Flow-field guided steering control for rigid autonomous ground vehicles in low-speed manoeuvring

ABSTRACT

This paper studies the low-speed manoeuvring problem for autono-mous ground vehicles operating in complex static environments. Making use of the intrinsic property of a fluid to naturally find its way to an outflow destination, a novel guidance method is proposed. In this approach, a reference flow field is calculated numerically through Computational Fluid Dynamics, based on which both the reference path topology and the steering reference to achieve the path are derived in a single process. Steering control considers three constraints: obstacle and boundary avoidance, rigidity of the vehicle, plus the non-holonomic velocity constraints due to the steering system. The influences of the parameters used during the flow field simulation and the control algorithm are discussed through numerical cases. A divergency field is defined to evaluate the quality of the flow field in guiding the vehicle. This is used to identify any problematic branching features of the flow, and control is adapted in the neighbourhood of such branching features to resolve possible ambiguities in the control reference. Results demonstrate the effectiveness of the method in finding smooth and feasible motion paths, even in complex environments.     

基于转角补偿的滑模驾驶员车道保持控制方法探究

针对智能车辆车道保持过程出现的转向不足或者转向过度而无法有效跟踪目标轨迹的情况,文章在滑模变结构控制基础上设计了汽车横向预瞄驾驶员模型,并引入前轮转角补偿控制建立车道保持模型,该模型能够及时修正和优化转角,确保车辆在转弯过程中保持稳定性。仿真结果表明,所提出的车道保持控制方法能够在车速较高的情况下实现车道保持功能,并能在连续弯道情况下实时修正并补偿前轮转角。高速下预瞄横向偏差和航向偏差分别降低38.4%和 37.6%,具有良好的路径跟踪性能。     

Emergency steering control of autonomous vehicle for collision avoidance and stabilisation

ABSTRACT

Collision avoidance and stabilisation are two of the most crucial concerns when an autonomous vehicle finds itself in emergency situations, which usually occur in a short time horizon and require large actuator inputs, together with highly nonlinear tyre cornering response. In order to avoid collision while stabilising autonomous vehicle under dynamic driving situations at handling limits, this paper proposes a novel emergency steering control strategy based on hierarchical control architecture consisting of decision-making layer and motion control layer. In decision-making layer, a dynamic threat assessment model continuously evaluates the risk associated with collision and destabilisation, and a path planner based on kinematics and dynamics of vehicle system determines a collision-free path when it suddenly encounters emergency scenarios. In motion control layer, a lateral motion controller considering nonlinearity of tyre cornering response and unknown external disturbance is designed using tyre lateral force estimation-based backstepping sliding-mode control to track a collision-free path, and to ensure the robustness and stability of the closed-loop system. Both simulation and experiment results show that the proposed control scheme can effectively perform an emergency collision avoidance manoeuvre while maintaining the stability of autonomous vehicle in different running conditions.     

Effect of drowsiness on driving performance variables of commercial vehicle drivers

Driver drowsiness is a major safety concern, especially among commercial vehicle drivers, and is responsible for thousands of accidents and numerous fatalities every year. The design of a drowsiness detection system is based on identifying suitable driver-related and/or vehicle-related variables that are correlated to the driver’s level of drowsiness. Among different candidates, vehicle control variables seem to be more promising since they are unobtrusive, easy to implement, and cost effective. This paper focuses on in-depth analysis of different driver-vehicle control variables, e.g., steering angle, lane keeping, etc. that are correlated with the level of drowsiness. The goal is to find relationships and to characterize the effect of a driver’s drowsiness on measurable vehicle or driving variables and set up a framework for developing a drowsiness detection system. Several commercial drivers were tested in a simulated environment and different variables were recorded. This study shows that drowsiness has a major impact on lane keeping and steering control behavior. The correlation of the number and type of accidents with the level of drowsiness was also examined. Significant patterns in lateral position variations and steering corrections were observed, and two phases of drowsiness-related degradation in steering control were identified. The two steering degradation phases examined are suitable features for use in drowsiness detection systems.     

A study on model fidelity for model predictive control-based obstacle avoidance in high-speed autonomous ground vehicles

This paper investigates the level of model fidelity needed in order for a model predictive control (MPC)-based obstacle avoidance algorithm to be able to safely and quickly avoid obstacles even when the vehicle is close to its dynamic limits. The context of this work is large autonomous ground vehicles that manoeuvre at high speed within unknown, unstructured, flat environments and have significant vehicle dynamics-related constraints. Five different representations of vehicle dynamics models are considered: four variations of the two degrees-of-freedom (DoF) representation as lower fidelity models and a fourteen DoF representation with combined-slip Magic Formula tyre model as a higher fidelity model. It is concluded that the two DoF representation that accounts for tyre nonlinearities and longitudinal load transfer is necessary for the MPC-based obstacle avoidance algorithm in order to operate the vehicle at its limits within an environment that includes large obstacles. For less challenging environments, however, the two DoF representation with linear tyre model and constant axle loads is sufficient.     

