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Nodes

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.