Planning Scene Monitor

The PlanningSceneMonitor is the recommended interface for maintaining an up to date planning scene. The relationship between RobotState, CurrentStateMonitor, PlanningScene, PlanningSceneMonitor, and PlanningSceneInterface can be really confusing at first. This tutorial aims to make clear these key concepts.

RobotState

The RobotState is a snapshot of a robot. It contains the RobotModel and a set of joint values.

CurrentStateMonitor

The CurrentStateMonitor (CSM) can be thought of as a ROS wrapper for the RobotState. It subscribes to a provided topic for JointState messages that provide up to date sensor values for single degree of freedom actuators such as revolute or prismatic joints and updates it’s internal RobotState with those joint values. In addition to the single degree of freedom joints, a robot can have joints with multiple degrees of freedom such as floating and planar joints. To maintain up-to-date transform information for links and other frames attached with multiple-degree-of-freedom joints, the CSM stores a TF2 Buffer that uses a TF2 TransformListener to set their transforms in its internal data.

PlanningScene

The PlanningScene can be seen as a snapshot of the world and that includes both the RobotState and any number of collision objects. The Planning Scene can be used for collision checking as well as getting information about the environment.

PlanningSceneMonitor

The PlanningSceneMonitor wraps a PlanningScene with ROS interfaces for keeping the PlanningScene up to date. To access the PlanningSceneMonitor’s underlying PlanningScene, you can use the provided LockedPlanningSceneRW and LockedPlanningSceneRO classes.

The PlanningSceneMonitor has the following objects which have their own ROS interfaces for keeping sub-components of the planning scene up to date:

  • A CurrentStateMonitor for tracking updates to the RobotState via a robot_state_subscriber_ and a tf_buffer_, as well as a planning scene subscriber for listening to planning scene diffs from other publishers.
  • An OccupancyMapMonitor for tracking updates an OccupancyMap via ROS topics and services.

The PlanningSceneMonitor has the following subscribers:

  • collision_object_subscriber_ - Listens to a provided topic for CollisionObject messages that might add, remove or modify collision objects in the planning scene and passes them into its monitored planning scene
  • planning_scene_world_subscriber_ - Listens to a provided topic for PlanningSceneWorld messages that may contain collision object information and or octomap information. This is useful for keeping planning scene monitors in sync
  • attached_collision_object_subscriber_ - Listens on a provided topic for AttachedCollisionObject messages that determine the attaching / detaching of objects to links in the robot state.

The PlanningSceneMonitor has the following services:

  • get_scene_service_ - Which is an optional service to get the full planning scene state.

The PlanningSceneMonitor is initialized with:

  • startSceneMonitor - Which starts the planning_scene_subscriber_
  • startWorldGeometryMonitor - Which starts the collision_object_subscriber_, the planning_scene_world_subscriber_, and the OccupancyMapMonitor.
  • startStateMonitor - Which starts the CurrentStateMonitor and the attached_collision_object_subscriber_.
  • startPublishingPlanningScene - Which starts another thread for publishing the entire planning scene on a provided topic for other PlanningSceneMonitor’s to subscribe to
  • providePlanningSceneService - Which starts the get_scene_service_.

PlanningSceneInterface

The PlanningSceneInterface is a useful class for publishing updates to a MoveGroup’s PlanningSceneMonitor through a C++ API without creating your own subscribers and service clients. It may not work without MoveGroup or MoveItCpp

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