DESIGN OF A SEMI-ACTIVE STEERING SYSTEM FOR A PASSENGER CAR

This thesis presents research into an improved active steering system technology for a passenger car road vehicle, based on the concept of steer-by-wire (SBW) but possessing additional safety features and advanced control algorithms to enable active steering intervention. An innovative active steering system has been developed as 'Semi-Active Steering' (SAS) in which the rigid steering shaft is replaced with a low stiffness resilient shaft (LSRS). This allows active steer to be performed by producing more or less steer angle to the front steered road wheels relative to the steering wheel input angle. The system could switch to either being 'active' or 'conventional' depending on the running conditions of the vehicle; e.g. during normal driving conditions, the steering system behaves similarly to a power-assisted steering system, but under extreme conditions the control system may intervene in the vehicle driving control. The driver control input at the steering wheel is transmitted to the steered wheels via a controlled steering motor and in the event of motor failure, the LSRS provides a basic steering function. During operation of the SAS, a reaction motor applies counter torque to the steering wheel which simulates the steering 'feel' experienced in a conventional steering system and also applies equal and opposite counter torque to eliminate disturbance force from being felt at the steering wheel during active control operation. The thesis starts with the development of a mathematical model for a cornering road vehicle fitted with hydraulic power-assisted steering, in order to understand the relationships between steering characteristics such as steering feel, steering wheel torque and power boost characteristic. The mathematical model is then used to predict the behaviour of a vehicle fitted with the LSRS to represent the SAS system in the event of system failure. The theoretical minimum range of stiffness values of the flexible shaft to maintain safe driving was predicted. Experiments on a real vehicle fitted with an LSRS steering shaft simulator have been conducted in order to validate the mathematical model. It was found that a vehicle fitted with a suitable range of steering shaft stiffness was stable and safe to be driven. The mathematical model was also used to predict vehicle characteristics under different driving conditions which were impossible to conduct safely as experiments. Novel control algorithms for the SAS system were developed to include two main criteria, viz. power-assistance and active steer. An ideal power boost characteristic curve for a hydraulic power-assisted steering was selected and modified and a control strategy similar to Steer-by-Wire (SBW) was implemented on the SAS system. A full-vehicle computer model of a selected passenger car was generated using ADAMS/car software in order to demonstrate the implementation of the proposed SAS system. The power-assistance characteristics were optimized and parameters were determined by using an iteration technique inside the ADAMS/car software. An example of an open-loop control system was selected to demonstrate how the vehicle could display either under-steer or over-steer depending on the vehicle motion. The simulation results showed that a vehicle fitted with the SAS system could have a much better performance in terms of safety and vehicle control as compared to a conventional vehicle. The characteristics of the SAS system met all the requirements of a robust steering system. It is concluded that the SAS has advantages which could lead to its being safely fitted to passenger cars in the future. Keywords: steer-by-wire, active steering, innovative, power-assisted steering, steering control, flexible shaft, steering intervention, system failure, safety features

