Integrated Control Of Vehicle Four-Wheel Steering And Active Suspension Systems

With the increasing demand of handling and stability, safety, ride performance of vehicle, the steering and suspension systems which affect those performances have a great development, such as Four-Wheel Steering and Active Suspension systems introduced in the last decades. Many research papers indicate that vehicle steering and suspension systems have strong coupling effect and interact with each other. The independent control of steering and suspension systems ignore the existence of other system theoretically and the effect to other system performance, the result is not the simple overlap of the two separate control systems, but usually weakened by each other. So it is necessary to develop integrated control of the two systems. Aiming at the main problems of this research area such as coupling mechanism and coordination strategy, developing a new method for the integrated control of 4WS and Active Suspension shows great significance on vehicle performance.A full vehicle dynamic model including steering and suspension systems is constructed. There are 9 degrees of freedom, i.e., lateral, yaw, heave, pitch, roll of sprung mass and vertical displacement of four unsprung masses. A semi-empirical tire lateral force model is used to describe tire non-linear characteristic and lateral vertical coupling characteristic. Several experiments were carried out to validate the model. At the base of 9dof full vehicle model, a 2dof 4WS model and 7dof Active Suspension model are built separately. A hardware in-loop simulation platform of 4WS is constructed, the actuator of rear steering system is a step motor, the front and rear steering angle is measured by two angular displacement sensors. Hardware in-loop experiment is carried out. The response time of rear wheel steering system is less than 30 milliseconds, almost has no influence on steering system response, and could be left out in the 4WS control algorithm.The coupling mechanism is studied by means of sensitivity analysis. Vehicle body rolls and lateral weight transfers due to centrifugal force of sprung mass. On one side, due to guide mechanism of suspension, roll motion affects roll steering and wheel camber, thus affects steady state cornering characteristics. On the other side, due to tire lateral vertical coupling characteristic, lateral weight transfer affects tire effective cornering stiffness of front and rear axles, and affects steering characteristic.With active suspension control, body roll angle decreases greatly, so does lateral forces produced by roll steering and wheel camber, and steady state cornering characteristic changes. Considering tire lateral vertical coupling characteristic, effect of 4WS and active suspension control on cornering characteristic is also studied.An coordinated control strategy of 4WS and active suspension is proposed based on body roll control. Control quantity of 4WS is adjusted when active suspension is introduced. On the basis of previous control strategy, another coordinated control strategy based on front and rear suspension stiffness distribution is proposed. And the two control strategies are integrated. Control quantity of active suspension is adjusted accoording to the error between the actual yaw rate and the demand yaw rate.By simulating steering wheel step input, the limit performance of vehicle with different control system is researched. 4WS system almost has no effect on vehicle limit performance. Active suspension system has great improvement on vehicle limit performance due to tire load and lateral force improvement. Coordinated control of 4WS and active suspension has further enhancement on vehicle limit performance because of lateral load improvement and force distribution between front and rear active suspension.