Research On The Methods Of Vehicle Nonlinear Tyre Forces Estimation

Vehicle Stability Control (VSC) is an active safety device, which lets the vehicle run following the driver's intension to prevent vehicle from losing stability. Now, VSC is an international research focus in the vehicle active safety field. With the development of control techniques and decrease of control system cost, VSC will replace ABS to be the standard device of modern vehicles. The state variables and enviorment parameters about the movement of vehicle is needed for the VSC system.Some of these variables or parameters can be measured directly, such as wheel speed,acceleration,brake pressure,etc.But others are hard to be measured because of the sensitivity or cost of the measurements,such as vehicle body silp angle,tyre force,etc.With the development of the estimation theory,people want to get these state variables or parameters through estimation.The dissertation pays attention to tyre force estimation of vehicle,especially the nonlinear tyre force when there is a severe steering maneuver.Two kinds of nonlinear system observer are researched,one is the Extended Kalman Filter,the other is the Sliding Mode Observer.Other issues are also discussed:modeling of vehicle and the model validation,simulation of vehicle system using the vehicle dynamics software Carsim,validation of the estimation methods using Carsim.There are five chapters in this dissertation.In Chapter One, many scholars'research achievements in the tyre force estimation area are reviewed.It can be seen that many exsisted methods are based on tyre force empirical or analytical models.They worked well only when the driver's steering maneuver is mild, besides that they require a priori knowledge of the road and tyre.So It's meaningful to research on a tyre force estimation method which doesn't require a tyre force model.In Chapter Two, Prof.Guo Konghui's tyre theory is firstly introduced.Based on this tyre model,an 8-DOF vehicle model is built,which includes vehicle body dynamics model and wheel dynamics model.The vehicle model is verified by thevehicle classic conditional test and adhesion limit condition simulation.It proves that the vehicle model we built can describe the vehicle's dynamic property,even when the tyres are at adhesion limit and tyre forces become saturated. It provides us a good vehicle model for tyre force estimation.Based on this model,the initial measurable data used in the estimation and the"real value"of the tyre force used to compare with the estimation results can be obtained.Based on the concept and traditional design methods of nonlinear system observer are introduced in Chapter Three. In Chapter Four,the Extended Kalman Filter and the Sliding Mode Observer are used to estimate tyre force separately.They are both based on the 7-DOF vehicle model,which is the simplification of the 8-DOF vehicle model. They don't require tyre force model and a priori knowledge of the tyre and road.It can be seen that the estimation result is good from the simulation which is carried out under classical conditional test.Even when there is a severe steering maneuver and the tyre force become saturated,the estimation result is also good. By comparing the estimation result and analyzing the concept of the two methods towards to nonlinear estimation, a conclusion is got that the nonlinear tyre force estimation method based on SMO is more efficient.In Chapter Five,the vehicle dynamics simulation software Carsim is used to validate the estimation methods.Some parameters of the 7-DOF vehicle model used in the estimation are from a car called Sedan, which is built in Carsim. The initial measurable data used in the estimation and the"real value"of the tyre force used to copare with the estimation results are obtained from Carsim.The simulation results show that the estimation methods are efficient and their practical application is significant.After analyzing the simulation results of the nonlinear tyre force estimation,the conclusions are as follows:1. The estimation methods that don't require tyre force model can work well not only when the steering is mild,but also when the steering issevere and the tyre force becomes saturated.The advantage also includes that no priori knowledge of tyre and road is needed.2. The estimation based on EKF shows a good result, but there are many parameters needed to be adjusted,so its design and application are complex.Compared with EKF,SMO has a better estimation result.And more importantly,its design and application are very easy.The disadvantage caused by switch function is that the estimation fluctuates in a high frequency.High order sliding mode observer may solve this problem.