In this post, we will see how to make a robot follow another robot. We’ll make the iRobot follow the big turtle all around the world when it moves, using ROS TF broadcaster and listener nodes.

PS: This ROS project is part of our ROS Mini Challenge series, which gives you an opportunity to win an amazing ROS Developers T-shirt! This challenge is already solved. For updates on future challenges, please stay tuned to our Twitter channel.

Step 1: Grab a copy of the ROS Project containing the code for the challenge

Click here to get your own copy of the project. If you don’t have an account on the ROS Development Studio, you will need to create one. Once you create an account or log in, we will copy the project to your workspace. That done, open your ROSject using the Open button. This might take a few moments, please be patient.

You should now see a notebook with detailed instructions about the challenge. This post includes a summary of these instructions as well as the solution to the challenge.

PS: Please ignore the Claim your Prize! section because…well…you are late the party 🙂

Step 2: Start the Simulation and get the robots moving

1. Click on the Simulations menu and then Choose launch file… . In the dialog that appears, select rotw7.launch under turtle_tf_3d package. Then click the Launch button. You should see a Gazebo window popup showing the simulation.
2. Get the robots moving. Pick a Shell from the Tools menu and run the following commands:

user:~$ source ~/catkin_ws/devel/setup.bash
user:~$ roslaunch rotw7_pkg irobot_follow_turtle.launch

At this point, you should already see the iRobot moving towards the big turtle.

Nothing happened? Heck, we gotta fix this! Let’s do that in the next section.

Step 3: Let’s find the problem

So the robots didn’t move as we expected. And we had this error message:

[INFO] [1580892397.791963, 77.216000]: Retrieveing Model indexes
[INFO] [1580892397.860043, 77.241000]: Robot Name=irobot, is NOT in model_state, trying again

The error message above says it cannot find the model name specified in the code, so let’s check that up. Fire up the IDE from the Tools menu and browse to the directory catkin_ws/src/rotw7_pkg/scripts. We have two Python scripts in there:

• turtle_tf_broadcaster.py
• turtle_tf_listener.py

The robot model names are specified on line 19 of turtle_tf_broadcaster.py:, in the publisher_of_tf function:

robot_name_list = ["irobot","turtle"]

Let’s check if we can find these robots in the simulation, using a Gazebo service:

user:~$ rosservice call /gazebo/get_world_properties "{}"
sim_time: 424.862
model_names: [ground_plane, coke_can, turtle1, turtle2]
rendering_enabled: True
success: True
status_message: "GetWorldProperties: got properties"

So we see that the names we specified are not in the simulation! The robots we need are turtle2 and turtle1.

Also, on line 19 of turtle_tf_listener.py, the code is publishing the “cmd_vel” (the topic that moves the robot) of the following robot:

turtle_vel = rospy.Publisher('/cmd_vel', geometry_msgs.msg.Twist,queue_size=1)

But, which of the turtles is the follower, and what is the correct topic for its “cmd_vel”? We have a hint from the launch file irobot_follow_turtle.launch:

<?xml version="1.0" encoding="UTF-8"?>
<launch>
    <include file="$(find rotw7_pkg)/launch/run_turtle_tf_broadcaster.launch"/>
    <include file="$(find rotw7_pkg)/launch/run_turtle_tf_listener.launch">
        <arg name="model_to_be_followed_name" value="turtle1" />
        <arg name="follower_model_name" value="turtle2" />
    </include>
</launch>

So the follower is turtle2. Now, let’s check what it’s “cmd_vel” topic is. It’s specified as /cmd_vel in the code, but is this true? Let’s check the list of topics:

user:~$ rostopic list
#...
/turtle1/cmd_vel
/turtle2/cmd_vel

Probably, it’s /turtle2/cmd_vel. How do we know? Let’s publish to both /cmd_vel and /turtle2/cmd_vel and see which works.

ser:~$ rostopic pub /cmd_vel
Display all 152 possibilities? (y or n)
user:~$ rostopic pub /turtle2/cmd_vel geometry_msgs/Twist "linear:
  x: 0.2
  y: 0.0
  z: 0.0
angular:
  x: 0.0
  y: 0.0
  z: 0.0"
publishing and latching message. Press ctrl-C to terminate
^Cuser:~$ rostopic pub /turtle2/cmd_vel geometry_msgs/Twist "linr:r
  x: 0.0
  y: 0.0
  z: 0.0
angular:
  x: 0.0
  y: 0.0
  z: 0.0"
publishing and latching message. Press ctrl-C to terminate

Publishing to /turtle2/cmd_vel works. /cmd_vel didn’t work.

Step 4: Let’s fix the problem

We saw the problems in Step 3, now let’s implement the fix and test again.

On line 19 of turtle_tf_broadcaster.py, change the list to reflect the real turtle names:

robot_name_list = ["turtle1","turtle2"]

Also, on line 19 of turtle_tf_listener.py, change /cmd_velto /turtle2/cmd_vel:

turtle_vel = rospy.Publisher('/turtle2/cmd_vel', geometry_msgs.msg.Twist,queue_size=1)

Now rerun the commands to move the robots:

user:~$ source ~/catkin_ws/devel/setup.bash
user:~$ roslaunch rotw7_pkg irobot_follow_turtle.launch

You should now see the iRobot moving towards the big turtle. Now you can start moving the Turtle using the keyboard. Pick another Shell from the Tools menu and run the following command:

user:~$ roslaunch turtle_tf_3d turtle_keyboard_move.launch

Move the big turtle around with the keyboard, and you should see that the iRobot follows it. Done, that’s an example of how to make a robot follow another robot.

Extra: Video of this post

We made a video showing how we solved this challenge and made the iRobot follow another robot. If you prefer “sights and sounds” to “black and white”, here you go:

Related Resources

• ROS Development Studio (ROSDS)
• Robot Ignite Academy
• ROS for Beginners Course (Python)

Feedback

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