Sensor fusion-based lane detection for LKS+ACC system

This paper discusses the market trends and advantages of a safety system integrating LKS (Lane Keeping System) and ACC (Adaptive Cruise Control), referred to as the LKS+ACC system, and proposes a method utilizing the range data from ACC for the sake of lane detection. The overall structure of lane detection is the same as the conventional method using monocular vision: EDF (Edge Distribution Function)-based initialization, sub-ROI (Region Of Interest) for left/right and distance-based layers, steerable filter-based feature extraction, and model fitting in each sub-ROI. The proposed method adds only the system for confining lane detection ROI to free space that is established by range data. Experimental results indicate that such a simple adaptive ROI can overcome occlusion of lane markings and disturbance of neighboring vehicles.     

改进的有限时间最优预瞄横向控制器设计

提出了有限时间最优预瞄横向控制算法,根据高速公路车道保持系统实际控制要求,使用了同时考虑车辆当前偏差、预瞄点偏差和控制变量的有限时间二次型性能指标函数。运用最优跟踪算法并进行预瞄距离内曲率恒定的假设,使得控制器参数可以离线求解,保证了车辆控制的实时性。通过仿真及实车试验验证,该控制算法具有较好的跟踪效果。     

Fault classification method for the driving safety of electrified vehicles

A fault classification method is proposed which has been applied to an electric vehicle. Potential faults in the different subsystems that can affect the vehicle directional stability were collected in a failure mode and effect analysis. Similar driveline faults were grouped together if they resembled each other with respect to their influence on the vehicle dynamic behaviour. The faults were physically modelled in a simulation environment before they were induced in a detailed vehicle model under normal driving conditions. A special focus was placed on faults in the driveline of electric vehicles employing in-wheel motors of the permanent magnet type. Several failures caused by mechanical and other faults were analysed as well. The fault classification method consists of a controllability ranking developed according to the functional safety standard ISO 26262. The controllability of a fault was determined with three parameters covering the influence of the longitudinal, lateral and yaw motion of the vehicle. The simulation results were analysed and the faults were classified according to their controllability using the proposed method. It was shown that the controllability decreased specifically with increasing lateral acceleration and increasing speed. The results for the electric driveline faults show that this trend cannot be generalised for all the faults, as the controllability deteriorated for some faults during manoeuvres with low lateral acceleration and low speed. The proposed method is generic and can be applied to various other types of road vehicles and faults.     

Stability control of 4WS vehicles using robust IMC techniques

A robust nonparametric approach to vehicle stability control by means of a four-wheel steer by wire system is introduced. Both yaw rate and sideslip angle feedbacks are used in order to effectively take into account safety as well as handling performances. Reference courses for yaw rate and sideslip angle are computed on the basis of the vehicle speed and the handwheel angle imposed by the driver. An output multiplicative model set is used to describe the uncertainty arising from a wide range of vehicle operating situations. The effects of saturation of the control variables (i.e. front and rear steering angles) are taken into account by adopting enhanced internal model control methodologies in the design of the feedback controller. Actuator dynamics are considered in the controller design. Improvements on understeer characteristics, stability in demanding conditions such as turning on low friction surfaces, damping properties in impulsive manoeuvres, and improved handling in closed loop (i.e. with driver feedback) manoeuvres are shown through extensive simulation results performed on an accurate 14 degrees of freedom nonlinear model, which proved to give good modelling results as compared with collected experimental data.     

自动驾驶汽车执行器故障冗余算法

提出一种自动驾驶汽车执行器故障冗余策略,包括转向和制动执行器故障。首先,利 用无迹卡尔曼对命令层的转角输入进行状态估计,并根据估计状态量与实际状态量的偏差判断执行器是否工作正常。在确定转向故障后,利用直接横摆力矩控制依托汽车电子稳定控制系统进行冗余转向控制;而确定制动故障后,通过 MAP 图查询法进行制动压力的获取,并借 助汽车电子稳定控制系统实现制动执行器的冗余。仿真结果表明,执行器故障能得到及时有效的诊断,并能获得相应的功能冗余,保证紧急工况下车辆安全。     

Design of safety-oriented control allocation strategies for overactuated electric vehicles

