First, we will install urdf-tutorial package from ROS2 using

sudo apt install ros-humble-urdf-tutorial

then, source ROS installation using

source /opt/ros/humble/setup.bash

We will use these examples to learn URDF generation and visualization essentials.

cd /opt/ros/humble/share # all of our packages are installed here!
cd urdf_tutorial/urdf   # here you will see all the existing examples of urdf

Let’s see one of these examples!

ros2 launch urdf_tutorial display.launch.py model:=urdf/08-macroed.urdf.xacro

You should be able to see the following,

Transformation (TFs)

Define the relative pose (position+orientation) of the links (and joints) of our Robot Model.

The RGB color scheme is a standard way of representing (x, y, z) axes in ROS, respectively. TF is not just the transformation between different joint or frames of the robot. It is the relationship beween how links move with respect to each other.

Relation between TFs, TF Tree

All the links have a parent-child relationship. For example, base_link is the parent of right_leg link. You will realize after running ros2 topic list and ros2 topic echo \\tf that \\tf is a topic in itself, which is publishing messages. However, we won’t use it by itself. We will use some other tools to visualize our TF Tree.

sudo apt install ros-humble-tf2-tools
source /opt/ros/humble/setup.bash

Now, to visualize this in addition to the above node running in the background,

ros2 run tf2_tools view_frames

This will generate a pdf file, perhaps called frames_{some-date-time}.pdf (or anything defaults) as well as some frames_{some-date-time}.gv file. The PDF file will help you see the relationship between all the frames present in the robot.