Investigation of Polynomial Trajectory Planner for Emergency Maneuver
Abstract: Getting an autonomous car to work is a complex task, and it is developed in steps. Today there are many semi-autonomous Advanced Driver Aid Systems in cars such as Adaptive Cruise Control and Lane Keeping Aid to improve comfort for the driver. Some systems will help brake a vehicle in case an accident is imminent, but this thesis will attempt to investigate solutions for doing emergency evading maneuvers when a static obstacle is late detected ahead of the car. When an obstacle is late detected an emergency routine will be triggered. The planner chosen is a polynomial based planner in a Frenet coordinate system. The planner is searching a state space for position in lateral and longitudinal direction. The algorithm will find qupintic and quartic polynomials between current and end positions. A Pure Pursuit path tracker will make the car follow these curves. The trajectory chosen is selected using a cost function. The cost function will assess different factors of the trajectories and give them a cost to compare. The goal of the thesis is to investigate how sensor range, replan rate and trajectory density in the trajectory planner affect the performance in terms of obstacle avoidance. The performance is judged by the portion of 50 trials that do not result in a collision. It was found that detection distance has a linear relation to success rate up to a range of 23 meters, after that it is basically collision free. The replan rate also has a proportional relation to success rate, until a point where the performance is capped. The last factor investigated was the trajectory density. It has little effect between the highest densities, but when it is more sparse will have a significant impact. An issue that was encountered was that accelerations were excessively large in the simulator. This causes the reliability of some maneuvers to be uncertain and reduced the legitimacy of the results. The sensor range where a maneuver can be made with reasonable accelerations is 25 meters.
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