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

Haptic Feedback Control Methods for Steering Systems

Haptic feedback from the steering wheel is an important cue that defines the steering feel in the driver-vehicle interaction. The steering feedback response in an electric power assisted steering is primarily dependent on its control strategy. The conventional approach is open loop control, where different functions are implemented in a parallel structure. The main drawbacks are: (a) limited compensation of the hardware impedance, (b) hardware system dependent steering feedback response and (c) limitation on vehicle motion control request overlay. This thesis investigates closed-loop control, in which the desired steering feedback response can be separated from the hardware dynamics. Subsequently, the requirements can be defined at the design stage. The closed-loop architecture constitutes of a higher and lower level controller. The higher level control defines the reference steering feedback, which should account for both driver and road excitation sources. This thesis focuses on the driver excitation, where a methodology is proposed for developing such a reference model using the standard vehicle handling maneuvers. The lower level control ensures: (a) reference tracking of the higher level control, (b) hardware impedance compensation and (c) robustness to unmodeled dynamics. These interdependent objectives are realized for a passive interaction port driving admittance. The two closed-loop possibilities, impedance (or torque) and admittance (or position) control, are compared objectively. The analysis is further extended to a steer-by-wire force-feedback system; such that the lower level control is designed with a similar criteria, keeping the same higher level control.The admittance control is found limited in performance for both the steering systems. This is explained by a higher equivalent mechanical inertia caused by the servo motor and its transmission ratio in electric power assisted steering; and for steer-by-wire force-feedback, due to the uncertainty in drivers\u27 arm inertia. Moreover, it inherently suffers from the conflicting objectives of tracking, impedance compensation and robustness. These are further affected by the filtering required in the admittance lower level control. In impedance control, a better performance is exhibited by its lower level control. However, the required filtering and estimation in the impedance higher level control is its biggest disadvantage. In closed-loop setting, the angular position overlay with a vehicle motion control request is also relatively easier to realize than open loop

Design and control of model based steering feel reference in an electric power assisted steering system

Electric Power Assisted Steering (EPAS) system is a current state of the art technology for providing the steering torque support. The interaction of the steering system with the driver is principally governed by the EPAS control method. This paper proposes a control concept for designing the steering feel with a model based approach. The reference steering feel is defined in virtual dynamics for tracking. The layout of the reference model and the control architecture is discussed at first and then the decoupling of EPAS motor dynamics using a feedback control is shown. An example of how a change in steering feel reference (as desired by the driver) creates a change in steering feedback is further exhibited. The ultimate goal is to provide the driver with a tunable steering feel. For this, the verification is performed in simulation environment

Comparison of Steering Feel Control Strategies in Electric Power Assisted Steering

This paper presents an objective comparison of two closed-loop steering feel control concepts in an electric power assisted steering (EPAS) system. The closed-loop methods, torque- and position-control, aim to compensate the EPAS motor inertia in an effective manner as compared to the open loop (feed-forward) solution. For a given steering feel reference, the feedback controllers are developed in a sequential manner ensuring coupled stability. Linear system theory is used for the analysis. For a comparable reference tracking and stability margin, higher haptic controller bandwidth is achieved in torque-control. The position controller stability and performance are limited due to feedback control filtering and high system inertia (from EPAS motor and driver arms), which further makes it more sensitive towards muscle co-contraction. Moreover, torque-control offers better road disturbance attenuation for low and high frequency spectrum, whereas position-control is better for mid-frequency range

Closed Loop Steering Feel Control Concept in EPAS

This work was presented at the Machine and Vehicle Systems Seminar, Chalmers University. It discusses regarding the closed loop steering feel controller which comprises of two parts; reference generator and feedback controller

Model based closed-loop position control for vehicle steering systems

The present disclosure relates to a method for operating a haptic system (100), the haptic system (100) comprising at least one actuator (110) and at least one haptic control device (120) adapted to control the at least one actuator(110) and to provide haptic feedback to a user, the method comprising the steps of: obtaining (S1), from a feedbackcomputational model, modelled feedback data, obtaining (S2), from a feedback estimator, estimated feedback databased on measurement data determined from measurement made on the haptic system, overlaying (S3) the modelledfeedback data and the estimated feedback data to generate blended feedback data, and providing (S4) the blendedfeedback data to control the haptic feedback to the user

Method and apparatus for operating a haptic system

The present invention relates to a method for operating a haptic system, such as a vehicle electric steering system, etc. We present a real-time disturbance observer based control strategy for a desired steering feedback, ranging from virtual to realistic behavior. The invention covers both the impedance (or torque) and admittance (or position) control concepts

Steering Feedback Transparency Using Rack Force Observer

The closed-loop electric power-assisted steering and steer-by-wire systems are non-transparent toward the environment, i.e., the tire-road interaction dynamics. To achieve driver–environment transparency, a rack force estimate is required. With the introduction of dynamic rack force observer feedback, a closed-loop interconnection is formed. Therefore, the driver coupled stability must be ensured with this interconnection. Consequently, an upper bound condition on transparency is derived using passivity for different control architectures. For selecting an observer with reasonable performance, two rack-force estimation schemes are investigated and compared, i.e., using vehicle motion signals from the inertial measurement unit sensor and the steering system sensors. Experiments indicate that the former approach has inferior performance due to vehicle inertia and signal latency, whereas in the latter approach, nonlinear estimation using second-order dynamics provides the best result; hence, it is selected to realize transparency. Finally, real-world experiments on an electric power-assisted steering vehicle illustrate the differences between non-transparent and transparent steering feedback when driving at the limits of handling on an icy road surface

Robust H-infinity Position Control for Vehicle Steering

This paper presents a robust position controller for an electric power assisted steering and a steer-by-wire force-feedback system. A typical position controller is required for the driving automation and the vehicle motion control functions. The same controller could also be used to realize the haptic functions for steering feedback. The driver’s physical impedance causes a parametric uncertainty during the steering wheel coupling. As a consequence, a classical (single variable) position controller becomes less robust and suffers a tracking performance loss. Therefore, a multi-variable robust position controller is proposed to mitigate the effect of uncertainty. An investigation is performed by including the sensed torque signal in a classical position controller. Finally, a robust solution is synthesized using the LMI-H-infinity optimization. With this, a desired loop gain shape is achieved: (a) a large loop gain at low frequencies for performance; and (b) a small loop gain at high frequencies for robustness. Frequency response comparison of different controllers on real hardware is presented. Experiments and simulation results clearly illustrate the improvements in reference tracking and robustness with an optimal torque feedback in the proposed H-infinity position controller

Musculoskeletal Driver Model for the Steering Feedback Controller

This paper aims to find a mathematical justification for the non-linear steady state steering haptic response as a function of driver arm posture. Experiments show that different arm postures, that is, same hands location on the steering wheel but at different initial steering angles, result in a change in maximum driver arm stiffness. This implies the need for different steering torque response as a function of steering angle, which is under investigation. A quasi-static musculoskeletal driver model considering elbow and shoulder joints is developed for posture analysis. The torque acting in the shoulder joint is higher than in the elbow. The relationship between the joint torque and joint angle is linear in the shoulder, whereas the non-linearity occurs in the elbow joint. The simulation results qualitatively indicate a similar pattern as compared to the experimental muscle activity results. Due to increasing muscle non-linearity at high steering angles, the arm stiffness decreases and then the hypothesis suggests that the effective steering stiffness is intentionally reduced for a consistent on-center haptic response.Intelligent Vehicle