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Behavior Trees & Task-Level Safety

Duration: 45 min · Level: Intermediate · Module: 8. Safety & Human-Robot Interaction · Focus: behavior-trees, task-planning, safety, ROS2

Learning objectives

By the end of this lesson you will be able to explain and apply:

  • Behavior Trees
  • Safety conditions as BT nodes
  • BTlib and BehaviorTree.CPP
  • Formal verification
  • SMACH and FlexBE

Why this matters​

Safety is not just a hardware concern — robot behavior must be safe at the task planning level.

Overview​

Safety is not just a hardware concern — robot behavior must be safe at the task planning level. Behavior Trees (BTs) provide a modular, verifiable framework for robot task execution with explicit failure handling and safety checks at every node.

Key concepts​

Key idea

Behavior Trees: directed acyclic graph of control nodes (Sequence, Fallback, Parallel) and leaf nodes (Actions, Conditions); composable and verifiable

  • Safety conditions as BT nodes: "CheckHumanInWorkspace" as a condition node that returns Failure if human too close; inserted as precondition for all motion sequences
  • BTlib and BehaviorTree.CPP: popular open-source BT implementations with ROS 2 integration; Nav2 (navigation) uses BTs for all decision-making
  • Formal verification: BTs can be model-checked against safety properties using formal methods; verify "robot never exceeds 65N contact force" across all possible execution paths
  • SMACH and FlexBE: alternative state machine frameworks; less modular than BTs but widely used; BTs preferred for safety-critical applications due to clear failure propagation
  • Healthcare task example: "Deliver medication" BT: verify patient identity → check medication → navigate to patient → confirm patient alert → hand medication → log action

Check your understanding​

Try to recall each answer before expanding it.

Q1. What do you know about Behavior Trees?

directed acyclic graph of control nodes (Sequence, Fallback, Parallel) and leaf nodes (Actions, Conditions); composable and verifiable

Q2. What do you know about Safety conditions as BT nodes?

"CheckHumanInWorkspace" as a condition node that returns Failure if human too close; inserted as precondition for all motion sequences

Q3. What do you know about BTlib and BehaviorTree.CPP?

popular open-source BT implementations with ROS 2 integration; Nav2 (navigation) uses BTs for all decision-making

Q4. What do you know about Formal verification?

BTs can be model-checked against safety properties using formal methods; verify "robot never exceeds 65N contact force" across all possible execution paths

Q5. What do you know about SMACH and FlexBE?

alternative state machine frameworks; less modular than BTs but widely used; BTs preferred for safety-critical applications due to clear failure propagation

← Previous: 8.2 Compliant Control & Force-Limiting Architecture · Next: 8.4 Ethical AI & Trust in Humanoid Systems →

Part of Module 8: Safety & Human-Robot Interaction.