The new vehicle platforms for electric vehicles (EVs) that are becoming available are characterised by actuator redundancy, which makes it possible to jointly optimise different aspects of the vehicle motion. To do this, high-level control objectives are first specified and solved with appropriate control strategies. Then, the resulting virtual control action must be translated into actual actuator commands by a control allocation layer that takes care of computing the forces to be applied at the wheels. This step, in general, is quite demanding as far as computational complexity is considered. In this work, a safety-oriented approach to this problem is proposed. Specifically, a four-wheel steer EV with four in-wheel motors is considered, and the high-level motion controller is designed within a sliding mode framework with conditional integrators. For distributing the forces among the tyres, two control allocation approaches are investigated. The first, based on the extension of the cascading generalised inverse method, is computationally efficient but shows some limitations in dealing with unfeasible force values. To solve the problem, a second allocation algorithm is proposed, which relies on the linearisation of the tyre–road friction constraints. Extensive tests, carried out in the CarSim simulation environment, demonstrate the effectiveness of the proposed approach.     

Sliding window method for vehicles moving on a long track

The rail is modelled as a simply supported beam in the vehicle–track coupled dynamics. The beam is formulated by a partial differential equation that is transformed into an ordinary differential equation by the method of mode superposition for numerical calculation. However, the size of the matrix that is formed by the mode-superposition method increases significantly with track length, which limits the calculation efficiency. Some methods have been developed to solve this calculation issue, but they diminish the merits of the vehicle–track coupled dynamics, which would systematically investigate the dynamics of a vehicle and a track from the entire vehicle–track system. A new method is developed to resolve this contradiction. First, a theory based on a sliding window is established to improve the computational stability with respect to the length and the window-movement ratio. Then, two methods, namely finite element method analysis and an analytical solution, are used to verify the accuracy of the new method, which is highly efficient when used in a vertical half-vehicle–track coupled model to calculate the vehicle response when the vehicle moves on a long track. The results of the vehicle response calculated with and without the sliding window show good consistency.     

Optimal design of adaptive shaking vibration control for electric vehicles

This paper provides a complete design solution about adaptive optimal shaking vibration control for electric vehicles. A general 4-DOF and 5-order linear torsional vibration model is established under given wheel speed, and the frequency characteristics of the vibration system are elaborately analysed in terms of variation of wheel speed and different model parameters. Aiming at decreasing the shaking vibration at the least sacrifice of acceleration loss, and improving the robustness of the system against external disturbance, a combination of feed-forward and feed-backward adaptive control structure is proposed. Further, a non-linear multi-constraint optimisation problem is formulated for solving the optimal adaptive control variables within the two-dimensional design space composed of the wheel speed and driver's torque command. Furthermore, the distribution of the optimal adaptive control variables within the design space and its extended application under different tyre road conditions are discussed. Eventually, several simulation test cases are particularly designed to verify the performances of the controller on all aspects. Test results show that the optimal adaptive controller achieves satisfactory anti-shaking vibration, acceleration maintaining and robustness performances within the whole adaptive design space as desired.     

Considerations on active control of crosswind stability of railway vehicles

The DLR research project Next Generation Train deals with concepts, methods and technologies for a very high-speed train in double-deck configuration and light-weight design. Due to these three key features, crosswind stability is a particular subject of study. It is shown that conventional approaches here fall short of guaranteeing safety in high-wind occurrences according to the given homologation standards. Therefore, this paper discusses the feasibility of different approaches to ensure crosswind stability by means of active control. Four different concepts are overviewed, the most promising one is then chosen und examined in detailed multibody simulations that are based on data from wind tunnel measurements of the Next Generation Train.     

Multivariable speed synchronisation for a parallel hybrid electric vehicle drivetrain

In this article, a new drivetrain configuration of a parallel hybrid electric vehicle is considered and a novel model-based control design strategy is given. In particular, the control design covers the speed synchronisation task during a restart of the internal combustion engine. The proposed multivariable synchronisation strategy is based on feedforward and decoupled feedback controllers. The performance and the robustness properties of the closed-loop system are illustrated by nonlinear simulation results.     

