Ubuntu Linux for Robotics · Lesson 54 of 96

URDF

Writing a robot description and checking it with the Ubuntu tools.

Estimated time · 60–90 min

Learning goals

What you will understand

Explain urdf in your own words
Use it in a small workstation example
Verify the result instead of assuming it worked

Understand the idea first

The mental model

URDF is an XML file that describes a robot's links (rigid bodies) and joints (how they move). Install the checking tools with sudo apt install liburdfdom-tools and validate a file with check_urdf robot.urdf. urdf_to_graphviz draws the link and joint tree as a PDF.

Use metres and radians, one root link, and give every movable joint a limit. Keep the description in its own package (robot_description) with urdf/, meshes/ and launch/ folders, and install them from CMake so other packages can find the files with package:// paths.

Guided lesson

Work through these steps

1.
Open a terminal and record the current state before changing anything.
2.
Read the urdf commands and predict what each one does.
3.
Run the commands one at a time and compare each output with your prediction.
4.
Write the commands that worked in a setup note you can repeat later.

Worked example

Read it before you run it

sudo apt install liburdfdom-tools
# Install check_urdf and urdf_to_graphviz.
check_urdf mini_arm.urdf
# Validate the file and print the link tree.
urdf_to_graphviz mini_arm.urdf
# Draw the tree into mini_arm.pdf.
xdg-open mini_arm.pdf
# Open the drawing.
ros2 launch urdf_tutorial display.launch.py model:=mini_arm.urdf
# View the robot in RViz2 with joint sliders.

What the example is doing

1.Lines without sudo only inspect or change files you own; sudo authorizes a system-wide change.
2.Read command output before moving to the next line. Stop if names, versions, disks, or paths differ from the lesson.
3.Comments beginning with # explain safety checks and are not executed by the shell.

Watch it run · Robot arm

See what this code does

The terminal types this lesson's practice solution and runs it. The scene shows the kind of robot behaviour that code drives. It plays by itself and loops.

Practice code

Now type it yourself

Open a terminal, run the starter line by line (or save it as practice.sh and run bash practice.sh), and replace every TODO with your own command.

# TODO: validate mini_arm.urdf with check_urdf

Expected output

robot name is: mini_arm

Stuck? Show the full solution (try it yourself first)

check_urdf mini_arm.urdf

Expected result

How you know it worked

Each command completes without an unexplained error.
Your notes contain the command, the important output, and what it proves.

Practical exercise

Apply the lesson

Write a URDF for a two-link arm, validate it with check_urdf, draw its tree, and view it in RViz2 with the joint sliders.

Before you continue

You recorded the starting state
Every command and output is understood
The change can be repeated from your notes

If something goes wrong

A command says permission denied.

Check ownership and permissions first. Do not add sudo unless the command changes a documented system location.

A command or package is not found.

Check spelling, Ubuntu version, enabled repositories, and whether the previous installation command succeeded.