Improving traction and control in electric vehicles

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  • Electric vehicles look set to increasingly dominate our roads in the coming years, but many remain unconvinced of their advantages over their more traditional, fossil fuel-burning counterparts. In his recent research, Professor Guoqing Xu at Shanghai University has proposed new ways in which electric vehicles can be improved beyond the capabilities of fossil fuel burners. Through innovations relating to both tyre adhesion stability and the recycling of braking energy, Professor Xu believes electric vehicles can realistically become safe and reliable features of our transport networks in the near future.

    As humanity begins to ramp up its battle against the critical issues of climate change and air pollution, electric vehicles will likely become an important part of a more environmentally sustainable society. Recently, countries including Norway, Sweden, and Denmark have set end-dates for sales of vehicles which run on fossil fuels, and in the coming years, other developed nations will be likely to follow. However, with many nations dragging their feet on their commitments to sustainability, it appears that electric vehicles may need to offer more incentives to consumers before more traditional vehicles can be phased out on global scales.

    In a series of studies carried out over a six-year period, Professor Xu and his colleagues at Shanghai University and Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, have identified several areas where electric vehicles offer improvements in safety, control, and energy efficiency over polluting vehicles; paying particular attention to maximising their performance on slippery road surfaces. Through both simulations and real experiments, the researchers have designed and rigorously tested an innovative range of new technologies for realising these improvements. This progress is a result of four key studies published between 2012 and 2016.

    A theory for improving adhesion
    In their first study, carried out in 2012, Professor Xu’s team developed a vehicle-integrated scheme for controlling traction between tyres and the road, avoiding loss of control on surfaces like ice or mud where friction is greatly reduced. Their research aimed to discover how this friction – named the ‘adhesion force’, can be controlled by the vehicle itself no matter the surface it is driving on. “This study proposed a new structure for vehicle adhesion control, in which feedback including the adhesion stability state and the maximum adhesion force, can be calculated with the electrical parameters of the drive motor,” Professor Xu explains.

    In the closed-loop feedback system Professor Xu describes, data from the vehicle’s motor is fed into an onboard computer, which calculates how the torque of the motor needs to adjust to increase the adhesion force, before sending its instructions back to the motor. Tests using computer simulations revealed that this integrated control scheme could realistically improve an electric vehicle’s driving and braking force, as well as its adhesion stability, even when a driver has lost control of the vehicle. The team’s simulations proved that the technology is entirely possible with electric vehicles, and paved the way for later physical experiments demonstrating its capabilities.

    A schematic of one possible set-up to improve traction and control.

    Detecting wheel slipping
    Following on from this earlier work, Professor Xu’s team showed that electric vehicles could be able to automatically detect rates of slipping in their tyres when driving across low-friction surfaces. Slipping occurs when a vehicle’s wheels are rotating either too quickly or too slowly to gain full traction with a low-friction surface, resulting in a reduction in its speed. From the results of a study published in 2013, the researchers patented a system for detecting slipping rates, simply using data gathered by the vehicle’s drive motor. “These studies proposed a real-time detection method of vehicle adhesion parameters for the first time, including the friction coefficient and wheel slip-ratio, using only the motor’s voltage and current,” says Professor Xu.

    The research includes an improvement of the energy efficiency… for electrically-driven vehicles, and enhancement of the adhesion stability.

    In the patented system, data from devices which detect the voltage and current of the vehicle’s motor are fed into a processing unit and compared with the speed of the vehicle, allowing the computer to calculate the rate at which wheel slipping is occurring. Through both simulations and, for the first time, physical experiments, Professor Xu’s team showed that slipping could be successfully calculated, even when the design of the vehicle’s motor is varied. This detection mechanism would become an important basis for the team’s future studies.

    The relationship between the speed of the wheel, the vehicle’s speed, brake torque and the tyre force or vehicle brake force is a crucial calculation.

    Improving vehicle stability
    Building on this work, Professor Xu and colleagues next worked towards developing systems which could automatically prevent slipping on low-friction surfaces. The team noted that although traction can be gained when a vehicle’s wheels are spinning at just the right speed, road surfaces have widely varying amounts of friction in reality, making it extremely difficult for drivers to manually maintain an optimal rotation. In the 2016 study, Professor Xu’s team aimed to gain new insights into the determination of the ‘Force transfer factor’ – a value which “directly determines the adhesion stability between the tyre and road,” as Professor Xu explains.

    “This study revealed a new detection mechanism of tyre-road adhesion,” Professor Xu continues. “A novel concept of force transfer factor and a new detection model using electrical parameters of motors were proposed, as well as a unified criterion for vehicle stability. The force transfer factor can be obtained by electrical parameters such as current and voltage of the drive motor.” On a road surface with unknown friction, the system could, therefore, determine the most stable wheel rotation speed, which would optimise wheel grip by transferring the most energy to the ground. Again, the researchers carried out both experiments and simulations to show that this could be achieved simply using data from an electric vehicle’s drive motor.