스티어 바이 와이어 시스템의 목표 조향감 재현을 위한 조향 반력 제어

학위논문(박사)--서울대학교 대학원 :공과대학 기계공학부,2020. 2. 이경수.This dissertation focused on the development of and steering assist torque control algorithm of Electric-Power-Steering (EPS) system from the conventional steering system perspective and Steer-by-Wire (SBW) system. The steering assist torque control algorithm has been developed to overcome the major disadvantage of the conventional method of time-consuming tuning to achieve the desired steering feel. A reference steering wheel torque map was designed by post-processing data obtained from target performance vehicle tests with a highly-rated steering feel for both sinusoidal and transition steering inputs. Adaptive sliding-mode control was adopted to ensure robustness against uncertainty in the steering system, and the equivalent moment of inertia damping coefficient and effective compliance were adapted to improve tracking performance. Effective compliance played a role in compensating the error between the nominal rack force and the actual rack force. For the SBW system, the previously proposed EPS assist torque algorithm has been also enhanced using impedance model and applied to steering feedback system. Stable execution and how to give the person the proper steering feedback torque of contact tasks by steering wheel system interaction with human has been identified as one of the major challenges in SBW system. Thus, the problem was solved by utilizing the target steering torque map proposed above. The impedance control consists of impedance model (Reference model with the target steering wheel torque map) and controller (Adaptive sliding mode control). The performance of the proposed controller was evaluated by conducting computer simulations and a hardware-in-the-loop simulation (HILS) under various steering conditions. Optimal steering wheel torque tracking performances were successfully achieved by the proposed EPS and SBW control algorithm.본 논문은 종래의 조향 시스템 관점에서 전동식 동력 조향 (EPS) 시스템과 스티어 바이 와이어 (SBW) 조향 보조 토크 제어 알고리즘의 개발을 중점으로 하였습니다. 기존 조향 보조 토크 제어 알고리즘은 원하는 조향감을 구현하기 위해 종래의 시간 소모적 인 튜닝 방법을 사용합니다. 이러한 주요 단점을 극복하기 위해 새로운 조향 보조 제어 알고리즘을 개발하였습니다. 목표 스티어링 휠 토크 맵은 정현파(Weave test) 및 등속도 스티어링 입력 (Transition test) 모두에 대해 높은 등급의 조향감을 차량 테스트에서 얻은 후 데이터 처리를 하여 설계되었습니다. 스티어링 시스템의 불확실성에 대한 강건성을 보장하기 위해 적응 형 슬라이딩 모드 제어가 채택되었으며, 관성 모멘트 감쇠 계수와 컴플라이언스 계수(Effective compliance)가 제어기 성능을 개선하도록 적응형 파라미터로 선정되었습니다. 컴플라이언스 계수는 계산된 랙 힘과 실제 랙 힘 사이의 차이를 보상하는 역할을 했습니다. SBW 시스템의 경우, 이전에 제안 된 EPS 지원 토크 알고리즘을 개선하고 향상시키기 위해 임피던스 모델을 사용하였으며 스티어링 피드백 시스템에 적용되었습니다. SBW 시스템의 주요 과제 중 하나는 사람과 스티어링 휠 시스템 상호 작용에 의해 안정적인 작동과 사람에게 적절한 스티어링 피드백 토크를 제공하는 방법입니다. 임피던스 제어는 임피던스 모델 (타겟 스티어링 휠 토크 맵)과 컨트롤러 (적응 슬라이딩 모드 제어)로 구성됩니다. 따라서, 상기 제안 된 목표 조향 토크 맵을 이용함으로써 스티어 바이 와이어에서 스티어링 피드백 토크를 절절히 적용 됨을 확인 하였습니다. 제안 된 컨트롤러의 성능은 다양한 조향 조건에서 컴퓨터 시뮬레이션 및 HILS (Hardware-in-the-loop) 시뮬레이션을 수행하여 평가되었습니다. 제안 된 EPS 및 SBW 제어 알고리즘을 통해 최적의 스티어링 휠 토크 추적 성능을 달성했습니다.Chapter 1 Introduction 1 1.1. Background and Motivation 1 1.2. Previous Researches 4 1.3. Thesis Objectives 9 1.4. Thesis Outline 10 Chapter 2 Dynamic Model of Steering Systems 11 2.1. Dynamic model of Hydraulic/Electrohydraulic Power-Assisted Steering Model 11 2.2. Dynamic model of Electric-Power-Assisted-Steering Model 17 2.3. Dynamic model of Steer-by-Wire Model 21 2.4. Rack force characteristic of steering system 23 Chapter 3 Target steering wheel torque tracking control 28 3.1. Target steering torque map generation 28 3.2. Adaptive sliding mode control design for target steering wheel torque tracking with EPS 30 3.2.1. Steering states estimation with a kalman filter 38 3.3. Impedance Control Design for Target Steering Wheel Torque Tracking with SBW 43 Chapter 4 Validation with Simulation and Hardware-in-the-Loops Simulation 49 4.1. Computer Simulation Results for EPS system 49 4.2. Hardware-in-the-Loops Simulation Results for EPS system 61 4.3. Computer Simulation Results for SBW system 77 4.4. Hardware-in-the-Loops Simulation Results for SBW system 82 Chapter 5 Conclusion and Future works 89 Bibliography 91 Abstract in Korean 97Docto

Neural Network Based Approach for the Generation of Road Feel in a Steer-By-Wire System

