Dwb Critics

name: dwb-critics description: 'Configure DWB critic plugins. Use when tuning GoalDist, PathDist, ObstacleFootprint, GoalAlign, PathAlign, or RotateToGoal critics.'

DWB Critics — Plugin Reference

Scoring Model

Each DWB critic plugin has a scale parameter that directly multiplies its raw score. The total trajectory score is the sum of all critics' weighted scores. The trajectory with the lowest total score wins.

Unlike MPPI's cost_weight / cost_power model, DWB uses a simple linear scale:

trajectory_score = Σ (critic_i.scale × critic_i.raw_score(trajectory))

Higher scale = more influence from that critic. The absolute values matter less than the ratios between critics.

PathDist

Purpose: Scores each trajectory based on distance to the nearest point on the global path. The primary path-tracking critic.

Behavior: For each trajectory point, computes distance to the closest point on the global plan. Aggregates (typically sums or averages) across all points. Lower distance = lower score = better.

Tuning: Highest-scaled critic for path following. Increase to 40+ for tight corridor tracking.

GoalDist

Purpose: Scores distance from the trajectory endpoint to the goal. Provides goal attraction.

Behavior: Measures Euclidean distance from the last simulated position in the trajectory to the goal. Ensures the robot makes progress toward the goal.

Tuning: Keep slightly lower than PathDist so the robot tracks the path rather than cutting directly to the goal.

GoalAlign

Purpose: Penalizes trajectories whose endpoint heading doesn't align with the goal orientation.

Behavior: Projects a point forward_point_distance ahead of the trajectory endpoint and measures angular deviation from the goal heading. Helps the robot arrive at the goal facing the correct direction.

Tuning: Increase if the robot consistently arrives with wrong heading. The forward_point_distance should roughly match the robot's length.

PathAlign

Purpose: Penalizes trajectories whose heading deviates from the path tangent direction. Keeps the robot oriented along the path.

Behavior: At each trajectory point, measures angular deviation between the robot's heading and the tangent of the nearest path segment. Prevents the robot from crabbing (moving laterally) along the path.

Tuning: High scale keeps the robot aligned. Lower for open areas where strict alignment isn't necessary.

BaseObstacle

Purpose: Checks trajectory points against the costmap for obstacle proximity.

Behavior: For each trajectory point, looks up the costmap value. With sum_scores: false, the trajectory's obstacle score is the maximum costmap value encountered along it. With sum_scores: true, it accumulates — penalizing trajectories that spend more time near obstacles.

Critical: A low scale (0.02) already provides strong obstacle avoidance because costmap values (0–254) are large numbers. Scale of 0.02 × costmap value 200 = 4.0, which is significant relative to other critics.

Tuning: Increase very cautiously. Too high and the robot refuses to navigate near any inflated cells.

ObstacleFootprint

Purpose: Full robot footprint collision checking against the costmap. More accurate than BaseObstacle's point check but significantly more expensive.

Behavior: At each trajectory point, stamps the robot's full footprint polygon onto the costmap and checks all cells underneath. If any lethal cell is under the footprint, the trajectory is rejected (infinite cost). Inscribed cells receive high but finite cost.

Use when: Robot has a non-circular or large footprint where point-based checking misses clipping corners on obstacles.

CPU cost: Scales with footprint polygon vertices × trajectory points. For a 6-vertex polygon over 40 trajectory points, it's ~240 costmap lookups per trajectory × number of trajectories.

RotateToGoal

Purpose: Near the goal, encourages the robot to rotate to match the goal heading.

Behavior: When the robot is within xy_goal_tolerance of the goal, this critic strongly favors trajectories that rotate to match the goal heading while slowing forward motion by slowing_factor.

Tuning: If robot overshoots the goal before rotating: decrease xy_goal_tolerance or increase slowing_factor.

Oscillation

Purpose: Detects and penalizes oscillatory behavior — the robot alternating between opposite velocity commands.

Behavior: Tracks recent velocity command history. If the robot has been alternating between positive and negative vx (forward/backward) or vtheta (left/right) for several consecutive cycles, trajectories continuing the oscillation are penalized.

Tuning: Usually leave at 1.0. The oscillation detector can be too sensitive in environments with multiple narrow gaps. Decrease scale if the robot hesitates at openings.

PreferForward

Purpose: Penalizes reverse and lateral motion. Essential for differential-drive robots.

Behavior: Assigns penalties to trajectories with negative linear velocity (reverse) and trajectories with significant lateral velocity components.

TwirlingCritic

Purpose: Prevents excessive in-place spinning during transit.

Behavior: Penalizes trajectories with high angular velocity, especially when combined with low linear velocity. Keeps the robot from spinning unnecessarily.

MapGrid

Purpose: General-purpose grid-based scoring using a precomputed navigation function on the costmap.

Behavior: Computes a wavefront propagation from the goal (or path) to create a cost grid. Each trajectory point is scored by its position in this grid. PathDist and GoalDist are special cases of MapGrid.

Recommended Scales for Indoor Differential Drive

# Balanced indoor configuration
BaseObstacle:
  scale: 0.02
  sum_scores: false
PathDist:
  scale: 32.0
GoalDist:
  scale: 24.0
PathAlign:
  scale: 32.0
  forward_point_distance: 0.325
GoalAlign:
  scale: 24.0
  forward_point_distance: 0.325
RotateToGoal:
  scale: 32.0
  slowing_factor: 5.0
Oscillation:
  scale: 1.0

Critic Ordering with Short-Circuit

When short_circuit_trajectory_evaluation: true, the controller stops evaluating a trajectory as soon as its partial score exceeds the current best total score. This means critic order affects performance:

1. Place collision critics first (BaseObstacle or ObstacleFootprint): quickly rejects infeasible trajectories.
2. Place expensive critics last: they only run on trajectories that survived cheaper checks.

Recommended ordering:

critics:
  - "BaseObstacle"    # Reject collisions immediately
  - "Oscillation"     # Cheap, eliminates oscillating
  - "RotateToGoal"    # Active near goal
  - "PathDist"        # Main path tracking
  - "GoalDist"        # Goal attraction
  - "PathAlign"       # Heading alignment with path
  - "GoalAlign"       # Final heading alignment