Improving heavy duty vehicle stability during split-μ braking situations with rear axle steering

Abstract

This thesis investigates the use of rear axle steering (RAS) to improve the stability of heavy-duty vehicles during split-μ braking scenarios. Split-μ braking, where the friction coefficient differs between the left and right wheel paths, generates asymmetric longitudinal forces and induces yaw moments that can compromise vehicle stability, control and safety. Controllers for the RAS actuator were developed and evaluated, including PID control, gain-scheduled Linear Quadratic Regulator (LQR), robust H∞ control, adaptive Model Predictive Control (MPC), and a brake pressure feed-forward approach. The controllers were implemented and assessed in a co-simulation environment using MATLAB/Simulink and IPG TruckMaker. The stability properties of the controllers were analysed using an induced-norm-based metric to assess their influence on the driver’s intended vehicle behaviour. Performance was evaluated across multiple scenarios, including straight-line braking, curved-road braking, and lane-change manoeuvres under various slit-μ conditions. Performance metrics included yaw rate, yaw angle, lateral deviation, braking distance, and driver steering effort. The results demonstrate that RAS can significantly improve lateral stability and reduce driver effort without notably increasing braking distance. Among the evaluated methods, the LQR and PID controllers provided the best overall performance in terms of stability and driver workload reduction, with the LQR offering a balance between performance and control effort. The H∞ controller showed robustness, while the MPC showed less consistent performance for short-duration, high-dynamics manoeuvres. The feed-forward approach proved effective in reducing initial yaw disturbances when combined with feedback control. Furthermore, it is shown that a more aggressive ABS strategy can be applied without compromising stability when combined with RAS control, thereby improving both braking performance and stability. The results also show that performance is primarily limited by the steering rate of the RAS actuator rather than by its maximum angle. Additionally, the Linear Parametric Varying (LPV) reference model used for control design can be further extended to improve controller performance and expand the operational capability of the RAS system. Overall, the thesis confirms that rear axle steering is a viable approach for enhancing vehicle stability during critical split-μ braking scenarios and can be implemented using existing vehicle signals without requiring predictive sensing technologies.