The motor industry continues to show a rapidly increasing interest in powertrain electrification, following the need for continued reductions in exhaust emissions and ever increasing pump fuel prices. This will soon overlap with another strong focus of the industry, namely the demand for active safety devices.
Electronic stability control (ESC), based on individual wheel braking, prevents the vehicle from spinning or drifting and has become the system of reference to improve safety and driveability at the limit of handling performance.
However, ESC interventions are not energy efficient and are intrusive to the driver, so its use within the
sub-limit driving region is limited. The potential for combining the above industry demands – achievable by using a hybrid powertrain for active vehicle dynamics control – is well known but has not been fully exploited yet. For cost and complexity reasons, the first generation of HEVs/EVs currently on the market has kept the same architecture of fuel driven vehicles, with one single central motor and power transferred to the wheels through a standard driveline. However, as market penetration increases it is reasonable to expect such architecture constraints to diminish and technologies such as in-wheel motors and distributed intelligence will start being adopted, with their inherent advantages for both packaging and vehicle control.
As an intermediate step towards these forthcoming technologies, we have developed a yaw motion control device based on torque vectoring by twin rear in-board electric motors.
Electric Dynamic Control (eDC) aims to provide the dynamic appeal that is lacking in most of today’s HEVs/EVs.
By adapting eDC to today’s hybrid technology and vehicle architecture, a number of vehicle dynamics features can be implemented, their cost/benefit ratio evaluated and their driver acceptance assessed. Our eDC system employs two high-torque independently controlled electric motors on the rear axle. The two motors are commanded in-phase to provide driveability features such as zero-emissions driving, acceleration support and regenerative braking. The motors are also commanded in opposite-phase to generate a yaw moment and control the vehicle yaw motion to a given target.
The vehicle retains its original drivetrain, with each motor mounted to the side of the final drive, in series with the driveshafts. Our eDC yaw control device presents a number of distinctive features. It provides fast and highly controllable actuation due to the fast response time and high control bandwidth of the motors. There is no corruption of driver feedback due to the differential actuation across the axle, with virtually no influence on the vehicle longitudinal state. Additionally, the system interventions do not negatively affect the driver’s steering feel, whereas this is a critical aspect for torque vectoring techniques on front axles. Importantly, there are no additional requirements on the battery, because the wheel with negative torque regenerates the power dissipated by the wheel with positive torque. Therefore, during system interventions in straight-ahead driving, the battery needs only to provide the power lost due to the regeneration efficiency.
The system possesses a high degree of yaw authority, in particular when compensating understeer. Interventions to correct oversteer can also be delivered providing they don’t saturate the rear tyres. However, in both cases, thanks to the smooth and energy-efficient interventions and unobtrusiveness to the driver, the system can be used seamlessly in the sub-limit region. This is an essential feature because it allows a high degree of yaw damping, thus increasing yaw stability, and offers many opportunities for customisation.
The torque vectoring concept also makes regenerative braking and traction support much more robust and less prone to the introduction of yaw instability. Further benefits include a reduced authority with vehicle speed, safe control by software if motor failure occurs, and an increase in vehicle parameter estimation from motor torque feedback.
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