This paper will present a form of mobility control for a 6x6 Hybrid Electric Vehicle (HEV). The vehicle concerned is a series configured HEV utilising Hub Mounted Electric Drives (HMED) at each of the six wheel stations to provide Individual Wheel Control (IWC). Whereas a conventional vehicle needs individual brake actuators or bulky differentials to vary individual wheel torques, IWC can be realised in this hybrid configuration through software control of each HMED, making it potentially more accurate, responsive and flexible than a mechanically implemented version.
Direct Yaw-moment Control (DYC) is a method of regulating individual wheel torque to control vehicle yaw motion, providing greater stability in cornering. By varying the torque applied to the left and right wheels, tyre forces can be controlled to produce a desired yaw moment. Not only can this be used to aid cornering, but also to reject disturbances, such as side winds, in straight line running. When combined with a Traction Control System (TCS), optimisation of these tyre forces are considered, ensuring that the vehicle handling characteristics remain stable while acceleration performance is improved. When integrating these two systems, consideration is given to the torque demands of each controller. This co-ordinated control ensures that the vehicle takes full advantage of the torque capabilities associated with the electric motor to provide improved vehicle handling, acceleration and stability.
The proposed control algorithms are implemented in MATLAB/SIMULINK on a basic non-linear vehicle handling model utilising a Dugoff tyre model to determine longitudinal and lateral tyre forces. The torque of each individual wheel is controlled to maintain a desired yaw rate and/or wheel slip. The model is then simulated on a number of road surfaces, undertaking a variety of test manoeuvres to assess the potential improvements that the combined controller can offer over a vehicle with fixed-torque distribution. The paper shows how the resultant controller offers a robust method of improving vehicle mobility, providing good stability under varying conditions.