| Steer-by-Wire system breaks the concept of traditional steering system. The mechanical joint between the steering wheel and the front wheel is cancelled in Steer-by-Wire system and the steering of the vehicle and road feeling simulation is achieved through by the steering motor and the steering feel motor respectively. This totally new concept provides new space for the development of steering system. Based on the project of National Natural Science Foundation "Research on Control methods and Key Technology for X-By-Wire Vehicle (No.50775096)" and "Research on Key Technology for Front Wheel Steer-By-Wire Vehicle (No.50475009)", and our group's research findings, this Ph.D. dissertation makes a deep study on the steering actuators of the Steer-by-Wire system, to improve reliability and responsiveness of the entire system. The control method proposed in this dissertation is tested with Hard-in-Loop simulation too. The main research work is summarized as follows:(1). Propose a Steer-by-Wire system structure with dual steering motorsIn order to prevent the invalidation of the entire system which caused by motor failure, the concept of dual steering motors is introduced in this dissertation. It will not only prevent system invalidation, but also improve the response characteristic. Taking into account increasing control signal and sensor signal and more complicated algorithm caused by dual steering motors, system controller will be overload. So one "Distributed Processing Architecture" is introduced in the system:motors are controlled by Motor Controller, all the algorithm is executed in Algorithm controller, and the communication among all the controller is realized through FlexRay Bus which has a lot of advantages such as high baud rate with fixed period and good fault-tolerance. Combining the Distributed Processing Architecture and the concept of dual steering motors, one Steer-by-Wire system structure with dual motors is built based on FlexRay Bus. This structure can raise the robustness and efficiency of the system and make the system extension and transplantation easier.(2). Proposed an optimization method for time allocation of the FlexRay Bus static segment and dynamic segmentFlexRay Bus is a robust, good fault-tolerance, time-triggered high-speed serial bus, and very suitable for Steer-by-Wire system. Except for the functionality of fault tolerance, FlexRay Bus can support several network topologies, and allow the dual-bus structure coexist with single-bus structure. Besides the basic software and hardware function, the fault tolerance control in FlexRay Bus communication layer is realized in this dissertation. Because that FlexRay Bus protocol does not give a definition of communication layer, after the analysis of physical-layer data flow and message response time, we proposed an optimization method of static slot allocation on the basis of message splitting. In addition, one optimization method of dynamic time configuration based on linear programming and the analysis of the most bad response time is introduced. And thus optimize the real-time performance of FlexRay Bus.(3). Considering different driver steering behavior, a dual-motor Control method under trouble-free circumstance is put forword."Both portability and sensitiveness" is always an important topic in the design of steering system, namely that steering ratio should be alterable with vehicle speed. Considering that steering ratio change and added steering angle can be get in Steer-by-Wire system, and dual steering motor can provide rapider response rate, two type of dual-motor control mode is introduced in this dissertation:One is for the regulation of steering ratio to realize fixed yaw rate gain, as driver steering at normal rate. And the other, driver steering quickly, the yaw rate feedback control is applied by using dual motor, and gets higher dynamic response. In the control algorithm, two types of dual-motor control mode are switched by the result of driver steering behavior recognition which is recognised with the application of the Hidden Markov Model. All the Multi-dimension Gauss Hidden Markov Model is trained for these two cases:driver steering normally and driver steering quickly, and then recognize driver steering behavior by using the trained model.(4). Establish the fault tolerant control method for steering motor and batterySteering motor and battery are the power source of the Steer-by-Wire system. Once there is something wrong with motor and battery, the entired system will become invalid. So, fault-tolerant control of motor and battery is very important. Aftering analysis the motor failure and battery failure, one fault tolerant control method for steering motor is established based on dual steering motor and one fault tolerant control method for battery is established through hardware redundancy technique. For motor fault tolerant control, estimate parameters of these two steering motor through adaptive Kalman filter algorithem, and judge motor fault, and then, determine the operation mode of the trouble-free motor according to the diagnose. For battery fault tolerant control, estimate remaining battery capacity through extended Kalman filter algorithem, and judge battery fault, then, according to the result, determine whether it is necessary to making compensate control through redundant battery.(5).Design and build Hardware-in-Loop test rig of dual-motor Steer-by-Wire system and verify all the control algolithm by experiment in the test rig.The Hardware-in-Loop test rig of dual-motor Steer-by-Wire system is built by combining software with hardware. Because the test rig can simulate a real car, so the control algolithms can be verified by Hardware-in-Loop experiment, without risk and distortion. Through the Hardware-in-Loop experiment mentioned in this dissertation, the experimental results demonstrate that the algorithm is feasible and accurate.So all in all, the innovative achievements made in this paper are shown as follow:(1). Considering the entire single-motor Steer-by-Wire system will break down if motor suddenly goes wrong, and the more control signal, more sensor signal and more complicated algorithm caused by the additional steering motor will make the system controller overload, so under the distributed processing architecture concept, dual-motors Steer-by-Wire system structure is built based on FlexRay Bus. After research on the real time property and fault tolerant property of FlexRay Bus, the fault tolerance control in FlexRay Bus communication layer is proposed and an optimization method for time allocation of the FlexRay Bus static segment and dynamic segment is put forward too.(2). Considering that steering ratio change and added steering angle can be get in Steer-by-Wire system, and dual steering motor can provide rapider response rate, two type of dual-motor control mode is introduced in this dissertation, and these two control mode are switched by the result of driver steering behavior recognition:One is for the regulation of steering ratio to realize fixed yaw rate gain, as driver steering at normal rate. And the other, driver steering quickly, the yaw rate feedback control is applied by using dual motor, and gets higher dynamic response. And the driver steering behavior recognition can be realized with Hidden Markov Model.(3).To avoid severe consequences caused by the motor failure and battery failure, the fault tolerant control method for steering motor and battery is proposed in this dissertation, including a fault-tolerant control method for steering motor based on dual steering motor and a fault tolerant control method for battery through hardware redundancy technique.
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