Drawing on the expertise of some of the world’s leading rally drivers, UK automotive company Prodrive has developed a system which modulates the torque distribution to individual wheels to increase the rate at which a car or vehicle will turn.
Prodrive claims that its Active Torque Dynamics system (ATD) provides a superior performance and feel when compared with conventional brake systems. The problem with road cars, explains engineer Damian Harty, is that they are driven in the ‘linear’ region. Under normal conditions, when the steering wheel is turned a certain amount, the vehicle will respond in a ‘known’ way every time. ‘This is all very well,’ says Harty, ‘but when the unexpected happens, drivers tend to apply more steering angle than the vehicle can actually follow and the vehicle behaviour changes significantly.
ATD extends the linear region, making the vehicle more controllable in emergencies’.
Using data from wheel speed sensors, a yaw rate sensor and a steering wheel angle sensor the system compares what the driver is requesting with what the vehicle is providing. If accelerating, a torque can be applied to the appropriate wheel to counter oversteer or understeer. Or, when braking, the system can brake the opposite wheel to enhance directional stability and reduce vehicle energy.
The system uses active centre, front, and rear differentials. It locks and releases to dynamically modulate the torque applied to each wheel, and a number of control strategies ensure that torque distribution is suited to each stage of different manoeuvres and that the system can be applied equally effectively to different vehicles.
In a basic implementation, the system will lock the rear differential as the vehicle oversteers, providing yaw damping and preventing the vehicle from spinning. In more sophisticated set-ups, ATD will modulate the torque at each of the wheels in real-time, allowing a vehicle to enter a corner like a 4wd car.
Harty says that the system is not only extremely responsive but also that ‘it predicts the vehicle’s reaction to changes before it calculates the correction required, achieving the required wheel speed in 70ms.’