The cross-coupling of lateral-longitudinal vehicle dynamics: Towards decentralized Fault-Tolerant Control Schemes

In recent years there has been an increasing interest in improving vehicle characteristics through the use of Vehicle Control Systems (VCS). In particular, VCS for the lateral (steering) and longitudinal (velocity) dynamics are used to improve the handling properties of a vehicle. Nonetheless, the introduction of the additional elements required for implementing these control systems also increases the possibility of faults. This problem can be mitigated by using Fault Tolerant Control (FTC) systems. The most common approach for steering FTC design is based on the use of a linear Bicycle Model (BM). Using this model decentralized steering controllers can be designed. However, the BM lacks significant lateral and longitudinal cross-coupling dynamics. In fact, the steering and velocity control problem could be viewed as a multivariable cross-coupled problem. In this article VCS for the steering and velocity are designed. The resulting controllers are decentralized and capable of practically eliminating the cross-coupling. A further problem, which has not been widely reported, is the propagation of the failure of one subsystem to other subsystems. It is shown that when the Velocity Control System (VelCS) fails, then the steering subsystem has a degraded performance due to cross-coupling. The main contribution of this article consists in showing that it is possible to detect and accommodate a failure of the VelCS within the steering control system, i.e. without requiring communication among subsystems. This enables a fully independent operation even if faults occur, that is a Decentralized Fault-Tolerant Control Scheme.