# Dwb Controller > --- name: dwb-controller description: 'Configure Nav2 DWB (Dynamic Window Based) controller. Use when setting velocity samples, forward simulation, or trajectory scoring.' --- # DWB Controller — Dynamic Window-Based Local Planner ## Algorithm Overview DWB (Dynamic Window approach, B-variant in Nav2) samples discrete velocity commands within the robot's dynamic window — the set of velocities reachable given current velocity and acceleration limits within one control cycle. Each sampled velocity is forward-simulated to produce a trajectory, and all trajectories are scored by critic plugins. Plugin: `dwb_core::DWBLocalPlanner` ## How It Differs from MPPI - DWB samples **velocities** (vx, vy, wz) and simulates forward. MPPI samples **trajectory perturbations** over the full horizon. - DWB evaluates each trajectory independently. MPPI uses Boltzmann-weighted averaging. - DWB is less computationally demanding but produces less-optimal trajectories in complex environments. - DWB critic evaluation order can matter when `short_circuit_trajectory_evaluation: true`. ## Key Parameters ### Velocity Limits | Parameter | Type | Description | Example | |-----------|------|-------------|---------| | `min_vel_x` | float | Minimum forward velocity (negative = reverse allowed) | -0.1 | | `max_vel_x` | float | Maximum forward velocity (m/s) | 0.5 | | `min_vel_y` | float | Minimum lateral velocity (0 for diff-drive) | 0.0 | | `max_vel_y` | float | Maximum lateral velocity (0 for diff-drive) | 0.0 | | `max_vel_theta` | float | Maximum angular velocity (rad/s) | 1.0 | | `min_speed_xy` | float | Minimum translational speed to be considered moving | 0.0 | | `max_speed_xy` | float | Maximum translational speed | 0.5 | | `min_speed_theta` | float | Minimum rotational speed | 0.0 | ### Acceleration Limits | Parameter | Type | Description | Example | |-----------|------|-------------|---------| | `acc_lim_x` | float | Forward acceleration limit (m/s²) | 2.5 | | `acc_lim_y` | float | Lateral acceleration (0 for diff-drive) | 0.0 | | `acc_lim_theta` | float | Angular acceleration limit (rad/s²) | 3.2 | | `decel_lim_x` | float | Deceleration limit (negative, m/s²) | -2.5 | | `decel_lim_y` | float | Lateral deceleration | 0.0 | | `decel_lim_theta` | float | Angular deceleration (negative) | -3.2 | ### Trajectory Generation | Parameter | Type | Description | Typical | |-----------|------|-------------|---------| | `sim_time` | float | Forward simulation time (seconds) | 1.7 | | `vx_samples` | int | Number of linear velocity samples | 20 | | `vy_samples` | int | Number of lateral velocity samples (1 for diff-drive) | 1 | | `vtheta_samples` | int | Number of angular velocity samples | 20 | | `transform_tolerance` | float | TF tolerance (seconds) | 0.1 | | `short_circuit_trajectory_evaluation` | bool | Stop scoring a trajectory once it's clearly worse than best so far | true | **Total trajectory count** = `vx_samples × vy_samples × vtheta_samples`. For 20×1×20 = 400 trajectories. More samples = better coverage of velocity space but more CPU. DWB typically evaluates 200–600 trajectories per cycle. ### Trajectory Generators The default generator is `dwb_plugins::StandardTrajectoryGenerator`. Alternative: `dwb_plugins::LimitedAccelGenerator` which strictly enforces acceleration constraints on samples. ```yaml trajectory_generator_name: "dwb_plugins::StandardTrajectoryGenerator" ``` ## Critics Critics are specified as a list. Each critic scores all evaluated trajectories. The order matters when `short_circuit_trajectory_evaluation: true` — place collision-checking critics first to reject infeasible trajectories early. ```yaml critics: ["RotateToGoal", "Oscillation", "BaseObstacle", "GoalAlign", "PathAlign", "PathDist", "GoalDist"] ``` See `dwb_critics.md` for details on each critic plugin. ## Complete YAML for Differential Drive ```yaml controller_server: ros__parameters: controller_frequency: 20.0 min_x_velocity_threshold: 0.001 min_y_velocity_threshold: 0.0 min_theta_velocity_threshold: 0.001 FollowPath: plugin: "dwb_core::DWBLocalPlanner" debug_trajectory_details: true # Velocity limits min_vel_x: 0.0 max_vel_x: 0.5 min_vel_y: 0.0 max_vel_y: 0.0 max_vel_theta: 1.0 min_speed_xy: 0.0 max_speed_xy: 0.5 min_speed_theta: 0.0 # Acceleration limits acc_lim_x: 2.5 acc_lim_y: 0.0 acc_lim_theta: 3.2 decel_lim_x: -2.5 decel_lim_y: 0.0 decel_lim_theta: -3.2 # Trajectory generation sim_time: 1.7 vx_samples: 20 vy_samples: 1 vtheta_samples: 20 transform_tolerance: 0.1 short_circuit_trajectory_evaluation: true # Trajectory generator trajectory_generator_name: "dwb_plugins::StandardTrajectoryGenerator" # Critics critics: - "RotateToGoal" - "Oscillation" - "BaseObstacle" - "GoalAlign" - "PathAlign" - "PathDist" - "GoalDist" 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 lookahead_time: -1.0 Oscillation: scale: 1.0 ``` ## Tuning Tips - **Robot wobbles/oscillates**: Increase `Oscillation` scale, reduce `vtheta_samples`, or lower `acc_lim_theta`. - **Robot clips corners**: Increase `PathDist` scale or `PathAlign` scale. - **Robot won't enter narrow spaces**: Reduce `BaseObstacle` scale; check inflation radius. - **Robot is sluggish**: Increase `vx_samples`, increase `max_vel_x`, or reduce `sim_time` (shorter lookahead can be more agile). - **CPU too high**: Reduce `vx_samples × vtheta_samples`, enable `short_circuit_trajectory_evaluation`. ## When to Use DWB - Middle ground between MPPI (heavy, optimal) and RPP (light, simple). - Good for environments that are moderately complex. - Familiar to users of the ROS1 `dwa_local_planner`. - Easier to reason about than MPPI because trajectories correspond to single velocity commands.