BT Use Cases

A set of use cases of Behavior Trees in ROS2 and C++. In this repo, we will use:

• The Groot graphical editor di design the BTs.
• The BehaviorTree.CPP library to execute the BTs.
• The Nav2
• Some of the design principles of the book Behavior Trees in Robotics and AI.
• Some of the nomenclature from RobMoSys, Composable Models and Software for Robotics Systems.
• Some utility functions from base classes from ros2_bt_utils.

Note This is not a tutorial on BehaviorTree.CPP, please see the tutorial of the library first. You may want to take a look at the TurtleBot3 Behavior Demos on demos on BTs nodes implementation as well. The use cases below focus on the BT design and the overall software structure.

Dependencies

• ROS2 (Humble)
• BehaviorTree.CPP
• Nav2
• ros2_bt_utils
• Groot (Optional)
• ZeroMQ (Optional)
• turtlebot3_simulations

Installation

1.0 - Install the dependencies above

sudo apt install libgazebo11 python3-pip
pip install transforms3d 
sudo apt install libzmq3-dev libboost-dev 
sudo apt install qtbase5-dev libqt5svg5-dev libzmq3-dev libdw-dev
sudo apt install ros-humble-gazebo-* 
sudo apt install ros-humble-behaviortree-cpp-v3 
sudo apt install ros-humble-navigation2 
sudo apt install ros-humble-nav2-bringup 

1.1 - pull and install the dependencies for these repos in your ROS2 workspace

cd <your-ws>
git clone https://github.com/miccol/ros2_bt_utils.git 
git clone https://github.com/miccol/bt_use_cases.git 
git clone https://github.com/BehaviorTree/Groot.git 
cd .. 
rosdep install --from-paths src --ignore-src 

2 - pull this repo in your ROS2 workspace

 cd <your-ws>
 git clone https://github.com/miccol/bt_use_cases

3 - build

colcon build

Don't forget to export the correct environmental variable for the simulation

export TURTLEBOT3_MODEL=waffle
export GAZEBO_MODEL_PATH=$GAZEBO_MODEL_PATH:/opt/ros/$ROS_DISTRO/share/turtlebot3_gazebo/models

Overall Software Structure

Each demo has the following folders:

• bts/descriptions: that contains the XML files that describe the BT.
• bts/src: that contains the source code for custom-made BT leaf nodes and the main executable of the BTs for the demo.
• components: that contains the source code for ROS nodes that provide the functionality to the system through formally defined services (e.g. in ROS are Topics, Services, and Actions).
• interfaces: that contains the custom interface descriptions (e.g. .msg for ROS messages, .srv for ROS services, .action for ROS Actions, etc) of the use case.

Why this structure?

We follow separation of concerns (and common sense) by writing in the components the code to implement the services required for the task (acting as the server side of the service), in the bts the code for the leaf nodes, that usually handles the requests and responses for services (acting as the client side of the service), and in the description the orchestration of the leaf nodes. Following this design, the code inside a leaf node becomes (hopefully) neat. Moreover, the behavior designer can test their BTs by implementing fake components with the same interface as the real ones. As done in the Use Cases below. Over the years, I found this separation quite useful.

The figure below depicts a schema with the two actors from the RobMoSys's Ecosystem Organization that are mainly involved in this design: The Behavior Developer and the Component Supplier

which, in ROS2 can translate to:

To recap BTs orchestrate leaf nodes, leaf nodes orchestrate services of components, and the components do the actual job.

BT Design Principles

With little surprise, I follow the Design Principles of the book Behavior Trees in Robotics and AI in particular, the ones in the following chapters:

• 3.1 Improving Readability using Explicit Success Conditions
• 3.4 Improving Safety using Sequences
• 3.5 Creating Deliberative BTs using Backchaining

Use Cases

Below you can find the list of use cases (click on the links)

Note For educational purposes, I duplicated some files so that each Use Case is self-contained.

Use Case 0: Simple Robot

Use Case 1: Tour Robot

More To Come