Stop Line

Role

This module plans velocity so that the vehicle can stop right before stop lines and restart driving after stopped.

Activation Timing

This module is activated when there is a stop line in a target lane.

Module Parameters

Inner-workings / Algorithms

• Gets a stop line from map information.
• insert a stop point on the path from the stop line defined in the map and the ego vehicle length.
• Sets velocities of the path after the stop point to 0[m/s].
• Release the inserted stop velocity when the vehicle stops at the stop point for stop_duration_sec seconds.

Flowchart

@startuml
title modifyPathVelocity
start

:find collision between path and stop_line;

if (collision is found?) then (yes)
else (no)
  stop
endif

:find offset segment;

:calculate stop pose;

:calculate distance to stop line;

if (state is APPROACH) then (yes)
  :set stop velocity;

  if (vehicle is within hold_stop_margin_distance?) then (yes)
    if (vehicle is stopped?) then (yes)
      :change state to STOPPED;
    endif
  endif
else if (state is STOPPED) then (yes)
  if (stopping time is longer than stop_duration_sec ?) then (yes)
    :change state to START;
  endif
else if (state is START) then (yes)
  if ([optional] far from stop line?) then (yes)
    :change state to APPROACH;
  endif
endif

stop
@enduml

This algorithm is based on segment. segment consists of two node points. It’s useful for removing boundary conditions because if segment(i) exists we can assume node(i) and node(i+1) exist.

First, this algorithm finds a collision between reference path and stop line. Then, we can get collision segment and collision point.

Next, based on collision point, it finds offset segment by iterating backward points up to a specific offset length. The offset length is stop_margin(parameter) + base_link to front(to adjust head pose to stop line). Then, we can get offset segment and offset from segment start.

After that, we can calculate a offset point from offset segment and offset. This will be stop_pose.

Restart prevention

If it needs X meters (e.g. 0.5 meters) to stop once the vehicle starts moving due to the poor vehicle control performance, the vehicle goes over the stopping position that should be strictly observed when the vehicle starts to moving in order to approach the near stop point (e.g. 0.3 meters away).

This module has parameter hold_stop_margin_distance in order to prevent from these redundant restart. If the vehicle is stopped within hold_stop_margin_distance meters from stop point of the module (_front_to_stop_line < hold_stop_margin_distance), the module judges that the vehicle has already stopped for the module’s stop point and plans to keep stopping current position even if the vehicle is stopped due to other factors.