AR-Tag tracking with Alvar package

An AR-Tag is a fiduciary marker system that can help with robot perception challenges, serving as a point of reference for autonomous tasks.

In this example, we will use ar_track_alvar package for detecting individual markers.

As sending raw images from the camera via wireless network may be insufficient, we will relay all the processing to the Raspberry Pi.

What to expect?

If all goes well, you should end up with something like this:

The detected AR Tags are also published to /ar_pose_marker topic, so you could use the output in your custom nodes.

Prerequisites

📄Connect via SSH

Learn how to establish an SSH connection with your Leo Rover and access its terminal using Putty or OpenSSH.

📄Connect to a local network and the Internet

Learn how to connect your Leo Rover to a local network and the internet to download files and forward internet to your computer.

Software integration

Start by logging in into your rover via SSH:

📄Connect via SSH

Learn how to establish an SSH connection with your Leo Rover and access its terminal using Putty or OpenSSH.

Create a workspace in your home directory if you don't have one yet:

mkdir -p ~/ros_ws/src
cd ~/ros_ws
catkin init
catkin config --extend /opt/ros/${ROS_DISTRO}

Create a new package that depends on ar_track_alvar:

cd ~/ros_ws/src
catkin create pkg leo_alvar_example --catkin-deps ar_track_alvar

Run rosdep to install dependent packages:

cd ~/ros_ws
rosdep update
rosdep install --from-paths src -i

Now, add launch/ and config/ directories inside your package:

cd ~/ros_ws/src/leo_alvar_example
mkdir launch config

Inside launch/ directory add alvar.launch file with the following content:

leo_alvar_example/launch/alvar.launch

<launch>
	<arg name="cam_image_topic" default="camera/image_raw" />
	<arg name="cam_info_topic" default="camera/camera_info" />

	<node name="ar_track_alvar" pkg="ar_track_alvar" type="individualMarkersNoKinect" respawn="false" output="screen">
		<rosparam command="load" file="$(find leo_alvar_example)/config/alvar.yaml" />

		<remap from="camera_image"  to="$(arg cam_image_topic)" />
		<remap from="camera_info"   to="$(arg cam_info_topic)" />
	</node>
</launch>

Inside config/ directory add alvar.yaml file:

marker_size: 10.0
max_new_marker_error: 0.08
max_track_error: 0.2
max_frequency: 8.0
output_frame: base_link

info

You will most likely need to change marker_size parameter depending on the actual size of your printed AR tag. You can read more about the parameters on the package wiki.

And build the package:

cd ~/ros_ws
catkin build
source devel/setup.bash

Launch

To start the Alvar tracking, type:

roslaunch leo_alvar_example alvar.launch

If you want to start the node when the rover boots, add this line to robot.launch file:

/etc/ros/robot.launch

<include file="$(find leo_alvar_example)/launch/alvar.launch"/>

Generate ARTags

Now, we need to create some markers, so go back to your computer.

Install the ar_track_alvar package:

sudo apt install ros-${ROS_DISTRO}-ar-track-alvar

And run the createMarker script:

rosrun ar_track_alvar createMarker 0 -s 10.0

This will create MarkerData_0.png file that stores a 10cm x 10cm marker with id 0. Print this file on a sheet of paper.

warning

Due to differences in printer setups, the actual size of the printed marker may be different. Make sure the marker_size parameter represents the actual size (in centimeters) of the AR tag.

Now to visualize detected AR Tags, you just need to:

• open RViz, by typing rviz in the terminal
• set Fixed Frame to base_link

• Click Add -> Marker and set Marker Topic to visualization_marker

• (optionally) Click Add -> RobotModel to visualize the Rover

• (optionally) Click Add -> Image, set Image Topic to /camera/image_raw and Transport Hint to compressed to open the image stream

What next?

With this tutorial completed you may be wondering what to do next. Our pick would be the ARTag follower. However, you can check out other examples from leo_examples repository (like line follower and object detection). You can also check out other integrations from our Integrations site.