Behavior Trees and State Machines in Robotics Applications

TL;DR

This paper studies the use of behavior trees and state machines in robotics, analyzing their language concepts, implementation in DSLs, and application in open-source ROS projects to understand their similarities, usage patterns, and growth.

Contribution

It provides a comparative analysis of behavior tree and state machine DSLs, and presents an empirical study of their application in real-world robotic open-source projects.

Findings

Behavior-tree DSL usage in open-source projects is rapidly increasing.

Similarities exist between behavior trees and state machines in design and application.

Behavior trees and state machines show comparable reuse patterns in robotics applications.

Abstract

Autonomous robots combine skills to form increasingly complex behaviors, called missions. While skills are often programmed at a relatively low abstraction level, their coordination is architecturally separated and often expressed in higher-level languages or frameworks. State machines have been the go-to language to model behavior for decades, but recently, behavior trees have gained attention among roboticists. Although several implementations of behavior trees are in use, little is known about their usage and scope in the real world.How do concepts offered by behavior trees relate to traditional languages, such as state machines? How are concepts in behavior trees and state machines used in actual applications? This paper is a study of the key language concepts in behavior trees as realized in domain-specific languages (DSLs), internal and external DSLs offered as libraries, and their use in open-source robotic applications supported by the Robot Operating System (ROS). We analyze behavior-tree DSLs and compare them to the standard language for behavior models in robotics:state machines. We identify DSLs for both behavior-modeling languages, and we analyze five in-depth.We mine open-source repositories for robotic applications that use the analyzed DSLs and analyze their usage. We identify similarities between behavior trees and state machines in terms of language design and the concepts offered to accommodate the needs of the robotics domain. We observed that the usage of behavior-tree DSLs in open-source projects is increasing rapidly. We observed similar usage patterns at model structure and at code reuse in the behavior-tree and state-machine models within the mined open-source projects. We contribute all extracted models as a dataset, hoping to inspire the community to use and further develop behavior trees, associated tools, and analysis techniques.