    Recycling braking energy
    In addition to these areas relating to slip prevention, Professor Xu’s team have also studied how the braking energy of electric vehicles can be recycled. As they slow down, vehicles convert their kinetic energy into heat in their brakes. In previous studies, researchers have proposed that electric vehicles could convert this energy back into electricity, allowing for longer journeys without a need for frequent recharging. Such a system would make vehicles highly safe and energy efficient, but presents significant challenges in ensuring that as much of a vehicle’s braking energy is fed back to its onboard energy storage system as possible.

    Two of the possible vehicle set-ups.

    The research has made a breakthrough compared with the traditional vehicle dynamics control method.

    In another 2016 study, Professor Xu and colleagues approached the problem by again considering how the transfer of force can be maximised, this time between the wheels and the brakes. “This study proposed a method for estimating the maximum adhesion based on a knowledge-based methodology in a hierarchical control structure, and a technique for achieving the deep energy recovery of electrified vehicles with the maximum adhesion control,” Professor Xu explains. “Results show that the energy recovery improves the driving range by more than 25%.”

    Again, using both simulations and experiments, Professor Xu’s team proved that onboard processing using data from a vehicle’s drive motor can be used to minimise slipping in its brakes. In a successful conclusion to their series of studies, the researchers demonstrated that a stable, high-performance energy recycling system can be integrated into electric vehicles, even in unknown road conditions.

    Testing in different conditions, e.g. a road with low-grip wet iron plates, gives different optimal operation points depending on the conditions.

    Promising potential for electric vehicles
    Through these four areas of study, Professor Xu and his colleagues have shown that electric vehicles could realistically allow for greatly improved braking systems and traction control when compared with traditional vehicles. “The significance of the research includes an improvement of the energy efficiency via braking energy recovery for electrically-driven vehicles, and the enhancement of the adhesion stability,” Professor Xu summarises. “The research has made a breakthrough compared with the traditional vehicle dynamics control method, which adjusts the driving braking force based on the mechanical braking force distribution.”

    The significant experiments and simulations carried out in the team’s studies prove that these improvements can push the capabilities of electric vehicles beyond those of their fossil fuel-burning counterparts. With the assurance that electric vehicles can be safer, more reliable, and more energy efficient than traditional vehicles, a transition away from traditional cars looks set to ramp up in the near future.

    Your research has made important strides towards the improvement of energy-saving and safety of electric-driven vehicles. What’s next for your research?


    We will promote the application of research results in the automotive industry. As it touches on the existing active safety system of automotive industry, choosing a reasonable industrialisation route is very important, such as integrating the new technology in the existing TCS/ABS system, or developing a new electrified TCS/ABS system. Furthermore, we will consider promoting the applications of research results in electric-driven rail vehicles.

    References

    • Xu G, Xu K, Li W (2013). Novel estimation of tyre-road friction coefficient and slip ratio using electrical parameters of traction motor for electric vehicles. International Journal of Vehicle Autonomous Systems, 11(2-3), 261-278.
    • Xu G, Xu K, Zheng C, Zahid T (2016). Optimal operation point detection based on force transmitting behavior for wheel slip prevention of electric vehicles. IEEE Transactions on Intelligent Transportation Systems, 17(2), 481-490.
    • Xu G, Xu K, Zheng C, Zhang X, Zahid T (2016). Fully electrified regenerative braking control for deep energy recovery and maintaining safety of electric vehicles. IEEE Transactions on Vehicular Technology, 65(3), 1186-1198.
    • Chen J, Xu G, Xu K, Li W (2012, June). Traction control for electric vehicles: A novel control scheme. In 2012 IEEE International Conference on Information and Automation, 367-372. IEEE.
  • Research Objectives
    Professor Xu’s research interests include electric vehicle control, energy processing, and automotive electronics.

    Funding
    Delt Environment & Educational Development Foundation

    Collaborators

    • Dr Xu Kun, Associate Professor, Shenzhen Institutes of Advanced Technology (SIAT), Chinese Academy of Sciences
    • Dr Yang Ying, Associate Professor, Shanghai University

    Bio
    Professor Xu received his PhD in electrical engineering from Zhejiang University before joining Tongji University, where he was awarded his Professorship in 2000. He was Research Professor at The Chinese University of Hong Kong, Director of the CAS/CUHK SZ Institute of Advanced Integration Technology, Shenzhen, China, until 2015. He has been a Professor at Shanghai University since 2016.

    Contact
    Prof Guoqing Xu
    Room 617 No 9, Lane 333
    Nanchen Rd
    Baoshan District
    Shanghai
    P.R. China

    E: gqxu@shu.edu.cn
    T: +(86)13671588619
    W:
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