Behavior Tree

Behavior Tree is a modular, hierarchical decision model that is widely used in robot control, and game development. It present some similarities to hierarchical state machines with the key difference that the main building block of a behavior is a task rather than a state. Behavior Tree have been shown to generalize several other control architectures (https://ieeexplore.ieee.org/document/7790863)

Core Concepts

Control Node

class MissionPlanning.BehaviorTree.behavior_tree.ControlNode(name)

Base class for all control nodes in a behavior tree.

Control nodes manage the execution flow of their child nodes according to specific rules. They typically have multiple children and determine which children to execute and in what order.

class MissionPlanning.BehaviorTree.behavior_tree.SequenceNode(name)

Executes child nodes in sequence until one fails or all succeed.

Returns:

• Returns FAILURE if any child returns FAILURE

• Returns SUCCESS when all children have succeeded

• Returns RUNNING when a child is still running or when moving to the next child

Example

<Sequence>
    <Action1 />
    <Action2 />
</Sequence>

class MissionPlanning.BehaviorTree.behavior_tree.SelectorNode(name)

Executes child nodes in sequence until one succeeds or all fail.

Returns:

• Returns SUCCESS if any child returns SUCCESS

• Returns FAILURE when all children have failed

• Returns RUNNING when a child is still running or when moving to the next child

Examples

<Selector>
    <Action1 />
    <Action2 />
</Selector>

class MissionPlanning.BehaviorTree.behavior_tree.WhileDoElseNode(name)

Conditional execution node with three parts: condition, do, and optional else.

Returns:

First executes the condition node (child[0]) If condition succeeds, executes do node (child[1]) and returns RUNNING If condition fails, executes else node (child[2]) if present and returns result of else node If condition fails and there is no else node, returns SUCCESS

Example

<WhileDoElse>
    <Condition />
    <Do />
    <Else />
</WhileDoElse>

Action Node

class MissionPlanning.BehaviorTree.behavior_tree.ActionNode(name)

Base class for all action nodes in a behavior tree.

Action nodes are responsible for performing specific tasks or actions. They do not have children and are typically used to execute logic or operations.

class MissionPlanning.BehaviorTree.behavior_tree.EchoNode(name, message)

Echo node that prints a message to the console.

Returns:

Returns SUCCESS after the message has been printed

Example

<Echo message="Hello, World!" />

class MissionPlanning.BehaviorTree.behavior_tree.SleepNode(name, duration)

Sleep node that sleeps for a specified duration.

Returns:

Returns SUCCESS after the specified duration has passed Returns RUNNING if the duration has not yet passed

Example

<Sleep sec="1.5" />

Decorator Node

class MissionPlanning.BehaviorTree.behavior_tree.DecoratorNode(name)

Base class for all decorator nodes in a behavior tree.

Decorator nodes modify the behavior of their child node. They must have a single child and can alter the status of the child node.

class MissionPlanning.BehaviorTree.behavior_tree.InverterNode(name)

Inverter node that inverts the status of its child node.

Returns:

• Returns SUCCESS if the child returns FAILURE

• Returns FAILURE if the child returns SUCCESS

• Returns RUNNING if the child returns RUNNING

Examples

<Inverter>
    <Action />
</Inverter>

class MissionPlanning.BehaviorTree.behavior_tree.TimeoutNode(name, timeout)

Timeout node that fails if the child node takes too long to execute

Returns:

If the timeout duration has been exceeded - Child’s status: Otherwise, passes through the status of the child node

Return type:

• FAILURE

Example

<Timeout sec="1.5">
    <Action />
</Timeout>

class MissionPlanning.BehaviorTree.behavior_tree.DelayNode(name, delay)

Delay node that delays the execution of its child node for a specified duration.

Returns:

• Returns RUNNING if the duration has not yet passed

• Returns child’s status after the duration has passed

Example

<Delay sec="1.5">
    <Action />
</Delay>

class MissionPlanning.BehaviorTree.behavior_tree.ForceSuccessNode(name)

ForceSuccess node that always returns SUCCESS.

Returns:

• Returns RUNNING if the child returns RUNNING

• Returns SUCCESS if the child returns SUCCESS or FAILURE

class MissionPlanning.BehaviorTree.behavior_tree.ForceFailureNode(name)

ForceFailure node that always returns FAILURE.

Returns:

• Returns RUNNING if the child returns RUNNING

• Returns FAILURE if the child returns SUCCESS or FAILURE

Behavior Tree Factory

class MissionPlanning.BehaviorTree.behavior_tree.BehaviorTreeFactory

Factory class for creating behavior trees from XML strings.

build_node(node)

Build a node from an XML element.

Parameters:

node (Element) – The XML element to build the node from.

Returns:

the built node

Return type:

BehaviorTree Node

build_tree(xml_string)

Build a behavior tree from an XML string.

Parameters:

xml_string (str) – The XML string containing the behavior tree.

Returns:

The behavior tree.

Return type:

BehaviorTree

build_tree_from_file(file_path)

Build a behavior tree from a file.

Parameters:

file_path (str) – The path to the file containing the behavior tree.

Returns:

The behavior tree.

Return type:

BehaviorTree

register_node_builder(node_name, builder)

Register a builder for a node

Parameters:

• node_name (str) – The name of the node.

• builder (function) – The builder function.

Example

factory = BehaviorTreeFactory()
factory.register_node_builder(
    "MyNode",
    lambda node: MyNode(
        node.attrib.get("name", MyNode.__name__),
        node.attrib["my_param"],
    ),
)

Behavior Tree

class MissionPlanning.BehaviorTree.behavior_tree.BehaviorTree(root)

Behavior tree class that manages the execution of a behavior tree.

print_tree()

Print the behavior tree.

Node print format:

Action: <Action> Decorator: (Decorator) Control: [Control]

Node status colors:

Yellow: RUNNING Green: SUCCESS Red: FAILURE

reset()

Reset the behavior tree.

tick()

Tick once on the behavior tree.

tick_while_running(interval=None, enable_print=True)

Tick the behavior tree while it is running.

Parameters:

• interval (float, optional) – The interval between ticks. Defaults to None.

• enable_print (bool, optional) – Whether to print the behavior tree. Defaults to True.

to_text(root, indent=0)

Recursively convert the behavior tree to a text representation.

Example

Visualize the behavior tree by xml-tree-visual.

Print the behavior tree

Behavior Tree
[Robot Main Controller]
    [Battery Management]
        (Low Battery Detection)
            <Check Battery>
        <Low Battery Warning>
        <Charge Battery>
    [Patrol Task]
        <Start Task>
        [Move to Position A]
            <Move to A>
            [Obstacle Handling A]
                [Obstacle Present]
                    <Detect Obstacle>
                    <Avoid Obstacle>
                <No Obstacle>
            <Position A Task>
        [Move to Position B]
            (Short Wait)
                <Prepare Movement>
            <Move to B>
            (Limited Time Obstacle Handling)
                [Obstacle Present]
                    <Detect Obstacle>
                    <Avoid Obstacle>
            <Position B Task>
        [Conditional Move to C]
            <Check Sufficient Battery>
            [Perform Position C Task]
                <Move to C>
                (Ensure Completion)
                    <Position C Task>
            <Skip Position C>
        <Complete Patrol>
        <Return to Charging Station>
Behavior Tree