Steer-by-wire is an advanced steering system which connects the steering wheel with the front wheel by using motors and sensors. Generating the road feel in steer by wire system is an important criterion since there is no mechanical connection between the steering wheel and the front wheel. In present work, Neural Network method is proposed for generating artificial road feel to the driver using the vehicle dynamic parameters such as vertical displacement, self-aligning moment and front wheel angle as inputs. Proposed neural network model was trained using the vehicle dynamic models for estimating the current to be supplied to the feedback motor according to the changing road conditions. Three different road profiles are selected such as dry, wet and icy for the simulation purpose and the estimated motor current values for the road surfaces using neural network are presented. From the simulation results for the sinusoidal road surface and sinusoidal steering angle driver input, it is clear that the neural network based method is able to produce the varying road feel to the driver for the different road conditions

스티어 바이 와이어 시스템 조향 반력 및 랙 위치 제어

학위논문 (석사)-- 서울대학교 대학원 : 공과대학 기계항공공학부, 2019. 2. 이경수.Steer by Wire (SBW) system is next generation steering apparatus for autonomous vehicle that it has no mechanical link between steering column and tire steering gearbox, the inner space of vehicle can be extended and utilized as the living room. SBW System is generally divided into two main parts, one is the steering reaction force control system and the other one is the rack position control system. Because of disconnection between column and gearbox, steering information like steering angle should be transmitted by electrical signal, and road and tire condition have no effect on steering system. So it needs to generate resistive torque for the driver to make appropriate steering feel like conventional power steering system which generates assist torque to the driver. The resistive torque can be obtained by setting reference torque based on measured data composed of 4-dimension which are steering angle, angular velocity and vehicle speed. It can be designed using system parameters and dynamics and set by optimization of tuning parameters. In terms of rack system, it is important that has to be controlled to precise position. SBW gearbox is generally high friction system which the nonlinearity is high that it is hard to control with linear system. And the road has various conditions so the force from tire changes continuously. This paper proposes methodology about steering reaction and rack position control using sliding mode control and the disturbance observer to compensate uncertainty caused by road conditions. And it also suggested the system performance results evaluated by hardware in the loop system(HILS).본 연구는 자율주행 차량에 적용이 요구되는 조향 신기술인 스티어 바이 와이어 시스템의 조향감을 생성하는 방법론 및 추종 알고리즘 그리고 차량의 모션을 제어하기 위한 랙 시스템의 위치 제어기법을 제안한다. 조향감을 생성하기 위해 차량의 데이터를 기반으로 하여 목표로 하는 조향감을 결정하고, 수식화된 조향감 모델을 구성하여 최적화 과정을 통해 목표 조향감을 생성해낸다. 이때 사용되는 각속도는 조향각 신호를 활용하여 칼만필터를 통해 계산되어 지며 목표 조향감을 조향각, 각속도, 차속에 대한 조향토크를 4차원 형태로 나타낼 수 있다. 목표 조향감을 임피던스 모델을 활용하여 목표 각속도 값을 도출하였으며 슬라이딩 모드 컨트롤 기법을 활용하여 모터 토크를 제어하였다. 차량의 모션을 결정하는 랙 시스템 모듈의 경우 차량의 노면조건, 부품의 마찰 특성에 따라 시스템 특성이 변화한다. 따라서 정확한 위치제어를 위해 기준 차량 조건에서의 시스템 파라메터 값을 Adaptation 기법을 활용하여 설정하였으며, 노면 조건에 의한 외란을 보상하기 위해 외란 추정기 (Disturbance Observer)를 활용하여 위치 제어 성능을 강건하게 확보 할 수 있는 알고리즘을 제안한다. 제안된 알고리즘을 검증하기 위해 Hardware In the Loop System(HILS)을 구성하였으며, 각 모듈 부품, 모터를 제어하기 위한 Autobox, 차량의 모션을 구현하기 위한 차량 모델과 모델에서부터 계산된 타이어 힘을 구현할 수 있는 유압식 액츄에이터로 구성이 되며, 실차와 유사한 HILS 환경에서 목표로 하는 조향감과 위치 제어 성능 검증 결과를 제시한다.Contents Abstract i List of Tables v List of Figures v Nomenclature vii Chapter 1 Introduction 1 1.1 Research Background 1 1.2 Research Overview 2 Chapter 2 Steer-by-wire System Architecture 4 2.1 Steering Reaction Module 5 2.2 Rack System Module 7 2.3 Overall system architecture 8 Chapter 3 State Estimation 10 3.1 System Requirement 10 3.2 Kalman Filter 11 Chapter 4 Steering Feel Target 13 4.1 Reference Torque 13 4.2 Target Torque Generation 15 Chapter 5 Control System 19 5.1 Steering Reaction Module 19 5.2 Rack System Module 22 Chapter 6 HILS Test Results 28 6.1 HILS System Configuration 28 6.2 Results of Steering Reaction Module 31 6.3 Results of Rack System Module 36 Chapter 7 Conclusions 42 Bibliography 43 국문초록 45Maste

