Research On Bilateral Control And Variable Ratio Characteristics For Steer-by-wire Automobile

Steering-By-Wire (SBW) system is a teleoperation system with force feedback, whichhas no mechanical linkage between the steering wheel and the front wheels. It is possible tocontrol the steering wheel actuator and the steering gear actuator getting an exact forcefeedback and steering independently. SBW system brings a broad space for the improvementof vehicle performance with the advantages of the modular structure and variable ratio. Theaccurate, stable coupling torque and position control between the blocks is the basis ofnormal operation and the variable ratio can directly affect the vehicle handing stability. Theyare the key technologies of the control method for SBW system.This dissertation is supported by the National Natural Science Foundation of Chinanamed “Research on Control Strategy and Key Technology for X-by-WireVehicle”(No.50775096) and 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 novel SBW bilateral control structure,the characteristics of vehicle steering and steering wheel force feedback, studying ondomestic and abroad research achievements, combined with simulation results andexperiment data of a SBW test vehicle, a bilateral control strategy and variable ratiocharacteristics of SBW system had been carried out in-depth research. The main researchwork is summarized as follows:1. Establishing simulation and experimental platforms for SBW systemA simulation platform for SBW system was established with Matlab/Simulink andCarSim. Based on some assumptions in the model, the steering system interface of Carsimwas set to match up it with the SBW steering block. The platform could be used for the design of SBW control structure and analyzing the effects of variable ratio characteristics onthe stability of vehicle SBW system. Based on a mass production passenger car, thisdissertation introduced the refit program of SBW system, the hardware selection, the motors’driver and the measurement&control system in detail. And analyzed the basis of actuatorselection and its impact on steering execution block. The SBW test car provided the mostpowerful and trusted platform for verifying the SBW control strategies. The least squaremethod was adopted to identify the parameters of the two electric motors. At the same time,the closed-loop system identification method was adopted to identify the mechanicalparameters of the steering wheel block and the steering execution block. The parameters notonly helped to improve the reliability of simulation results, but also be convenient fordebugging, analysis and application of SBW bilateral control strategy.2. Research on a novel bilateral control structure of SBW systemBased on the detailed analysis of the coupling control requirements of the force andposition between the steering wheel and front wheels, a new bilateral control structure wasput forward, in which the system was driven by steering motor torque and feedback bypinion angle. The steering wheel was controlled by position closed-loop. The torquedisturbance of driver could be measured by torsion bar sensor, and steering block wascontrolled by torque closed-loop. This idea simplified the control structure, meanwhileavoided the measurement or estimation of the steering resistance. This dissertationestablished a necessary bilateral control model and analyzed its transparency, and introducedthe position controller of the steering wheel block and the torque controller of the steeringexecution block of the SBW test vehicle. The precision of the position control of the steeringwheel block had been greatly improved by angle feedback, acceleration feedback, frictioncompensation and torque disturbance control. The equivalent parameters of the friction,damping and inertia of the SBW system could be adjusted by inertia compensation, dampingcompensation and friction compensation control. The friction compensation control in thetwo blocks considered the dynamic process of dynamic-static friction, which improved theprecision of the steering wheel position control. The dissertation analyzed the passivity ofmechanical steering system and SBW system combined the theory of passive stability, and established passive observer and controller according to the bilateral teleoperation properties.The effectiveness of the stability control of the bilateral control structure was verified usingthe SBW test vehicle.3. Research on variable angle ratio considering the steering characteristicsAfter the analysis of classification of passenger car chassis performance and the effectson vehicle handling of the angle ratio, two objective evaluation indexes of the steady stateyaw rate gain and steering sensitivity were adopted. They are closely related to the staticangle ratio characteristics. Combined with the research results and test data both domesticand abroad, the reasonable range of the indexes was summarized and the general variationsof angle ratio versus speed or pinion angle were discussed. A new angle ratio design methodwith variable steering gain was put forward. The angle ratio at high speed was based on thesteering sensitivity. The ratio at medium speed considered the angle of the steer wheel andyaw rate gain in general condition. The ratio at low speed was based on the results ofsubjective evaluation of the drivers. When the speed was constant, the angle ratio can also“expand” the “linear zone” of vehicle response. Objective experiments were conducted tocompare the characteristics of the SBW vehicle model with different ratio. The experimentresults showed that the SBW vehicle in common mode can steer flexible at low and mediumspeed and steer steadily at high speed. Sport mode improved the path tracking response andthe performance of obstacle avoidance at medium and high speed.4. Research on torque ratio based on the steering wheel torque characteristicsAccording to the classification of the steering wheel torque characteristics of passengercars, Objective evaluation indexes were picked to analyze the torque characteristics of thesteering wheel in3aspects including driving on-center, at different lateral acceleration andparking. After the analysis of the relationship between the resistance torque and road feelingintensity of SBW system and the torque ratio, combined with bilateral control structure andreferring to Electric Power Steering system, this paper defined the concept of “equivalentassist torque” instead of the torque ratio of SBW system. On this basis, the effect of thecharacteristics of the equivalent assist torque and friction compensation control on the torquecharacteristics, especially the on-center handling, was analyzed in detail. Variable friction compensation control and negative equivalent assist torque map were put forward to improvethe road feeling returnability. According to the academic achievements and experiment dataof domestic and abroad, a design method of equivalent assist torque was summarized, and amethod of speed interval division of the equivalent assist torque characteristics was putforward. According to the indexes’ values of a reference vehicle and the ideal steering wheeltorque, combined with the characteristics of angle ratio in different driving mode, the designmethod of equivalent assist torque characteristics in general mode and sport mode at thespeed of100km/h were discussed. Coordinated with the angle ratio in corresponding mode,the general mode had a similar steering wheel force characteristics with the reference vehicle,and the sport mode had a relatively clearer road feeling feedback, higher steering forcesensitivity and smaller steering lag.The innovative contributions of the dissertation are summarized as follows:1. SBW system is essentially a bilateral teleoperation control system with forcetelepresence. A novel torque driven/position feedback bilateral control structure and a newstability control method based on passivity theory were put forward to deal with theproblems of transparency and stability that came from the torque and position couplingcontrol between the steering wheel block and the steering execution block. This not onlysimplified the control structure of SBW system, avoided the observation or measurement ofthe steering resistance torque, also realized the stability control between the master and slaveblocks of SBW system.2. A unique design method of angle ratio with variable steering gain in general modeand sport mode was put forward based on the effects of the static angle ratio and the relatedresearch. The angle ratio of high speed was based on the steering sensitivity. The design ofangle ratio at medium speed considered the input amplitude of the steering wheel angle andthe steady yaw rate gain. The ratio at low speed came from the results of subjectiveevaluation experiments. When the speed was a constant, the ratio declined with a largerpinion angle, which could “expand” the “linearization zone” of vehicle motion response.3. Based on the analysis of the effects of the SBW torque ratio on the characteristics ofthe steering wheel torque, the dissertation defined the “equivalent assist torque” characteristics. In order to improve the definition of road feeling, enhance the indensity ofroad feeling and returnability on center, this dissertation put forward a variable frictioncompensation control method and negative equivalent assist torque characteristics.According to research results and test data domestic and abroad, this dissertationsummarized the design basis of the equivalent assist torque, and put forward a speed intervaldivision method of the equivalent assist torque map based on the static lateral accelerationgain, that simplified the calibration of equivalent power-assisted at the full range of speed.