Along with the innovation of vehicle technology, the active steering system has an excellent effect on the prevention of uncontrolled steering events due to its significant advantage in optimising handling stability. Meanwhile, the safety boundary is an important judgement basis for the stable operation of the vehicle, and based on the safety boundary, the controller can help the driver to keep the vehicle in the stable region of the state space. In this paper, an active rear wheel steering model prediction controller is proposed based on the safety boundary to control the rear wheel steering angle to assist the front wheel steering, and constrain the actual cross-swing angular angular velocity and centre-of-mass lateral deflection angle of the vehicle within the safety boundary of the state space, so as to ensure the stable operation of the vehicle, and the main research contents are as follows:
1. Aiming at the problem that the linear two-degree-of-freedom model of the vehicle can't show the actual running state of the vehicle well, the thesis starts from the two aspects of the vehicle's degree of freedom and the vehicle's tyre model, and selects the magic formula tyre model and the vehicle's two-degree-of-freedom model at the modelling level, and makes the stability controller better capture the vehicle's stability in the range of the normal sideways speed and the front wheel angle steering in the case of not much amplitude, thus demonstrating the vehicle's actual motion state. The stability controller can better capture the stability of the vehicle in the normal lateral speed range and the front wheel steering angle is not too large, so as to show the actual motion state of the vehicle.
2. Facing the problem that the controller cannot control the vehicle in real time according to the position and type of the vehicle balance in the system, the thesis carries out research on the phase diagram and the safety boundary, by drawing the plane phase diagram of the vehicle, observing the relationship between the stable balance point and the saddle point under different steering angles, as well as the region of attraction or the stability region about these balance points, and then delineating the safety boundary of the vehicle that can be operated in a stable manner, so as to Help the controller to identify whether the vehicle is in the stable region or not.
3. Aiming at the problem of steering instability caused by the mismatch between controller and control strategy, in view of the advantages of model predictive control rolling optimisation and combination with feedback control, an active rear-wheel steering system based on model predictive control is designed, where the controller controls the rear wheels to steer and assists the front wheels in steering so that the vehicle's centre-of-mass lateral deflection angle and the traverse angular velocity are kept within the safety boundaries for the vehicle's stable steering operation.