Aldo Sorniotti

Wheel Torque Distribution Criteria for Electric Vehicles With Torque-Vectoring Differentials

The continuous and precise modulation of the driving and braking torques of each wheel is considered the ultimate goal for controlling the performance of a vehicle in steady-state and transient conditions. To do so, dedicated torque-vectoring (TV) controllers that allow optimal wheel torque distribution under all possible driving conditions have to be developed. Commonly, vehicle TV controllers are based on a hierarchical approach, consisting of a high-level supervisory controller that evaluates a corrective yaw moment and a low-level controller that defines the individual wheel torque reference values. The problem of the optimal individual wheel torque distribution for a particular driving condition can be solved through an optimization-based control-allocation (CA) algorithm, which must rely on the appropriate selection of the objective function. With a newly developed offline optimization procedure, this paper assesses the performance of alternative objective functions for the optimal wheel torque distribution of a four-wheel-drive (4WD) fully electric vehicle. Results show that objective functions based on the minimum tire slip criterion provide better control performance than functions based on energy efficiency.

Comparison of Feedback Control Techniques for Torque-Vectoring Control of Fully Electric Vehicles

Fully electric vehicles (FEVs) with individually controlled powertrains can significantly enhance vehicle response to steering-wheel inputs in both steady-state and transient conditions, thereby improving vehicle handling and, thus, active safety and the fun-to-drive element. This paper presents a comparison between different torque-vectoring control structures for the yaw moment control of FEVs. Two second-order sliding-mode controllers are evaluated against a feedforward controller combined with either a conventional or an adaptive proportional-integral-derivative (PID) controller. Furthermore, the potential performance and robustness benefits arising from the integration of a body sideslip controller with the yaw rate feedback control system are assessed. The results show that all the evaluated controllers are able to significantly change the understeer behavior with respect to the baseline vehicle. The PID-based controllers achieve very good vehicle performance in steady-state and transient conditions, whereas the controllers based on the sliding-mode approach demonstrate a high level of robustness against variations in the vehicle parameters. The integrated sideslip controller effectively maintains the sideslip angle within acceptable limits in the case of an erroneous estimation of the tire-road friction coefficient.

Integral Sliding Mode for the Torque-Vectoring Control of Fully Electric Vehicles: Theoretical Design and Experimental Assessment

This paper presents an integral sliding mode (ISM) formulation for the torque-vectoring (TV) control of a fully electric vehicle. The performance of the controller is evaluated in steady-state and transient conditions, including the analysis of the controller performance degradation due to its real-world implementation. This potential issue, which is typical of sliding mode formulations, relates to the actuation delays caused by the drivetrain hardware configuration, signal discretization, and vehicle communication buses, which can provoke chattering and irregular control action. The controller is experimentally assessed on a prototype electric vehicle demonstrator under the worst-case conditions in terms of drivetrain layout and communication delays. The results show a significant enhancement of the controlled vehicle performance during all maneuvers.

Analysis and simulation of the gearshift methodology for a novel two-speed transmission system for electric powertrains with a central motor:

Electric vehicle powertrains traditionally consist of a central electric motor drive, a single-speed transmission and a differential. This electric powertrain layout, for use in either fully electric vehicles or through-the-road parallel hybrid electric vehicles, will be extensively adopted in the next few years, despite the ongoing research in electric vehicles with individually controlled motors. However, current research suggests that electric powertrains with a central electric motor drive can still be widely improved. For example, the installation of a seamless multiple-speed transmission instead of a single-speed transmission can cause an increase in the vehicle performance, together with an enhancement in the overall efficiency of the electric powertrain. These novel transmission systems for electric powertrains require a specific design, in order to be efficient, compact, easy and robust to control and cheap to manufacture. This article presents the mechanical layout and the control system of a novel two-speed transmission system designed by the present authors, with particular focus on the achievement of optimal gearshift dynamics. The torque characteristics of typical electric motor drives require a different actuation of the seamless gearshifts, in comparison with the equivalent operation for a dual-clutch transmission within a powertrain driven by an internal combustion engine.

The effect of half-shaft torsion dynamics on the performance of a traction control system for electric vehicles

This article deals with the dynamic properties of individual wheel electric powertrains for fully electric vehicles, characterised by an in-board location of the motor and transmission, connected to the wheel through half-shafts. Such a layout is applicable to vehicles characterised by significant power and torque requirements where the adoption of in-wheel electric powertrains is not feasible because of packaging constraints. However, the dynamic performance of in-board electric powertrains, especially if adopted for anti-lock braking or traction control, can be affected by the torsional dynamics of the half-shafts. This article presents the dynamic analysis of in-board electric powertrains in both the time domain and the frequency domain. A feedback control system, incorporating state estimation through an extended Kalman filter, is implemented in order to compensate for the effect of the half-shaft dynamics. The effectiveness of the new controller is demonstrated through analysis of the improvement in the performance of the traction control system.

