dwb_critics

This package contains plugin implementations of dwb_local_planner::TrajectoryCritic sufficient to replicate the behavior of dwa_local_planner.

Obstacle Avoidance

There are two options for critics that examine the contents of the costmap and reject trajectories that come close to obstacles.

• BaseObstacleCritic assumes a circular robot, and therefore only needs to check one cell in the costmap for each pose in the trajectory (assuming the costmap is properly inflated).
• ObstacleFootprintCritic uses the robot’s footprint and checks all of the cells along the outline of the robot’s footprint at each pose.

Progress Toward the Goal along the Path

There are two critics which evaluate the robot’s position at the end of the trajectory relative to the goal pose and the global plan.

• GoalDistCritic estimates the distance from the last pose to the goal pose.
• PathDistCritic estimates the distance from the last pose to the closest pose in the global plan.

Alignment

There are also two critics for keeping the robot pointed in the right direction. They use a point on the front of the robot as a proxy to calculate which way the robot is pointed.

• GoalAlignCritic estimates the distance from the front of the robot to the goal pose. This score will be higher if the robot is pointed away from the goal.
• PathAlignCritic estimates the distance from the front of the robot to the closest point in the global plan. This score will be higher if the robot is pointed away from the global plan.

Rotating to the Goal

There is a special case in the navigation when the robot reaches the correct XY position, but still needs to rotate to the proper yaw. The standard critics will not be useful in this case. Instead we have RotateToGoalCritic which operates in a couple different modes.

• If the robot is not yet in the correct XY position, it has no effect on the trajectory score.
• If the robot is at the correct XY position but still moving, this critic will score the trajecotries such that the robot slows down.
• If the robot is at the correct XY position and stopped its forward motion, the critic will A) Disallow trajectories with forward motion B) Score trajectories (rotations) based on how close to the goal yaw they get.

Other Critics

• OscillationCritic detects oscillations by looking at the sign of the commanded motions. For example, if in a short window, the robot moves forward and then backward, it will penalize further trajectories that move forward again, as that is considered an oscillation.
• PreferForwardCritic was implemented but not used by DWA and penalize trajectories that move backwards and/or turn too much.
• TwirlingCritic penalizes trajectories with rotational velocities