Local Planner

Local Planner under ROS 1

Under ROS 1, the navigation stack provides a BaseLocalPlanner interface used by MoveBase. The BaseLocalPlanner component is expected to take a path and current position and produce a command velocity. The most commonly used implementation of the local planner uses the DWA algorithm, however, the Trajectory Rollout algorithm is also available in the ROS 1 navigation stack.

The navigation repository provides two implementations of the DWA algorithm: base_local_planner and dwa_local_planner. In addition, there is another DWA based planner in the Robot Navigation repository called DWB. The dwa_local_planner was meant to replace base_local_planner and provides a better DWA algorithm, but failed to provide the Trajectory Rollout algorithm, so was unable to completely replace base_local_planner.

DWB was meant to replace both the planners in the navigation repository and provides an implementation of both DWA and Trajectory Rollout.

Migrating to ROS 2

Rather than continue with 3 overlapping implementations of DWA, the DWB planner was chosen to use as the default controller plugin.

The advantages of DWB were:

• it was designed with backwards compatibility with the other controllers
• it was designed with clear extension points and modularity

Changes to DWB

For the most part, the DWB codebase is unchanged, other than what was required for ROS2 and the nav2_core interface for Controllers.

New local planner interface

See nav2_core Controller interface, which defines an interface for the controller plugins. These plugins are loaded into the nav2_controller_server to compute control commands from requests to the ROS2 action server.

DWB Plugins

DWB is highly configurable through the use of plugins. There are three types of plugins used. For each of them, a few implementations are available but you can create custom ones if you need to.

Trajectory Generator Plugins

These plugins generate the set of possible trajectories that should be evaluated by the critics. The trajectory with the best score determines the output command velocity.

There are two trajectory generators provided with Navigation 2. Only one can be loaded at a time.

• StandardTrajectoryGenerator - This is similar to the trajectory rollout algorithm used in base_local_planner in ROS 1.
• LimitedAccelGenerator - This is similar to DWA used in ROS 1.

Goal Checker Plugins

These plugins check whether we have reached the goal or not. Again, only one can be loaded at a time.

• SimpleGoalChecker - This checks whether the robot has reached the goal pose
• StoppedGoalChecker - This checks whether the robot has reached the goal pose and come to a stop.

Critic Plugins

These plugins score the trajectories generated by the trajectory generator. Multiple plugins can be loaded and the sum of their scores determines the chosen command velocity.

• BaseObstacle - Scores a trajectory based on where the path passes over the costmap. To use this properly, you must use the inflation layer in costmap to expand obstacles by the robot's radius.
• ObstacleFootprint - Scores a trajectory based on verifying all points along the robot's footprint don't touch an obstacle marked in the costmap.
• GoalAlign - Scores a trajectory based on how well aligned the trajectory is with the goal pose.
• GoalDist - Scores a trajectory based on how close the trajectory gets the robot to the goal pose.
• PathAlign - Scores a trajectory based on how well it is aligned to the path provided by the global planner.
• PathDist - Scores a trajectory based on how far it ends up from the path provided by the global planner.
• PreferForward - Scores trajectories that move the robot forwards more highly
• RotateToGoal - Only allows the robot to rotate to the goal orientation when it is sufficiently close to the goal location
• Oscillation - Prevents the robot from just moving backwards and forwards.
• Twirling - Prevents holonomic robots from spinning as they make their way to the goal.