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ackermann_pure_pursuit package from wato_monorepo repo

ackermann_pure_pursuit action_bringup behaviour_planning freeroam_planner global_planning lattice_planning lattice_planning_markers lattice_planning_msgs model_predictive_control wato_trajectory_msgs infrastructure_bringup tf_static_relay vision_msgs_markers wato_lifecycle_manager ackermann_mux camera_calib lidar_lidar_calib can_state_estimator dummy_ackermann eve_description interfacing_bringup joystick_interfacing oscc_interfacing pid_control roscco_msg sensor_interfacing camera_object_detection depth_estimation feature_extraction patchworkpp patchworkpp_vendor perception_bringup perception_utils bytetrack_cpp_vendor track_viz_2d tracking traffic_light_detection carla_bringup carla_common carla_control carla_lifecycle carla_localization carla_msgs carla_perception carla_pygame carla_ros_bridge carla_sample_description carla_scenarios carla_teleop simulation_bringup wato_test behaviortree_ros2_vendor behaviour behaviour_markers behaviour_msgs costmap costmap_markers costmap_msgs lanelet_markers lanelet_msgs lidar_localization localization prediction simple_prediction world_model world_model_markers world_model_msgs world_modeling_bringup

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Package Summary

Version	0.1.0
License	Apache 2.0
Build type	AMENT_CMAKE
Use	RECOMMENDED

Repository Summary

Description	Dockerized ROS2 stack for the WATonomous Autonomous Driving Software Pipeline
Checkout URI	https://github.com/watonomous/wato_monorepo.git
VCS Type	git
VCS Version	main
Last Updated	2026-03-01
Dev Status	UNKNOWN
Released	UNRELEASED
Contributing	Help Wanted (-) Good First Issues (-) Pull Requests to Review (-)

Package Description

Pure pursuit controller for ackermann vehicles

Maintainers

WATonomous

Authors

No additional authors.

src/action/ackermann_pure_pursuit/README.md

Ackermann Pure Pursuit

ROS2 lifecycle node that tracks a trajectory using the pure pursuit algorithm, publishing Ackermann drive commands for vehicle steering and speed control.

Overview

The pure pursuit node receives a planned trajectory, transforms each waypoint into the vehicle’s base frame, selects a lookahead point, and computes the steering angle and speed via the pure pursuit geometric algorithm. It also publishes an idle signal when no valid trajectory is available or the trajectory has gone stale.

Current Status: Functional pure pursuit controller with TF-based wheelbase measurement and speed scaling proportional to steering magnitude.

ROS Interface

Subscribed Topics

Topic	Remapped From	Type	Description
`trajectory`	`/action/freeroam_planner/trajectory`	`wato_trajectory_msgs/Trajectory`	Planned trajectory from the freeroam planner

Published Topics

Topic	Type	Description
`/action/ackermann`	`ackermann_msgs/AckermannDriveStamped`	Steering angle and speed command
`/action/is_idle`	`std_msgs/Bool`	`true` when no valid trajectory is being tracked

Configuration

Parameters are loaded from config/params.yaml under the namespace action/pure_pursuit/pure_pursuit_node/ros__parameters.

Parameter	Type	Default	Description
`base_frame`	string	`"base_footprint"`	Robot base frame for trajectory transforms
`rear_axle_frame`	string	`"rear_axle"`	Rear axle TF frame for wheelbase measurement
`front_axle_frame`	string	`"front_axle"`	Front axle TF frame for wheelbase measurement
`lookahead_distance`	double	`3.5`	Target lookahead distance for pure pursuit (m)
`min_lookahead_distance`	double	`2.0`	Minimum accepted lookahead distance (m)
`max_speed`	double	`5.0`	Maximum commanded speed (m/s)
`min_speed`	double	`0.5`	Minimum commanded speed while tracking (m/s)
`control_rate_hz`	double	`20.0`	Frequency at which control commands are published (Hz)
`wheelbase_fallback`	double	`2.5667`	Fallback wheelbase if TF lookup fails (m)
`max_steering_angle`	double	`0.5`	Maximum steering angle magnitude (rad)
`idle_timeout_sec`	double	`2.0`	Time since last trajectory before declaring idle (s)
`invert_steering_`	bool	`true`	Invert the steering angle sign if needed [true - positive cw]

Algorithm Details

Pure Pursuit

The node transforms trajectory waypoints into base_frame and selects the first point ahead of the vehicle (x > 0) that is at least min_lookahead_distance away and at least lookahead_distance from the vehicle, or the last point if none qualify.

Steering and speed are then computed as:

\[\delta = \arctan\left(\frac{L \cdot 2y}{x^2 + y^2}\right)\]

where (L) is the wheelbase, and ((x, y)) is the lookahead point in base_frame.

Speed Scaling

Speed is reduced proportionally to steering demand:

\[v = v_{\text{target}} \cdot \left(1 - 0.5 \cdot \frac{|\delta|}{\delta_{\max}}\right)\]

and clamped between min_speed and max_speed. If the target trajectory point has max_speed ≤ 0, the vehicle is commanded to stop.

Wheelbase Measurement

Wheelbase is resolved at runtime via a TF lookup between rear_axle_frame and front_axle_frame. On failure, wheelbase_fallback is used and the result is cached for subsequent control cycles.

Dependencies

ROS 2 (tested on Humble)
tf2_ros, tf2_geometry_msgs
ackermann_msgs, std_msgs, geometry_msgs
wato_trajectory_msgs (custom trajectory message)
rclcpp_lifecycle

License

CHANGELOG

No CHANGELOG found.

Package Dependencies

Deps	Name
	ament_cmake
	rclcpp
	rclcpp_lifecycle
	geometry_msgs
	std_msgs
	ackermann_msgs
	tf2
	tf2_ros
	tf2_geometry_msgs
	wato_trajectory_msgs

System Dependencies

No direct system dependencies.

Dependant Packages

Name	Deps
action_bringup

Launch files

No launch files found

Messages

No message files found.

Services

No service files found

Plugins

No plugins found.

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