios and parallel linkages. A transmission transforms efforts/flow variables such that their product - power - remains constant. Multiple actuators may be linked to multiple joints through complex transmission.

Here is an example of a transmission element:

{{{#!xml
<transmission name="simple_trans">
  <type>transmission_interface/SimpleTransmission</type>
  <joint name="foo_joint">
    <hardwareInterface>EffortJointInterface</hardwareInterface>
  </joint>
  <actuator name="foo_motor">
    <mechanicalReduction>50</mechanicalReduction>
    <hardwareInterface>EffortJointInterface</hardwareInterface>
  </actuator>
</transmission>
}}}
=== <transmission> Attributes ===
The transmission element has one attribute:

 . '''name''' (required)
  . Specifies the unique name of a transmission

=== <transmission> Elements ===
The transmission has the following elements:

 . '''<type>''' (one occurrence)
  . Specifies the transmission type.
 . '''<joint>''' (one or more occurrences)
  . A joint the transmission is connected to. The joint is specified by its '''name''' attribute, and the following sub-elements:
  . '''<hardwareInterface>''' (one or more occurrences)
   . Specifies a supported joint-space [[ros_control#Hardware_Interfaces|hardware interface]]. Note that the value of this tag should be '''EffortJointInterface''' when this transmission is loaded in Gazebo and '''hardware_interface/EffortJointInterface''' when this transmission is loaded in RobotHW.
 . '''<actuator>''' (one or more occurrences)
  . An actuator the transmission is connected to. The actuator is specified by its '''name''' attribute, and the following sub-elements:
  . '''<mechanicalReduction>''' (optional)
   . Specifies a mechanical reduction at the joint/actuator transmission. This tag may not be needed for all transmissions.
  . '''<hardwareInterface>''' (optional) (one or more occurrences)
   . Specifies a supported joint-space [[ros_control#Hardware_Interfaces|hardware interface]]. Note that '''<hardwareInterface>''' tag should only be specified here for ROS releases prior to Indigo. The correct place to specify this tag is in '''<joint>''' tag. More details about this can be found [[https://github.com/ros-controls/ros_control/issues/164|here]].

=== Development Notes ===
Currently, only the [[ros_control]] project uses this transmission elements. Developing an updated transmission format that is extensible to all use cases was complicated endeavor that can be reviewed [[https://github.com/ros-simulation/gazebo_ros_pkgs/issues/52|here]].