urdf transmission for simulation and hardware_interface

[fjp]

I am trying to get a differential drive robot working using ros_control. My plan is to setup everything in simulation first and then write my own hardware_interface for the real robot. According to the Gazebo tutorials on ROS control, we have to use the transmission tag in the urdf.

When it comes to writing the hardware_interface I don't know how I should take care of the transmission or if it is even required. I thought about reading and writing velocity states and commands (via the JointVelocityInterface) directly without using transmissions.

Could someone please tell me how to incorporate the transmission in the hardware_interface or how I can avoid using it when working with the real robot hardware? Do we need to define the transmission tag even if we are not using it in gazebo? Thank you.