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Technical Paper

Unified Control of Brake- and Steer-by-Wire Systems Using Optimal Control Allocation Methods

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.
Journal Article

Stability and Control Considerations of Vehicle-Trailer Combination

In this paper, dynamics and stability of an articulated vehicle in the yaw plane are examined through analysis, simulations, and vehicle testing. Control of a vehicle-trailer combination using active braking of the towing vehicle is discussed. A linear analytical model describing lateral and yaw motions of a vehicle-trailer combination is used to study the effects of parameter variations of the trailer on the dynamic stability of the system and limitations of different control strategies. The results predicted by the analytical model are confirmed by testing using a vehicle with a trailer in several configurations. Design of the trailer makes it possible to vary several critical parameters of the trailer. The test data for vehicle with trailer in different configurations is used to validate the detailed non-linear simulation model of the vehicle-trailer combination.
Technical Paper

Influence of Chassis Characteristics on Sustained Roll, Heave and Yaw Oscillations in Dynamic Rollover Testing

In dynamic rollover tests many vehicles experience sustained body roll oscillations during a portion of road edge recovery maneuver, in which constant steering angle is maintained. In this paper, qualitative explanation of this phenomenon is given and it is analyzed using simplified models. It is found that the primary root cause of these oscillations is coupling occurring between the vehicle roll, heave and subsequently yaw modes resulting from suspension jacking forces. These forces cause vertical (heave) motions of vehicle body, which in turn affect tire normal and subsequently lateral forces, influencing yaw response of vehicle. As a result, sustained roll, heave and yaw oscillations occur during essentially a steady-state portion of maneuver. Analysis and simulations are used to assess the influence of several chassis characteristics on the self-excited oscillations. The results provide important insights, which may influence suspension design.
Technical Paper

Exploring the Trade-Off of Handling Stability and Responsiveness with Advanced Control Systems

Advanced chassis control systems enable a vehicle to achieve new levels of performance in handling stability and responsiveness. In recent work by NHTSA and others, the performance of Electronic Stability Control (ESC) systems has been studied with focus on yaw stability and roll stability of vehicles on high friction surfaces. However, it is recognized that vehicle handling responsiveness is also an important aspect that should be maintained. This paper explores the trade-offs between yaw rate, side slip, and roll motions of a vehicle, and their relationships to handling stability and handling responsiveness. This paper further describes how various control systems are able to manage these motions. The paper also discusses methods to assess vehicle stability and responsiveness using specific maneuvers and measurements, and it includes data from vehicle tests on a slippery surface.
Technical Paper

Estimation of Vehicle Side Slip Angle and Yaw Rate

An algorithm for estimation of vehicle yaw rate and side slip angle using steering wheel angle, wheel speed, and lateral acceleration sensors is proposed. It is intended for application in vehicle stability enhancement systems, which use controlled brakes or steering. The algorithm first generates two initial estimates of yaw rate from wheel speeds and from lateral acceleration. A new estimate is subsequently calculated as a weighted average of the two initial ones, with the weights proportional to confidence levels in each estimate. This preliminary estimate is fed into a closed loop nonlinear observer, which generates the final estimate of yaw rate along with estimates of lateral velocity and side slip angle. Parameters of the observer depend on the estimated surface coefficient of adhesion, thus providing adaptation to changes in road surface coefficient of adhesion.
Technical Paper

Estimation of Vehicle Roll Angle and Side Slip for Crash Sensing

Estimation of vehicle roll angle, lateral velocity and side slip angle for the purpose of crash sensing is considered. Only roll rate sensor and the sensors readily available in vehicles equipped with ESC (Electronic Stability Control) systems are used in the estimation process. The algorithms are based on kinematic relationships, thus avoiding dependence on vehicle and tire models, which minimizes tuning efforts and sensitivity to parameter variations. The estimate of roll angle is obtained by blending two preliminary estimates, each valid in different conditions, in such a manner that the final estimate continuously favors the more accurate one. The roll angle estimate is used to compensate the gravity component in measured lateral acceleration due to vehicle roll or road bank angle. This facilitates estimation of lateral velocity and side slip angle from fundamental kinematic relationships involving the gravity-compensated lateral acceleration, yaw rate and longitudinal velocity.
Technical Paper

Effects of Brake Actuator Error on Vehicle Dynamics and Stability

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.
Technical Paper

Control of Brake- and Steer-by-Wire Systems During Brake Actuator Failure

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.
Technical Paper

Closed Loop Yaw Control of Vehicles Using Magneto-Rheological Dampers

A new vehicle suspension control system that enhances vehicle stability and handling in fast evasive maneuvers performed close to the limit of adhesion is evaluated. The central idea is to use continuously variable magneto-rheological (MR) dampers to distribute the damping forces between front and rear axles in order to bring the vehicle yaw rate as close as possible to the desired yaw rate. This mitigates the vehicle oversteer or understeer tendencies during quick transient maneuvers. The basic principle of system operation is explained using known dynamic properties of MR dampers, vehicles and tires. The available control authority and the effect of MR damper settings on vehicle yaw response is then evaluated using computer simulations. The results of vehicle tests are presented. They demonstrate the benefits of the proposed control method in terms of improved vehicle response and reduced driver steering effort.