Analysis of the properties of a steering shaft used as a back-up for a steer-by-wire system during system failure

YesAn analysis is presented to determine the best selection criteria for the properties of a steering shaft to be used as a back-up apparatus for a steer-by-wire (SBW) system during system failure. The properties of interest are the steering-shaft stiffness and its damping coefficient. A mathematical model representing the failed state of an SBW system is derived, and a set of experiments to validate the model is presented. Once the model had been validated, further predictions of the car¿s handling behaviour for a range of steering-shaft properties and different road speeds were completed by simulations in MATLAB/Simulink. A minimum stiffness which did not cause the car to become unstable owing to overshoot was determined, and the minimum acceptable damping coefficient value was derived. It is concluded that the suggested stiffness and damping coefficient values increased the steering ratio, and the results of further investigations are presented, which confirm that the vehicle is safe to be driven in the event of SBW system failure if the recommended shaft properties are used

The Vehicle Steer by Wire Control System by Implementing PID Controller

The latest technology of vehicle steer-by-wire (VSBW) system has promised significant improvement in vehicle safety, dynamics, stability, comfort and maneuverability. Due to complete separation between steering wheel and the front wheels gives the practical problems for steering control especially on directional control and wheel synchronization of vehicle. This paper presents investigations into the development of PID control scheme for directional control and wheel synchronization of a VSBW system. Two PID controllers are used to control the steering wheel angle and front wheel angle. The PID controllers use the front wheel tracking error to generate controlled steering angle. The Ziegler Nichols method is used for tuning the PID parameters. The implementation environment is developed within Matlab/Simulink software for evaluation of performance of the control scheme. Implementation results of the response of the VSBW system with the PID controller are presented in time domains. The performances of control schemes are examined in terms of input tracking capability, wheel synchronization and time response specifications with the absence of disturbances

Cooperative lateral vehicle guidance control for automated vehicles with Steer-by-Wire systems

With the global trend towards automated driving, fault-tolerant onboard power supply systems are introduced into modern vehicles and the level of driving automation is continuously increasing. These advancements contribute to the applicability of Steer-by-Wire systems and the development of automated lateral vehicle guidance control functions. For the market acceptance of automated driving, the lateral vehicle guidance control function must hereby be cooperative, that is it must accept driver interventions. Existing approaches for automated lateral vehicle guidance commonly do not consider driver interventions. If unconsidered in the control loop, the driver intervention is interpreted as an external disturbance that is actively compensated by feedback. This thesis addresses the development of a cooperative lateral vehicle guidance control concept, which enables a true coexistence between manual steering control by the driver and automated steering control. To this end, the subordinate controls of the Steer-by-Wire system for the manual and automated driving mode are initially presented. These include the steering feel generation and steering torque control of the Steer-by-Wire Handwheel Actuator for the manual driving mode, which is structurally extended to a cascade steering position control for the automated driving mode. Subsequently, a superposition control is introduced, which fuses steering torque and position control. The resulting cooperative Handwheel Actuator control achieves precise tracking of the reference steering position in automated driving mode but accepts driver interventions. Thus, the driver can override the active control and experiences a natural steering feel. The transitions hereby are seamless as no blending, gain scheduling or controller output saturation is required. Subsequently, the superimposed lateral vehicle guidance controller for the automated driving mode is described, which computes the reference steering position for the respective Steer-by-Wire controls. In contrast to existing approaches, the plant model equations are rearranged to isolate the vehicle speed dependent dynamics. Thereafter, the concept of inverse nonlinearity control is employed, using a virtual control loop and feedback linearization for an online inversion of the nonlinear plant dynamics. The remaining plant is fully linear and independent of vehicle speed. Consequently, one controller can be synthesized that is valid for all vehicle speeds. The closed and open loop system thereby have the same dynamics independent of vehicle speed, which significantly simplifies control synthesis, analysis, and performance tuning in the vehicle. For considering the future reference path information and constraints on the maximum steering position within the control law, a linear Model Predictive Controller synthesis is selected. The combination of inverse nonlinearity control and linear Model Predictive Controller thus results in a Nonlinear Adaptive Model Predictive Control concept, which makes commonly applied gain scheduling fully obsolete. The controller is structurally extended by a cooperative dynamic feedforward control for considering driver interventions within the control loop. Consequently, the driver can override the active control and seamlessly modify the lateral vehicle motion. A variety of nonlinear simulation analyses and real vehicle tests demonstrate the effectiveness of the proposed control concept

