Publishing the TF Tree

Written by: Erick Mejia Uzeda

NOTE: This section assumes you have an existing URDF describing your robot.

To understand how the robot moves and the relationship between the various components, we must publish (broadcast) such data so that it can be appropriately converted from one coordinate frame to another. For some examples of why this is important, see the tf Robot Setup.

A default robot_setup.launch file looks as follow:

<launch>
    <arg name="model" default="$(find caffeine_description)/urdf/caffeine.urdf.xacro"/>

    <!-- Parse and Condense URDF using xacro -->
    <param name="robot_description" command="xacro --inorder '$(arg model)'"/>
	
    <!-- Publish Joint States from URDF -->
    <node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher"/>
	
    <!-- Publish Robot State from URDF -->
    <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"/>
</launch> 

Xacro

NOTE: xacro.py is deprecated, use xacro --inorder instead

Python script that is used to parse and condense the urdf.xacro file into XML for use within the .launch file.

• NOTE: we specify robot_description as a top level param such that is available to both joint_state_publisher and robot_state_publisher.

Joint State Publisher

From Joint_State_Publisher wiki:

Reads the robot_description (URDF) and parses it to find non-fixed joint and publishes all of them to onesensor_msgs/JointState message.

As the name suggests, this node publishes the states of joints which is important for determining the kinematics of the robot.

Robot State Publisher

From the Robot_State_Publisher wiki:

Reads the robot_description (URDF) and subscribes to the joint_states topic to calculate the forward kinematics of the robot and publish the results via tf.

In short, robot_state_publisher publishes (broadcasts) the necessary transforms (coordinate frames) that need to be available for all your components while also taking into account your robot's joints.

Reference:

• joint state publisher and robot state publisher