Hierarchical Control Strategy Research For Direct Yaw-moment Control Based On Tire Model

Direct Yaw-moment Control (DYC)is an important part of the vehicle ElectronicStability Program. It often works with the early developed longitudinal stabilitycontrol systems such as Anti-lock Braking System (ABS) and Traction ControlSystem (TCS) to achieve a global protection of the vehicle and the passengers.Compared with the Active Front Steering (AFS) and4Wheel Steering who are alsodeveloped to keep the vehicle lateral stability, DYC can still use the margin of thelongitudinal force to adjust the lateral motion of the vehicle, when the tire lateral forceis almost saturated. So the effective area where DYC can work is much larger thanthat of AFS or4WS. When the electric vehicle attracts more and more attention fromour government and society, many researchers focus on the research of the applicationof the differential-driving DYC to the distributed motors drive electric vehicle.Compared with the traditional DYC which adopts differential braking to achieve theexternally required yaw moment, differential-driving DYC can not only keep thevehicle stability, but also reduce the disturbance to the driver caused by active braking.Actually, the control strategy of the differential-driving DYC and the tradition one arealmost the same. The realization of the control input and the state observation of thetire are even easier for the differential-driving DYC. As a result, it is necessary tohave a deeper research on the traditional differential-braking DYC. Especially, thedesign and optimization of the DYC strategy from the aspects of the tire dynamicsand vehicle dynamics can drastically reduce the difficulty of debugging the program.Firstly, a literature review of the Magic Formula (MF) tire model ispresented.How the MF model deals with the transient tire property, tire camber,different normal loads and tire-road frictions has been specificallydescribed.Validation and evaluation have been made via polynomial fitting andsimulation test. The overall calculating process of the MF tire forcesusing physicallybased model under the combined slip condition has been summarized clearly, and theerror easy to occur in using the physically based modelis pointed out and analyzed.These works are the basis for the following hierarchical control strategy based on theon-board tire model.Then, the estimation method of the tire-road friction which is a key state for thereal vehicle control is introduced.Two indexes reflecting the nonlinearity of the vehicle are proposed to identify the tire-road friction in lateral vehicle dynamicscontrol.Based on the identified friction, a simple but efficient engine control strategyfor the steady circular test is proposed. Field tests validated the identification strategyand proved that the engine control strategy can response to the driver input mostly oncondition that the vehicle is stable.Further optimization of the yaw-rate prediction via a nonlinear vehicle model isstudied. Additional feedback compensation makes the prediction algorithm morerobust. Off-line simulations show that the proposed prediction algorithm is effective.To obviously embody the promotion for DYC by using predicted yaw rate,combinedsimulation of Carsim and Simulink is adopted.At last, the whole flow of the DYC hierarchical control strategy is studied, andmuch attention is paid to the acquisition of the target wheel slip in the lower controller.The target wheel slip is determined using on-board tire model directly, and theabnormal states caused by the tire model error are also considered. Transient andsteady state vehicle tests under different frictions are conducted by combinedsimulation of Carsim and Simulink to validate the effectiveness of the proposedhierarchical control strategy.