A driver-adaptive stability control strategy for sport utility vehicles

Conventional vehicle stability control (VSC) systems are designed for average drivers. For a driver with a good driving skill, the VSC systems may be redundant; for a driver with a poor driving skill, the VSC intervention may be inadequate. To increase safety of sport utility vehicles (SUVs), this paper proposes a novel driver-adaptive VSC (DAVSC) strategy based on scaling the target yaw rate commanded by the driver. The DAVSC system is adaptive to drivers’ driving skills. More control effort would be exerted for drivers with poor driving skills, and vice versa. A sliding mode control (SMC)-based differential braking (DB) controller is designed using a three degrees of freedom (DOF) yaw-plane model. An eight DOF nonlinear yaw-roll model is used to simulate the SUV dynamics. Two driver models, namely longitudinal and lateral, are used to ‘drive’ the virtual SUV. By integrating the virtual SUV, the DB controller, and the driver models, the performance of the DAVSC system is investigated. The simulations demonstrate the effectiveness of the DAVSC strategy.     

A fundamental investigation of tilt control systems for narrow commuter vehicles

One way of addressing traffic congestion is by efficiently utilizing the existing highway infrastructure. Narrow tilting vehicles that need a reduced width lane can be part of the solution if they can be designed to be safe, stable, and easy to operate. In this paper, a control system that stabilizes the tilt mode of such a vehicle without affecting the handling of the vehicle is proposed. This control system is a combination of two different types of control schemes known as steering tilt control (STC) and direct tilt control (DTC) systems. First, different existing variations of both STC and DTC systems are considered and their shortcomings analysed. Modified control schemes are then suggested to overcome the deficiencies. Then a new method of integrating these two control schemes that guarantees smooth switchover between the controllers as a function of vehicle velocity is proposed. The performance of the proposed STC, DTC, and integrated systems is evaluated by carrying out simulations for different operating conditions and some experimental work. The design of a second-generation narrow tilting vehicle on which the developed control system has been implemented is presented.     

Fidelity of using scaled vehicles for chassis dynamic studies

There are many situations where physical testing of a vehicle or vehicle controller is necessary, yet use of a full-size vehicle is not practical. Some situations include implementation testing of novel actuation strategies, analysing the behaviour of chassis feedback control under system faults, or near-unstable situations such as limit handling under driver-assist feedback control. Historically, many have advocated the use of scale vehicles as surrogates for larger vehicles. This article presents analysis and experimental testing that examines the fidelity of using scaled vehicles for vehicle chassis dynamics and control studies. In support of this effort, this work introduces an experimental system called the Pennsylvania State University Rolling Roadway Simulator (the PURRS). In the PURRS, a custom-built scale-sized vehicle is freely driven on a moving roadway surface. While others have used scale-vehicle rolling roadway simulators in the past, this work is the first to attempt to directly match the planar dynamic performance of the scale-sized vehicle to a specific full-sized vehicle by careful design of the scale vehicle. This article explains details of this effort including vehicle dynamic modelling, detailed measurement of model parameters, conditions for dynamic similitude, validation of the resulting experimental vehicle in the time, frequency, and dimensionless domains. The results of the dynamic comparisons between scale- and full-sized vehicles clearly illustrate operational regimes where agreement is quite good, and other regimes where agreement is quite poor. Both are useful to understand the applicability of scale-vehicle results to full-size vehicle analysis.     

Asymptotic sideslip angle and yaw rate decoupling control in four-wheel steering vehicles

This paper shows that, for a four-wheel steering vehicle, a proportional-integral (PI) active front steering control and a PI active rear steering control from the yaw rate error together with an additive feedforward reference signal for the vehicle sideslip angle can asymptotically decouple the lateral velocity and the yaw rate dynamics; that is the control can set arbitrary steady state values for lateral speed and yaw rate at any longitudinal speed. Moreover, the PI controls can suppress oscillatory behaviours by assigning real stable eigenvalues to a widely used linearised model of the vehicle steering dynamics for any value of longitudinal speed in understeering vehicles. In particular, the four PI control parameters are explicitly expressed in terms of the three real eigenvalues to be assigned. No lateral acceleration and no lateral speed measurements are required. The controlled system maintains the well-known advantages of both front and rear active steering controls: higher controllability, enlarged bandwidth for the yaw rate dynamics, suppressed resonances, new stable cornering manoeuvres and improved manoeuvrability. In particular, zero lateral speed may be asymptotically achieved while controlling the yaw rate: in this case comfort is improved since the phase lag between lateral acceleration and yaw rate is reduced. Also zero yaw rate can be asymptotically achieved: in this case additional stable manoeuvres are obtained in obstacle avoidance. Several simulations, including step references and moose tests, are carried out on a standard small SUV CarSim model to explore the robustness with respect to unmodelled effects such as combined lateral and longitudinal tyre forces, pitch, roll and driver dynamics. The simulations confirm the decoupling between the lateral velocity and the yaw rate and show the advantages obtained by the proposed control: reduced lateral speed or reduced yaw rate, suppressed oscillations and new stable manoeuvres.