Viability of differential braking based steering redundancy for an autonomous vehicle

Abstract

The exploration of differential braking as an alternative steering method for autonomous vehicles is a pivotal theme of this research, particularly in the context of steering system failure. Differential braking, which applies varying braking forces to the wheels, has been integral to enhancing vehicle stability and maneuverability in systems like Electronic Stability Control and Differential Braking Assisted Steering. This thesis underscores the importance of redundant systems in autonomous vehicles, given the rapid advancements in autonomous driving technologies and the inherent risks of steering system failures, especially at high speeds. Differential braking emerges as a promising safety net in such critical situations. The historical progression of differential braking is examined, from its initial role in cornering stability to its integration with advanced control strategies that enable sophisticated vehicle control tasks. The technology has evolved significantly, now being combined with active steering systems to improve vehicle handling. The research methodology involves assessing differential braking as an emergency steering system through double lane change maneuvers based on NATO standard. A reference trajectory is generated for the vehicle, and two PID controllers are optimized to minimize trajectory and yaw angle deviations. The optimization process uses a heuristic algorithm inspired by natural swarms, Particle Swarm Optimization, which iteratively updates to find the optimal solution. The algorithm's parameters are carefully chosen to ensure stability and convergence, with the optimization process tailored to reduce computation time. A pseudo-ABS system is introduced to manage wheel slip during intense braking, ensuring optimal braking performance and preventing loss of traction. The system adjusts braking force based on longitudinal slip, with control margins customized to the tire and ground characteristics. A co-simulation approach is employed, integrating a high-fidelity vehicle model with the controller, to evaluate the system. The simulations are conducted at different vehicle speeds, with the optimized controller gains derived from the higher speed. The vehicle model used is well-documented and validated, ensuring the reliability of the simulation results. The findings reveal that at higher speeds, the vehicle can successfully navigate the maneuver within the course boundaries, though with some response lag. The pseudo-ABS system effectively controls longitudinal slip, maintaining tire traction. At lower speeds, the vehicle performs the maneuver with minimal delay, but the gains tuned for higher speeds may be too aggressive, indicating the need for speed-dependent tuning. The impact of front wheel geometry on differential braking performance is also explored, with the scrub radius identified as a critical suspension parameter. Simulations with varying scrub radii show that there is a threshold below which the necessary steering torque for successful maneuvers is not produced. While larger scrub radii improve differential braking effectiveness, they are not typically preferred for road vehicles due to the negative implications for steering effort, tire wear, and road feel. The study concludes that the default scrub radius offers an optimal balance, allowing the vehicle to follow the reference path with minimal steering wheel oscillations. In conclusion, the research confirms the viability of differential braking as a redundant steering system for autonomous vehicles, as demonstrated by successful simulations of obstacle avoidance maneuvers. The simple yet effective PID controllers could be further enhanced by optimizing control parameters for specific vehicle speeds. The significant influence of front wheel geometry, particularly the scrub radius, on the success of differential steering is highlighted, with an optimal radius being necessary to generate sufficient steering torque while minimizing unwanted steering vibrations. The potential of differential steering as a reliable redundant steering system is underscored, with future work aimed at leveraging predictive model-based control systems to refine such systems in autonomous vehicles.