Optimal Wheel Torque Distribution for a Four-Wheel-Drive Fully Electric Vehicle

Vehicle handling in steady-state and transient conditions can be significantly enhanced with the continuous modulation of the driving and braking torques of each wheel via dedicated torque-vectoring controllers. For fully electric vehicles with multiple electric motor drives, the enhancements can be achieved through a control allocation algorithm for the determination of the wheel torque distribution. This article analyzes alternative cost functions developed for the allocation of the wheel torques for a four-wheel-driven fully electric vehicle with individually controlled motors. Results in terms of wheel torque and tire slip distributions among the four wheels, and of input power to the electric drivetrains as functions of lateral acceleration are presented and discussed in detail. The cost functions based on minimizing tire slip allow better control performance than the functions based on energy efficiency for the case-study vehicle.

Electro-Hydraulic Brake Systems: Design and Test Through Hardware-in-the-Loop Simulation

Are the new electro-hydraulic systems really advantageous in comparison with the conventional brake systems? The first part of the paper is devoted to the description of some possible configurations of an Electro-Hydraulic Brake (EHB) system, then the design and test methodology followed in the activity is presented. Through simulation it was possible to determine the main design specifications of EHB and the advantages over conventional brake systems, both from the point of view of the base brake function, anti-lock brake system function and electronic stability control function. The failsafe algorithm for the EHB system was implemented through simulation too. On the basis of the simulation results, by using, where possible, the components of conventional brake systems, a hardware-in-the-loop test bench for a prototype EHB was implemented. The main simulation and experimental results are presented and discussed.

Active Roll Control to Increase Handling and Comfort

The paper deals with the elaboration of an Active Roll Control (ARC) oriented both on comfort and handling improvement. The ARC determines hydraulically the variation of the equivalent stiffness of the anti-roll bars. Thecontrol strategies conceived were extensively validated through road tests managed on an Alfa Romeo sedan. The first part of the paper deals with comfort improvement, mainly consisting in an absence of bar effect during straight-ahead travel and in a modification of the roll characteristic of the car. To increase drivers handling feeling, it was necessary to optimise the ratio between front and rear roll stiffness. This purpose can be reached through control strategies based exclusively on lateral acceleration. Some control strategy corrections were necessary to optimise roll damping and front/rear roll stiffness balancing. A control logic, based on yaw rate arid vehicle sideslip angle estimation, was introduced, with the main purpose of optimise yaw damping during transient manoeuvres. Road tests extensively managed validated the efficiency of active roll control strategies conceived. Finally the paper presents some example of integrations with other chassis control systems, like Vehicle Dynamics Control (VDC). The most important results obtained, both in simulation and during road tests, are presented and discussed.

Optimization of a multiple-speed transmission for downsizing the motor of a fully electric vehicle

The research presented in this paper focuses on the effects of downsizing the electric motor drive of a fully electric vehicle through the adoption of a multiple-speed transmission system. The activity is based on the implementation of a simulation framework in Matlab / Simulink. The paper considers a rear wheel drive case study vehicle, with a baseline drivetrain configuration consisting of a single-speed transmission, which is compared with drivetrains adopting motors with identical peak power but higher base speeds and lower peak torques coupled with multiple-speed transmissions (double and three-speed), to analyze the benefits in terms of energy efficiency and performance. The gear ratios and gearshift maps for each multiple-speed case study are optimized through a procedure developed by the authors consisting of cost functions considering energy efficiency and performance evaluation. The cost functions are explained in the paper along with the models adopted for the research. Copyright

Reducing the motor power losses of a four-wheel drive, fully electric vehicle via wheel torque allocation

Individually controlled electric motors provide opportunities for enhancing the handling characteristics and the energy efficiency of fully electric vehicles. Online power loss minimisation schemes based on the electric motor efficiency data may, however, be impractical for real-time implementation owing to the heavy computational demand. In this paper, the optimal wheel torque distribution for minimal power losses from the electric motor drives is evaluated in an offline optimisation procedure and then approximated using a simple function for online control allocation. The wheel torque allocation scheme is evaluated via a simulation approach incorporating straight-ahead driving at a constant speed, a ramp manoeuvre and a sequence of step steer manoeuvres. The energy-efficient wheel torque allocation scheme provides motor power loss reductions and yields savings in the total power utilisation compared with a simpler method in which the torques are evenly distributed across the four wheels. The method does not rely on complex online optimisation and can be applied on real electric vehicles in order to improve the efficiency and thus to reduce power consumption during different manoeuvres.