Task3/line_follower

line_follower

ROS Noetic visual line following for the U-CAR. The active implementation is split into perception and control so that the camera can be inspected without moving the vehicle.

Active nodes

• line_follow_debug.py: subscribes to /usb_cam/image_raw, applies the calibrated inverse-perspective transform and HSV white-line extraction, fits the lane centreline, and publishes a metric lookahead target.
• line_follow_control.py: subscribes to the target and uses pure pursuit to publish /cmd_vel only when ~enabled is explicitly set to true.

The metric target topic is:

/line_follow_debug/lookahead_target  geometry_msgs/PointStamped

It follows base_link conventions: point.x is metres forward from the vehicle control centre and point.y is metres to the left. The controller uses:

curvature = 2 * point.y / (point.x^2 + point.y^2)
angular.z = linear_speed * curvature_gain * curvature

This is geometric lookahead control. ipm_origin_ahead_of_control_m is an extrinsic offset used while converting the IPM curve into the control-centre frame; it is not a time or odometry delay.

Start safely

Start the base driver and the single shared camera first. Then run:

source ~/ucar_ws/devel/setup.bash
roslaunch line_follower line_follow_debug.launch

Confirm that the green centre curve and purple lookahead target are stable. In another terminal run:

source ~/ucar_ws/devel/setup.bash
roslaunch line_follower line_follow_control.launch

The controller starts disabled. On a clear test route:

rosparam set /line_follow_control/enabled true

The integrated task state machine uses the equivalent service interface:

rosservice call /line_follow_control/set_enabled "data: true"

Stop immediately with:

rosparam set /line_follow_control/enabled false

Main tuning parameters

• config/line_follow_debug.yaml
  • lookahead_distance_m: requested target distance from the control centre.
  • ipm_origin_ahead_of_control_m: forward offset from the control centre to the calibrated IPM origin.
  • max_near_target_extrapolation_m: maximum tangent-only extension when the observed centreline ends farther away than the requested target.
  • metric_scale_px_per_m and metric_origin_px: metric IPM calibration.
• config/line_follow_control.yaml
  • linear_speed: commanded forward speed.
  • curvature_gain: pure-pursuit steering gain.
  • max_angular_speed: steering safety limit.
  • target_alpha: frame-to-frame target smoothing.

The current tuning is lookahead_distance_m=0.30 m, linear_speed=0.15 m/s, curvature_gain=1.8, and target_alpha=0.65. The angular velocity remains limited to 0.20 rad/s.

During the integrated task, normal_lookahead_distance_m=0.30 m is used for LEFT/RIGHT routes. The latched task state automatically selects after_second_lookahead_distance_m=0.35 m in FOLLOW_AFTER_SECOND. In that state only, curved targets are shifted 0.025 m toward the detected curve's inside once lateral target displacement exceeds 0.015 m; straight targets and LEFT/RIGHT routes remain centred.

The controller also switches from normal_linear_speed=0.15 m/s to after_second_linear_speed=0.10 m/s only in FOLLOW_AFTER_SECOND. Other task states automatically restore 0.15 m/s.

If no fresh valid target is available, the controller publishes a zero velocity command instead of reusing old geometry.