Fail-safe control allocation for a distributed brake-by-wire system considering the driver's behaviour

摘要

A distributed brake-by-wire system provides flexible and precise braking force control with shorter or no brake pipes. Fail-safe control is a critical part of the system which guarantees braking safety during failure of the brake actuators. The purpose of this paper is to develop a fail-safe control strategy that cooperates better with the driver so as to enhance the fail-safe control performance. Therefore, the driver's behaviour should be taken into account during the design of fail-safe control strategies. Two design goals considering the driver's behaviour are proposed. The first concerns the braking performance; the second concerns whether the driver can handle the failure situation easily. A qualified fail-safe control strategy should meet both the design goals. The driver model used includes an anti-wind-up proportional-integral controller braking operation model and an optimal preview control steering operation model. Two different fail-safe control strategies, namely strategy I and strategy II, are proposed and examined with the design goals. Strategy I is designed to follow the nominal motions of the vehicle in all the three degrees of freedom in the yaw plane. The pseudo-control vector is determined by sliding-mode controllers and allocated to follow the nominal motions. Strategy II is designed to minimize the braking force imbalance between the left side and the right side of the vehicle on the premise that the brake deceleration demand has been achieved. Simulation results show that strategy I generates a longer braking distance and requires a harder brake pedal force, which violates both the design goals. Strategy II meets both the design goals and therefore is a qualified strategy to cooperate with the driver model. Its capability to be implemented in real time is verified on a hardware-in-the-loop test bench. Its advantages considering the unmodelled behaviour of the driver is also discussed. The distributed electrohydraulic braking system developed is used as the distributed brake-by-wire system.