turtlesim_controller

Turtlesim Controller

Position controller to move the turtle simulatoin. Turtlesim is a lightweight simulator for learning ROS 2. It illustrates what ROS 2 does at the most basic level to give you an idea of what you will do with a real robot or a robot simulation later on;

About

Here, the turtle represents a robot that we want to control. Given a goal position $(x_g, y_g)^T$, we want to take the robot from an arbitrary position $(x, y)^T$ to the goal. To do so, we need to eliminate the position error $(\tilde{x},\tilde{y})=(x_g - x, y_g - y)^T$. We can also define the problem using polar coordinates, where the position error is given by:

$$ \begin{pmatrix}p \\ \alpha \end{pmatrix} = \begin{pmatrix} \sqrt{\tilde{x}^2 + \tilde{y}^2} \\ \arctan(\frac{\tilde{y}}{\tilde{x}}) - \Psi \end{pmatrix} $$

Where $p$ is the euclidean distance to the desired point, $\Psi$ is the robot orientation and $\alpha$ is the angle between the robot's current orientation and the vector $p$. Then, we implement a simple control law to see if we can control the robot:

$$ \begin{pmatrix}v \\ \omega \end{pmatrix} = \begin{pmatrix} k_{v}\tanh(p) \\ k_{\omega}\alpha \end{pmatrix} $$

Where $k_{v}$ is a positive constant that represents the maximum linear speed and $k_{\omega}$ is a positive constant that represents the maximum angular speed. So, we developed an action server following these mathematical formulations. The task of moving the turtle until reaching a final goal is a long running task and actions are the proper way of handling such situations in ROS. You can view the source code of the action server at turtlesim_controller/controller.py. We also provide an action client at turtlesim_controller/controller_client.py.

Note that to send the robot to a goal position, receive feedback and the final result, it is necessary to implement an interface for the action server. Check the turtlesim_interfaces package for more details.

Demo

Below, its a recording of a working demonstration. Note that we just sent the turtle some random positions to go.

Demo

Building and runinng

Just clone this project in your ROS2 workspace:

cd ~/ros_ws/src
git clone https://github.com/luizcarloscf/autonomous-robots.git
cd ~/ros_ws

Then, build this package with:

rosdep install -i --from-path src --rosdistro humble -y
colcon build --packages-up-to=turtlesim_controller
source install/setup.bash

Finally, you can launch with:

ros2 launch turtlesim_controller turtlesim_controller_launch.py

The above launch script also starts the turtlesim controller. You can test it by opening another terminal and executing (just remenber to source the workspace in this new terminal):

source install/setup.bash
ros2 action send_goal /turtletask turtlesim_interfaces/action/TurtleTask "{x: 1.0, y: 10.0}"

The turtle controller exposes several configuration parameters, you can check them with:

ros2 param list

To modify the parameter:

ros2 param set /turtlesim_controller kp_angular 6

For more information about each configuration parameter, you can describe each one with:

ros2 param describe /turtlesim_controller kp_angular

Running with Docker

You can run the turtle simulation and its controller with Docker and a launch script:

docker run -it -e DISPLAY=$DISPLAY -v /tmp/.X11-unix:/tmp/.X11-unix:rw --rm luizcarloscf/autonomous-robots:main ros2 launch turtlesim_controller turtlesim_controller_launch.py

This program will run the turtle simulation and also run its controller. Using an action client, you can send the turtle wherever you want with, e.g.:

docker run -it --rm luizcarloscf/autonomous-robots:main ros2 run turtlesim_controller client -x 5 -y 8