Nodes

A behavior tree is a tree-shaped data structure consisting of nodes, each of which contain a logical method which executes code. The behavor tree is evaluated recursively starting at the root node. Each node has the abilility to execute code which will either run a script, or execute all of its children. Each node will also return one of 3 outputs to its parent: "success", "failure", or "running". There are two main types of nodes: the control-flow (parent nodes) nodes and the leaf nodes.

Control-Flow Nodes

• Selector

  • The Selector executes its children sequentially from left to right.

  • If one of its children returns either "success" or "running", it will halt execution of its children and it will return the result of the child it stopped on.

  • If all of its children return "failure", the Selector will also return "failure".

• Sequencer

  • The Sequencer executes its children sequentially from left to right.

  • The Sequencer will not halt execution of its children unless one of them returns "failure" or "running", in which case it will also return "failure" or "running".

  • If all children return "success" the Sequencer will return "success"

• Multitasker

  • The Multitasker runs all of its children concurrently, each in a separate thread.

  • The Multitasker will return "success" only if all of it's children return "success".

  • If any of its children return "running" but none return "failure", the Multitasker will return "running".

  • If any of its children return "failure", the Multitasker will return "failure".

Leaf Nodes

• Action Nodes

  • Action nodes send ROS topic messages from the behavior tree to the robot.

  • Often the type of message sent from an Action node is a cmd_vel message which encodes movement instructions for the robot.

• Update Nodes

  • Update nodes are designated for updating data in the blackboard.

  • Often times the types of data updates performed by Update nodes include preprocessing or processing of message data from the robot.

• Conditional Nodes

  • Conditional nodes will return either "success" or "failure", corresponding to the boolean values "true" and "false" respectively

  • Conditional nodes will access data in the blackboard and return one of the two values listed above based on if a particular condition is met within the data.