Steering control for haptic feedback and active safety functions

Steering feedback is an important element that defines driver–vehicle interaction. It strongly affects driving performance and is primarily dependent on the steering actuator\u27s control strategy. Typically, the control method is open loop, that is without any reference tracking; and its drawbacks are hardware dependent steering feedback response and attenuated driver–environment transparency. This thesis investigates a closed-loop control method for electric power assisted steering and steer-by-wire systems. The advantages of this method, compared to open loop, are better hardware impedance compensation, system independent response, explicit transparency control and direct interface to active safety functions.The closed-loop architecture, outlined in this thesis, includes a reference model, a feedback controller and a disturbance observer. The feedback controller forms the inner loop and it ensures: reference tracking, hardware impedance compensation and robustness against the coupling uncertainties. Two different causalities are studied: torque and position control. The two are objectively compared from the perspective of (uncoupled and coupled) stability, tracking performance, robustness, and transparency.The reference model forms the outer loop and defines a torque or position reference variable, depending on the causality. Different haptic feedback functions are implemented to control the following parameters: inertia, damping, Coulomb friction and transparency. Transparency control in this application is particularly novel, which is sequentially achieved. For non-transparent steering feedback, an environment model is developed such that the reference variable is a function of virtual dynamics. Consequently, the driver–steering interaction is independent from the actual environment. Whereas, for the driver–environment transparency, the environment interaction is estimated using an observer; and then the estimated signal is fed back to the reference model. Furthermore, an optimization-based transparency algorithm is proposed. This renders the closed-loop system transparent in case of environmental uncertainty, even if the initial condition is non-transparent.The steering related active safety functions can be directly realized using the closed-loop steering feedback controller. This implies, but is not limited to, an angle overlay from the vehicle motion control functions and a torque overlay from the haptic support functions.Throughout the thesis, both experimental and the theoretical findings are corroborated. This includes a real-time implementation of the torque and position control strategies. In general, it can be concluded that position control lacks performance and robustness due to high and/or varying system inertia. Though the problem is somewhat mitigated by a robust H-infinity controller, the high frequency haptic performance remains compromised. Whereas, the required objectives are simultaneously achieved using a torque controller

Nonlinear Modeling and Control of Driving Interfaces and Continuum Robots for System Performance Gains

