Variable Steering Ratio Design For Vehicle Steer-by-wire System With Joystick

Steering-By-Wire (SBW) system removes the linkage between the steering wheel andthe road wheels, which realized driver’s steering purposes by electrical signals. SBW systemmakes the car not constrained by the steering wheel control. Joystick, button and touchscreen can all be used for automobile steering control. Joystick has its unique advantages.For example, The joystick drive system allows a handicapped person who have obstacles onone side of the body to drive a car using a joystick interface with a small force and shortstroke. It also has other advantages such as more casual install and higher utilizationefficiency of interior space, diminishing big angle under low speed to reduce driver’sbusyness, and so on. But the tilt range of joystick is much smaller than the steering wheel(smaller than25°).So how to design steering ratio of SBW system based on joystickstructure to make the vehicle more stability when steering is the core content of this paper.This dissertation is supported by the Young Foundation of the National Natural ScienceFoundation of China named Research on Joystick and Bilateral Control Strategy forSteer-by-Wire Vehicle (No.51105165). Based on the study of Steer-by-Wheel system as wellas the SBW system with joystick at home and abroad, variable ratio characteristics of SBWsystem based on joystick structure had been carried out in-depth research.The main research work is summarized as follows:Firstly, the research on the development of joystick structure, steering wheel structureas well as variable ratio characteristics both at home and abroad was carried out. Thisdissertation compared the two structures and pointed the practical significance of joysticksteering. Due to the difference angle rang between the steering wheel and joystick, thedesign of steering ratio aimed at steering wheel structure cannot be directly applied to thejoystick steering system. Thus, in order to ensure the joystick still can achieve the effect thattraditional steering system achieves, the variable ratio based on joystick steering system hasbeen designed.Secondly, we established a joint simulation control model. On the basis of structure characteristics of SBW system, established SBW vehicle simulation platform withMatlab/Simulink and CarSim. Their seamless connection was realized by setting the modelinterface of the two software. The platform was mainly used for contrast emulation ofdifferent joystick variable transmission ratio control strategy.Thirdly, Combined with the existing research results, considering difference betweenjoystick system and steering wheel system, this paper designed variable ratio characteristicaffect by vehicle speed, variable ratio characteristic affect by speed as well as tilt angle, ratiocharacteristic with fixed yaw velocity gain. Studies found that the ratio characteristic withfixed yaw velocity gain couldn’t meet the basic requirements of joystick steering. So thisdissertation proposed a new control strategy of variable ratio, which is variable yaw velocitygain control strategy. This dissertation extracted steering sensitivity and vehicle yawingangular velocity gain as the design principle of angle ratio at high speed and moderate speedsegment respectively. The control strategy was verified by software simulation platformproving variable gain ratio characteristic on SBW system has certain advantages comparingwith other control strategies.Finally, hardware in-the-loop test bench of SBW based on dSPACE system wasestablished, and driving simulator experiments were carried out. Considering the constraintsfrom law and the existing experimental condition, there is no vehicle or test vehicle forjoystick steering, we set up hardware in loop simulation test bench with joystick for furtherresearch on the effectiveness of the control strategy proposed. Using hardware in-the-looptest bench of SBW based on dSPACE, we contrast the drivers’ lane tracking capability andevasive maneuvering capability between steering wheel and joystick steering. The resultsshowed that variable ratio characteristic on SBW system with joystick was superior to thesteering wheel, and it could guarantee the driving stability of vehicle more reliable. Inaddition, variable ratio characteristic on SBW system is superior to conventional steering,especially in the process of vehicle obstacle avoidance. Experiments show that withreasonable variable angle transmission ratio design, joystick control steering system canrealize good vehicle handling stability.