Rollover Stability And Anti-Roll Control For Sport Utility Vehicles

Vehicle rollover, as a serious safety problem, has been received more attention in recent years. An investigation by National Highway Traffic Safety Administration indicated that most fatal accidents owned to rollover, just less than crashes. Yet, the dynamic rollover stability and rollover warning of vehicle in cornering with a large steering angle and at a high speed have not been touched on in current publications. Therefore, the dynamic rollover stability and the control strategy based on rollover warning for sport utility vehicles (SUV) have been studied in this paper. The results and the main contributions of the dissertation are as the following.(1) A linear model is established for a kind of SUV with three degrees of freedom (DOF) in a frame of coordinates fixed on the vehicle body, including lateral, yaw, and roll motion, for given longitudinal motion. The main parameters that have an influence on the rollover performance of vehicle in critical driving manoeuvre are included in the model. Also, a linear dynamic model of tire is established after taking the roll deformation of the tires and suspension into account. From the above models, the balance conditions and the stability conditions of vehicle rollover are determined on the basis of the Routh-Hurwitz criterion. Then, a so-called dynamic stability factor (DSF) is defined to reveal the effects of main parameters on the stability of vehicle rollover. Compared with the traditional static stability factor, DSF includes not only the height of center of gravity and track width of vehicle, but also the vehicle parameters and manipulation parameters, such as the vehicle speed, the front-wheel angle, the sprung mass, the height of roll center, the longitudinal location of center of gravity, the roll stiffness and damping of suspension, the tire cornering stiffness, the suspension and tire deformation. In order to demonstrate the theoretical results, numerical examples are given for four typical driving manoeuvres. The results show that the rollover response is closer to the actual response by DSF than the static's.(2) To analyze the stability of the car-driver closed loop system, a linear model for driver's reaction to rollover is established in the closed loop system. The effects of driver's parameters on the stability and frequency performance of vehicle rollover are analyzed under the assumption that the closed loop system has no time delay. Then, taking the time-delay into account, the influence of driver's parameters on delay-independent stability of vehicle rollover is discussed on the basis of the generalized Sturm criterion; and the critical time delay of the closed loop system is determined when the delay-independent stability conditions does not hold. The theoretical results are vilified by numerical simulations of SUV with the driver's parameters variation in a reasonable range.(3) A rollover warning algorithm is proposed based on the rollover dynamic stability factor. From the dynamic stability factor, a new dynamic rollover indicator is defined, and the critical rollover condition is treated as that the absolute value of the new indicator is larger than 1. According to the rollover warning algorithm, the dangerousness of vehicle can be described by time-to-rollover (TTR), which is defined as the time which takes for the vehicle to reach the critical rollover condition. Then the performance of rollover warning algorithm is analyzed by the relationship between the maximum value of TTR and vehicle structure parameters and manipulate parameters. In order to demonstrate the features of rollover warning algorithm, a case study of SUV is presented. The results show that the rollover warning algorithm can calculate TTR and detect the rollover dangerousness of vehicle timely and accurately.(4) A fuzzy control strategy based on rollover warning with differential braking is proposed, and the effectiveness and robustness of the control algorithm are analyzed. To realize the anti-roll control algorithm based on rollover warning, the dynamic rollover indicator is treated as feedback variable in a 3-DOF vehicle model. Then, the principle of differential braking for anti-roll control is applied to a nonlinear vehicle model of 4-DOF after taking the longitudinal motion into account. The fuzzy PD control algorithm based on rollover warning with differential braking is simulated in Matlab/Simulink. Also, the robustness of the control algorithm is discussed with limited variation of uncertainty parameters, such as the tire pressure. The results show that the control strategy can not only prevent vehicle rollover effectively, but also improve the work efficiency of brake system.(5) To validate the performance of the anti-roll control system, the hardware in loop simulation (HILS) technology is studied. An experimental platform is established, which consists of a personal computer (PC), a multifunction data acquisition card (PCI-6024E), a desktop signal conditioner (SC-2075), a warning/controller, an electro-mechanical braking (EMB) system, and sensors. The graphical user interface and algorithm of HILS system are programmed by using both VC++ and MATLAB. Hence, the vehicle model, driving environment, and some un-designed mechanical actuators can be simulated by the algorithm on a PC. An ARM7.0 microprocessor is applied to compute the TTR and output voltage of the anti-roll control system. The EMB system is composed of a DC motor, a reducer, a ball screw, a hydraulic cylinder, and a brake clamp and disc (un-designed). Under the selected driving manoeuvre, the variables'values of vehicle model are determined. Then the analog signal of vehicle speed and front-wheel angle converted by D/A converter are sent to warning/controller. The current and torque of DC motor, and the pressure in the cylinder are measured by sensors connected with the acquisition card. These analog signals are converted to digital signals by an A/D converter, and then sent to the PC. Finally, the performance of the EMB system, warning/controller, and the anti-roll control system is validated by the test data acquired from the HILS platform. Also, the effectiveness and robustness of the anti-roll control strategy based on rollover warning can be testified on the experimental platform.