Study Of Direct Yaw Moment Control For In-wheel Motor Electric Vehicles

In the context of the global energy crisis, environmental degradation, and batterytechnology improvements, the electric vehicle (EV) has recently emerged and flourished.The in-wheel motor electric vehicle (IEV) is one form of EV, which is driven by fourin-wheel motors, each of which independently drives one wheel. Since the structure of IEVchanges greatly, the dynamics characteristics and stability control of IEV are of a largechallenge.Direct yaw moment control (DYC) is an advanced active safety control system focusingon the vehicle stability control. The advantages of developing DYC for IEV are divides intotwo aspects.On one hand, DYC solutes the vehicle stability issues, including the vehiclestates estimation and staibilty control. On the other hand, IEV provides a better platform forDYC. Due to the advantageous structure of IEV, the driving and the combined DYC systemare easily implemented on IEV without any structural alteration. Therefore, the performanceof the DYC system can be upgraded on IEV.This dissertation develops advanced DYC systems using the model based design(MBD)method for the purpose of vehicle stability improvement. It is funded as a sub program byNational Basic Research Program (Program973)‘Research on Key Basic Issues of HighPerformance In-wheel Motor Electric Vehicles’(No.2011CB711201). The main work of thisdissertation is as follows:According to the MBD method, a14DOF vehicle dynamics model is built in thisdissertation to provide a simulation platform for the DYC system design. In the14DOFvehicle model, the tire model ’UniTire’ by Prof. Guo is utilized in order to improve the modelprecision, and the driver model ’Optimal Preview Lateral Acceleration Driver Model’ by Prof.Guo is used to realize the driver-vehicle closed-loop simulation. The validation results show that the vehicle model herein well matches the experimental data. Furthermore, a simplifiedin-wheel motor characteristics model is built, with the parameters identified from thein-wheel motor experimental data.Two nonlinear vehicle states observers are developed in this dissertation to accuratelyestimate the vehicle slip angle and tire forces. Firstly, the sliding mode observer (SMO) isadopted after the comparison with several popular nonlinear observers, whose feasibility isstrongly restricted in a vehicle system. Secondly, the SMO design methodology is detailedand the system damping terms are added into SMO to derive the higher convergence speed.Finally, a full order SMO (FO-SMO) and a reduced order SMO (RO-SMO) are developedbased on a7DOF vehicle model. The FO-SMO uses the four wheel angular speed as theinputs, while the RO-SMO employs the four longitudinal tire forces as the inputs. The resultsshow that the error dynamics converge fairly fast, and the vehicle slip angle and tire forcesare well estimated by both the FO-SMO and RO-SMO.The controller design in this dissertation is performed in terms of two differentdirections: the slip angle based control and the yaw rate based control. The slip angle basedcontroller DSCDYC, which is from the angle of vehicle stability, has the control goal ofminimizing the vehicle slip angle. The yaw rate based controller DMbDYC, which is fromthe angle of vehicle response, has the control goal of making the yaw rate meet the driver’sintention.The hierarchical control architecture is utilized in the DSCDYC system. In the uppercontroller, the desired yaw moment is determined by means of the dynamic surface control(DSC) method, which is proposed based on the sliding mode control. DSC is an advancednonlinear control method for mismatch system and amends the ’term explosion’ issue of themultiple sliding surface (MSS) control dealing with the mismatch systems. Furthermore,DSC is a perfect control law for the vehicle slip angle control, since the vehicle system withthe slip angle to be stabilized is a mismatch system. In the lower controller, the longitudinalforces dynamic distribution strategy (LoFDDS) is developed on the basis of the deepanalysis of the tire mechanics. Three major advantages are given by LoFDDS:(1) LoFDDS to some extent ensures the vehicle longitudinal response according with the driver’sintention;(2) LoFDDS makes the vehicle steady state steering characteristics assist tostabilize the vehicle;(3) LoFDDS computes a precise control boundary using an embedded‘UniTire’ tire model. The simulation results show that the vehicle slip angle is perfectlyrestricted by the DSCDYC system, and the vehicle stability is therefore improved greatly.The current yaw rate based DYC system calculates the driver’s intention throughmeasuring the steering wheel angle. However, in some emergency conditions, due to panic,the driver may make an incorrect maneuver. In this case, this method is meaningless,because the steering wheel angle cannot reflect the driver’s intention. Unlike common DYCsystem, the DMbDYC system predicts the driver’s intention via a driver model. The drivermodel ‘Single Point Preview Optimal Lateral Acceleration Driver Model’ is modified toreal-timely calculate the control goal: the driver’s desired yaw rate. The sliding mode control(SMC) and the LoFDDS strategy are then utilized to complete the DMbDYC system.Subsequently, a general real-time test platform is built, including hardware-in-the-loop (HIL)mode, software-in-the-loop (SIL) mode, and simulator mode. Finally, the SIL mode of theplatform is used to validate the DMbDYC system. The ’virtual vehicle’ with the DMbDYCsystem is maneuvered by a real driver. The results show (1) the driver model is able toprecisely predict the driver’s intention;(2) the vehicle response is regulated according to thedriver’s intention by the proposed DMbDYC system;(3) the vehicle stability is greatlyimproved.The main innovation points of this dissertation are as follows:1. The sliding mode observer (SMO) for IEV is developed to real-timely estimate theslip angle and tire forces. The SMO embeds the7DOF vehicle model and UniTire model.The system damping terms are added into the nonlinear observer to speed the convergenceand to inhibit the SMO chattering.2. The DSCDYC system is developed, which directly controls the vehicle slip anglebased on the dynamic surface control (DSC) method. DSC is a perfect control law for thevehicle slip angle control, since the vehicle system with the slip angle to be stabilized is a mismatch system.3. The LoFDDS strategy is developed to perfect the proposed DSCDYC and DMbDYCsystems. The tire model ‘UniTire’ is embedded in the LoFDDS to precisely compute the tireadhesive limit and to maximally explore the tire potential. Besides, LoFDDS takes advantageof the vehicle steady state characteristics to stabilize the vehicle and ensures the vehiclelongitudinal response according with the driver’s intention.4. The innovative DMbDYC system, which embeds the driver model, is developed. Bypredicting the driver’s intention real-timely, the DMbDYC system improves the currentcontrol target. It reduces the affect of the driver’s mistaken maneuver, and applies the controltorque earlier and timelier.