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A new optimal control strategy for dealing with braking actuator failures in a vehicle equipped with a brake-by-wire and steer-by- wire system is described. The main objective of the control algorithm during the failure mode is to redistribute the control tasks to the functioning actuators, so that the vehicle performance remains as close as possible to the desired performance in spite of a failure. The desired motion of the vehicle in the yaw plane is determined using driver steering and braking inputs along with vehicle speed. For the purpose of synthesizing the control algorithm, a non-linear vehicle model is developed, which describes the vehicle dynamics in the yaw plane in both linear and non-linear ranges of handling. A control allocation algorithm determines the control inputs that minimize the difference between the desired and actual vehicle motions, while satisfying all actuator constraints.

In this paper a method of mitigating the consequences of potential brake actuator failure in vehicles with brake-by-wire (BBW) and possibly with steer-by-wire (SBW) systems is described. The proposed control algorithm is based on rules derived from general principles of vehicle dynamics. When a failure of one actuator is detected, the algorithm redistributes the braking forces among the remaining actuators in such a way that the desired deceleration of vehicle is followed as closely as possible, while the magnitude and the rate of change of the yaw moment caused by asymmetric braking are properly managed. When vehicle is equipped with BBW system only, or when the desired deceleration can be obtained by redistributing of braking forces, without generating an undesired yaw moment, no steering correction is used. Otherwise, a combination of brake force redistribution and steering correction (to counter the yaw moment generated by non-symmetric braking) is applied.

In this paper the effects of rear brake imprecision on vehicle braking performance and yaw dynamics are investigated for a vehicle with individually controlled brake actuators. The effects of side to side brake force imbalance on vehicle yaw rate and path deviation during straight line braking and in braking in turn maneuvers are examined through analysis, simulations and vehicle testing. These effects are compared to the influences of disturbances encountered during normal driving such as side winds and bank angles of the road. The loss of brake efficiency due to imprecision in generating actuating force is evaluated for different types of vehicles and different levels of vehicle deceleration. Requirements regarding path deviation during straight line braking and braking efficiency on low friction surfaces were found to lead to the most stringent specifications for actuator accuracy in realizing the desired braking forces.