Vehicle Dynamics Control for Active Safety Functions using Electrified Drivelines

University dissertation from ; Chalmers tekniska högskola; Gothenburg

Abstract: Studies have shown that even considering pledges and commitments made by various governments and organisations, the growth in electrified vehicle sales is likely to be insufficient to reduce CO2 emissions for mitigating global warming. Some form of added incentive is needed that can help drive electrified vehicle sales in the open market. On the other hand, there is an increased need for traffic safety due to customer demand and the adoption of ambitious goals such as the Vision Zero. This thesis attempts to identify vehicle dynamic opportunities to improve vehicle safety that are enhanced or enabled by electrified drivetrains, thereby offering an opportunity to add value to electrified vehicles and make them more attractive to consumers. As an example of low hanging fruit, the possibility of accelerating an electrified lead vehicle to mitigate the consequences of, or prevent being struck from behind was investigated. A hypothetical Autonomous Emergency Acceleration (AEA) system (analogous to the Automatic Emergency Braking (AEB) system) was envisioned and the safety benefit due to the same was estimated. It was found that the AEA system offers significant opportunities for preventing or reducing injuries in rear-end collisions. The possibility of using propulsion to improve safety in an obstacle avoidance scenario in the presence of oncoming traffic was also investigated. In order to better understand the manoeuvre kinematics, a point mass based optimal control analysis is done, in which a characteristic parameter is identified that correlates well with the need to increase or decrease speed in the manoeuvre for mitigating the risk of collision with the oncoming vehicle. After verification through experiments, an integrated motion controller is formulated, implemented and tested in a high-fidelity simulation environment. Results showed that consistent reductions in collision risk to the oncoming vehicle could be achieved using the integrated controller. Specifically, the results showed that the availability of electric drives consistently reduced collision risk by enabling greater torque vectoring magnitudes and mitigating the deceleration side effect of differential braking. The integrated controller was then evaluated for robustness to steering effort in simulations followed by real-time implementation of the controller and testing using a Volvo XC90 test vehicle. Intersection accidents are then investigated with regards to the possibility of crossing the intersection ahead of a bullet vehicle for collision avoidance. Optimal manoeuvres for the same were derived using analytical optimal control theory and it was seen that optimal manoeuvres could be represented as a maximisation of the tyre forces in a fixed global direction. Based on this finding an integrated motion controller is implemented and tested. Simulation results showed that collision risk can be reduced significantly over a passive vehicle even in limit scenarios where the tyre forces are saturated. In summary, several vehicle dynamic opportunities for improving safety using electrified drivetrains were identified. Detailed investigations of select cases showed that significant safety benefit potentially stands to be gained by appropriate control of electrified drivetrains in the accident scenarios. Consequently, a strong opportunity is seen for adding safety related value to electrified vehicles at little to no extra cost.