Index

Dynamic Obstacle Stop

Role

dynamic_obstacle_stop is a module that stops the ego vehicle from entering the immediate path of a dynamic object.

The immediate path of an object is the area that the object would traverse during a given time horizon, assuming constant velocity and heading.

Activation Timing

This module is activated if the launch parameter launch_dynamic_obstacle_stop_module is set to true in the motion planning launch file.

Inner-workings / Algorithms

The module insert a stop point where the ego trajectory collides with the immediate path of an object. The overall module flow can be summarized with the following 4 steps.

1. Filter dynamic objects.
2. Calculate immediate path rectangles of the dynamic objects.
3. Find earliest collision where ego collides with an immediate path rectangle.
4. Insert stop point before the collision.

In addition to these 4 steps, 2 mechanisms are in place to make the stop point of this module more stable: an hysteresis and a decision duration buffer.

The hysteresis parameter is used when a stop point was already being inserted in the previous iteration and it increases the range where dynamic objects are considered close enough to the ego trajectory to be used by the module.

Filter dynamic objects

An object is considered by the module only if it meets all of the following conditions:

• it is a vehicle (pedestrians are ignored);
• it is moving at a velocity higher than defined by the minimum_object_velocity parameter;
• it is not too close to the current position of the ego vehicle;
• it is not unavoidable (only if parameter ignore_unavoidable_collisions is set to true);
• it is close to the ego trajectory.

An object is considered unavoidable if it is heading towards the ego vehicle such that even if ego stops, a collision would still occur (assuming the object keeps driving in a straight line).

For the last condition, the object is considered close enough if its lateral distance from the ego trajectory is less than the threshold parameter minimum_object_distance_from_ego_trajectory plus half the width of ego and of the object (including the extra_object_width parameter). In addition, the value of the hysteresis parameter is added to the minimum distance if a stop point was inserted in the previous iteration.

Calculate immediate path rectangles

For each considered object, a rectangle is created representing its immediate path. The rectangle has the width of the object plus the extra_object_width parameter and its length is the current speed of the object multiplied by the time_horizon.

Find earliest collision

We build the ego trajectory footprints as the set of ego footprint polygons projected on each trajectory point. We then calculate the intersections between these ego trajectory footprints and the previously calculated immediate path rectangles. An intersection is ignored if the object is not driving toward ego, i.e., the absolute angle between the object and the trajectory point is larger than \(\frac{3 \pi}{4}\).

The collision point with the lowest arc length when projected on the ego trajectory will be used to calculate the final stop point.

Insert stop point

Before inserting a stop point, we calculate the range of trajectory arc lengths where it can be inserted. The minimum is calculated to satisfy the acceleration and jerk constraints of the vehicle. If a stop point was inserted in the previous iteration of the module, its arc length is used as the maximum. Finally, the stop point arc length is calculated to be the arc length of the previously found collision point minus the stop_distance_buffer and the ego vehicle longitudinal offset, clamped between the minimum and maximum values.

Duration buffer

To prevent chatter caused by noisy perception, two duration parameters are used.

• add_stop_duration_buffer represents the duration of consecutive collision detection with an object for the corresponding stop point to be added.
• remove_stop_duration_buffer represents the duration of consecutive non-detection of collision with an object for the corresponding stop point to be removed.

Timers and collision points are tracked for each dynamic object independently.

Module Parameters

Parameter	Type	Description
extra_object_width	double	[m] extra width around detected objects
minimum_object_velocity	double	[m/s] objects with a velocity bellow this value are ignored
stop_distance_buffer	double	[m] extra distance to add between the stop point and the collision point
time_horizon	double	[s] time horizon used for collision checks
hysteresis	double	[m] once a collision has been detected, this hysteresis is used on the collision detection
add_stop_duration_buffer	double	[s] duration where a collision must be continuously detected before a stop decision is added
remove_stop_duration_buffer	double	[s] duration between no collision being detected and the stop decision being remove
minimum_object_distance_from_ego_trajectory	double	[m] minimum distance between the footprints of ego and an object to consider for collision
ignore_unavoidable_collisions	bool	[-] if true, ignore collisions that cannot be avoided by stopping (assuming the obstacle continues going straight)