With the rise of (semi)autonomous vehicles and continuum robotics technology and applications, there has been an increasing interest in controller and haptic interface designs. The presence of nonlinearities in the vehicle dynamics is the main challenge in the selection of control algorithms for real-time regulation and tracking of (semi)autonomous vehicles. Moreover, control of continuum structures with infinite dimensions proves to be difficult due to their complex dynamics plus the soft and flexible nature of the manipulator body. The trajectory tracking and control of automobile and robotic systems requires control algorithms that can effectively deal with the nonlinearities of the system without the need for approximation, modeling uncertainties, and input disturbances. Control strategies based on a linearized model are often inadequate in meeting precise performance requirements. To cope with these challenges, one must consider nonlinear techniques. Nonlinear control systems provide tools and methodologies for enabling the design and realization of (semi)autonomous vehicle and continuum robots with extended specifications based on the operational mission profiles. This dissertation provides an insight into various nonlinear controllers developed for (semi)autonomous vehicles and continuum robots as a guideline for future applications in the automobile and soft robotics field. A comprehensive assessment of the approaches and control strategies, as well as insight into the future areas of research in this field, are presented.First, two vehicle haptic interfaces, including a robotic grip and a joystick, both of which are accompanied by nonlinear sliding mode control, have been developed and studied on a steer-by-wire platform integrated with a virtual reality driving environment. An operator-in-the-loop evaluation that included 30 human test subjects was used to investigate these haptic steering interfaces over a prescribed series of driving maneuvers through real time data logging and post-test questionnaires. A conventional steering wheel with a robust sliding mode controller was used for all the driving events for comparison. Test subjects operated these interfaces for a given track comprised of a double lane-change maneuver and a country road driving event. Subjective and objective results demonstrate that the driver’s experience can be enhanced up to 75.3% with a robotic steering input when compared to the traditional steering wheel during extreme maneuvers such as high-speed driving and sharp turn (e.g., hairpin turn) passing. Second, a cellphone-inspired portable human-machine-interface (HMI) that incorporated the directional control of the vehicle as well as the brake and throttle functionality into a single holistic device will be presented. A nonlinear adaptive control technique and an optimal control approach based on driver intent were also proposed to accompany the mechatronic system for combined longitudinal and lateral vehicle guidance. Assisting the disabled drivers by excluding extensive arm and leg movements ergonomically, the device has been tested in a driving simulator platform. Human test subjects evaluated the mechatronic system with various control configurations through obstacle avoidance and city road driving test, and a conventional set of steering wheel and pedals were also utilized for comparison. Subjective and objective results from the tests demonstrate that the mobile driving interface with the proposed control scheme can enhance the driver’s performance by up to 55.8% when compared to the traditional driving system during aggressive maneuvers. The system’s superior performance during certain vehicle maneuvers and approval received from the participants demonstrated its potential as an alternative driving adaptation for disabled drivers. Third, a novel strategy is designed for trajectory control of a multi-section continuum robot in three-dimensional space to achieve accurate orientation, curvature, and section length tracking. The formulation connects the continuum manipulator dynamic behavior to a virtual discrete-jointed robot whose degrees of freedom are directly mapped to those of a continuum robot section under the hypothesis of constant curvature. Based on this connection, a computed torque control architecture is developed for the virtual robot, for which inverse kinematics and dynamic equations are constructed and exploited, with appropriate transformations developed for implementation on the continuum robot. The control algorithm is validated in a realistic simulation and implemented on a six degree-of-freedom two-section OctArm continuum manipulator. Both simulation and experimental results show that the proposed method could manage simultaneous extension/contraction, bending, and torsion actions on multi-section continuum robots with decent tracking performance (e.g. steady state arc length and curvature tracking error of 3.3mm and 130mm-1, respectively). Last, semi-autonomous vehicles equipped with assistive control systems may experience degraded lateral behaviors when aggressive driver steering commands compete with high levels of autonomy. This challenge can be mitigated with effective operator intent recognition, which can configure automated systems in context-specific situations where the driver intends to perform a steering maneuver. In this article, an ensemble learning-based driver intent recognition strategy has been developed. A nonlinear model predictive control algorithm has been designed and implemented to generate haptic feedback for lateral vehicle guidance, assisting the drivers in accomplishing their intended action. To validate the framework, operator-in-the-loop testing with 30 human subjects was conducted on a steer-by-wire platform with a virtual reality driving environment. The roadway scenarios included lane change, obstacle avoidance, intersection turns, and highway exit. The automated system with learning-based driver intent recognition was compared to both the automated system with a finite state machine-based driver intent estimator and the automated system without any driver intent prediction for all driving events. Test results demonstrate that semi-autonomous vehicle performance can be enhanced by up to 74.1% with a learning-based intent predictor. The proposed holistic framework that integrates human intelligence, machine learning algorithms, and vehicle control can help solve the driver-system conflict problem leading to safer vehicle operations