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Version: 6

Key Terminology

Objective: A collection of Behaviors structured as a Behavior Tree. The order of execution is determined by control flow logic within the tree.

Behavior Tree: a way to structure the switching between different tasks in an autonomous agent. An efficient way of creating complex systems that are both modular and reactive.

Behavior: The smallest building block in MoveIt Pro. Represents a discrete sensing, planning, or motion step.

Behavior Configuration Parameter: Value used to configure a Behavior when it is initialized. Does not change while the Objective is executed. Saved in YAML files.

Behavior Data Port: Input into or output from a Behavior. Generated dynamically while the Objective is executed. Defined in the BehaviorTree.CPP XML format, sometimes with default values.

MTC (MoveIt Task Constructor): The MoveIt Task Constructor framework provides a flexible and transparent way to define and plan actions that consist of multiple interdependent subtasks.

Objective Behavior Terminology Diagram

Defining Objectives​

Objectives are defined in the BehaviorTree.CPP XML format.

To demonstrate this, here are the contents of an example Objective XML file:

<?xml version="1.0"?>
<root BTCPP_format="4" main_tree_to_execute="Move to Known Pose">
    <!-- ////////// -->
    <BehaviorTree ID="Move to Known Pose">
        <Sequence name="root">
            <SubTree ID="Move to Named Pose" waypoint_name="move_to_known_pose.target_state_name"/>
        </Sequence>
    </BehaviorTree>
    <!-- ////////// -->
    <BehaviorTree ID="Move to Named Pose">
        <Sequence name="move_to_waypoint_main">
            <Action ID="RetrieveWaypoint" waypoint_joint_state="{target_joint_state}" waypoint_name="{waypoint_name}" joint_group_name="manipulator"/>
            <Action ID="InitializeMTCTask" task_id="move_to_waypoint" controller_names="/joint_trajectory_controller /robotiq_gripper_controller" task="{move_to_waypoint_task}"/>
            <Action ID="SetupMTCCurrentState" task="{move_to_waypoint_task}"/>
            <Action ID="SetupMTCMoveToJointState" joint_state="{target_joint_state}" planning_group_name="manipulator" task="{move_to_waypoint_task}"/>
            <Action ID="PlanMTCTask" solution="{move_to_waypoint_solution}" task="{move_to_waypoint_task}"/>
            <Action ID="ExecuteMTCTask" solution="{move_to_waypoint_solution}"/>
        </Sequence>
    </BehaviorTree>
</root>

The main_tree_to_execute field in the root node is treated as the name of the Objective. The name provided in the /do_objective action goal request is expected to match this.

Objectives can consist of multiple subtrees. In this example, the top-level Move to Known Pose tree includes the MoveToWaypoint subtree.

In the top-level Move to Known Pose tree, the value of the waypoint_name port exposed by the MoveToWaypoint subtree is expected to be set by a Behavior parameter override.

Behaviors Architecture​

Behaviors are the smallest building block in MoveIt Pro, and correspond to a node in a Behavior Tree. MoveIt Pro provides a library of pre-built Behaviors for motion planning, computer vision, basic hardware control, and more. In addition, anyone can create their own Behaviors as extensions / plugins. For more information, see Creating a Custom Behavior.

A Behavior's implementation must not be blocking to the Behavior Tree's execution flow, so asynchronous or long-lived tasks need their own threads. Behaviors that require ROS services or actions can have a reference to a shared ROS node, from which they can create publishers, call actions, etc. But it is up to your Behavior implementation to manage that.

A Behavior is run as a plugin within the MoveIt Pro Objective Server. This is advantageous in that you don't need to create separate launch files to start new Behaviors. This also allows developers to create a ROS package that contains your Behavior that can be side-loaded.

However, a Behavior can be a shallow call out to a separate ROS node, that may even be on a separate machine or container. This can be very advantageous to software with a lot of exact dependencies, such as TensorFlow.