Study On Vehicle Chassis Control Integration Based On General Actuator-Plant Structure

Vehicle chassis control integration is a research focus of vehicle dynamics in recent years. Its contents are continuously enriched with the developments of modern control theories, x-by-wire technology, sensor fusion technology and so on. Therefore, it is worthwhile to investigate the potential performance benefits of control integration from theoretical aspects, as well as the limitations of individual subsystem control. This thesis is attempting to investigate the effects of vehicle motion control integration based on couplings of tyre characteristics and vehicle motion states.Since all vehicle dynamics control problems can be essentially summarized to the control process of general force (longitudinal, lateral forces and yaw moment) introduced from tyre-road interface to vehicle motion states, a"general actuator-plant structure"can thus be used to describe them. Here, the"actuator"includes the force generation from tyre-road interface and the particular utilization approaches, while the"plant"is corresponding to the vehicle motion states of interest.Based on the proposed structure, tyre force control and vehicle motion control can be decoupled into a servo loop and a main loop respectively. In the servo loop, based on the nonlinear tyre characteristics, Sequential Quadratic Programming (SQP) has been used in tracking control strategy to realize the target general force among wheels by minimizing a properly defined cost function. The control outputs have then been converted to the physical variables that can be implemented by the coordination of steering control (4WS) and longitudinal slip control (LSC).Since the vehicle stability control problems in general can be represented in terms of the reference control for yaw rate and regulator control for vehicle sideslip angle, the classical two-degree-of-freedom feedforward plus feedback framework is used for the main loop based on a simplified vehicle model for control design purpose. For the tyre nonlinearity and the variability of the friction capability, in the main loop, the errors due to servo loop tracking process has been treated as source of uncertainty, thus modern robust control techniques can be applied in consideration of perturbations due to uncertainty and measurement noise.Based on different treatments for the uncertainty perturbations, standard H_∞ optimal control, mixed sensitivity H∞loop shaping and structured singular valueμ-synthesis have been respectively used for the main loop feedback control design. Particularly, standard H∞control is used to reduce the H∞norm of the nominal system directly; mixed sensitivity loop shaping is to shape the frequency responses of system loop transfer functions, leading to reasonable trade-off between system robustness and nominal performances, whileμ-synthesis is to reduce system structured singular value, then system robust stability and performance can be improved based on Small Gain Theorem and by utilizing the structured information of the uncertainties.The control integration for steering control and longitudinal slip control derived from the above procedure can utilize tyre forces (longitudinal and lateral forces) in tyre-road contact patches to improve vehicle active safety. Then, the effect of Active Suspension has been introduced to form the global chassis control integration. Hence, tyre longitudinal, lateral and vertical characteristics can be utilized simultaneously with a coordinated manner, so both ride quality and handling performance improvements can be expected.The significance of control integration can be verified and analyzed through open and closed loop handling maneuver simulations, from which the following conclusions can be drawn: although theoretically subsystem individual control can be put into the proposed separated-loop structure, the resultant control systems can only utilize one particular aspect of tyre coupled characteristics, and exhibits evident force tracking errors which usually exceeds the tolerable uncertainty bound of the control laws. Therefore, system robust performance can not be maintained and satisfactory vehicle handling performance can not be obtained. While for control integration, since tyre characteristics can be fully utilized by subsystem coordination, the uncertainty perturbation caused by tracking error can be remained in an acceptable level so that the robust stability and robust performance can be guaranteed. Therefore, vehicle stability performance under various driving conditions can be expected. By introducing the effect of active suspension, global chassis integrated control can effectively reduce the interactions of control activities for different motion control, thus, control energy requirement is reduced and ride comfort quality can be improved.