ZF develops prototype with combined front and rear axle steering

ZF Friedrichshafen AG is currently proving with a new test vehicle how the different fields of competences of the two formerly independent companies ZF and TRW can be combined to generate solutions with a high customer value. The prototype combines the Dual Pinion Electrically Powered Steering (EPS) by ZF TRW on the front axle with the Active Kinematics Control (AKC) by ZF on the rear axle. The degree of safety can be enormously enhanced: Despite the slippery ground, the passenger car stays perfectly on track even when changing lanes or overtaking.

"The new test vehicle shows that by combining our extended ZF product portfolio, we can create systems with a high customer value," says Dr Stefan Sommer, ZF's CEO. "Driving comfort and more importantly driving safety can be significantly increased when well-established and proven systems are perfectly tuned to work together."

The test vehicle is equipped with ZF’s AKC active rear axle kinematics which is connected to the efficient Dual Pinion EPS by ZF TRW via joint control electronics. The benefits of combining these two systems can be compared as the active rear axle kinematics can also be de-activated on this vehicle. At low speeds, AKC moves the rear wheels in the opposite direction to the steering angle of the front wheels. This reduces the vehicle's turning radius helping to improve driver comfort. At higher speeds, the rear and front wheels steer in the same direction improving the vehicle's stability. When all wheels steer into the same direction, the vehicle rotation around the vertical axis is reduced, enabling safer driving.

The system provides various responses depending on the vehicle's road speed – to increase passenger car driving safety, comfort, maneuverability and driving dynamics.

AKC integration in modular rear axle system
The original rear axle system of the test vehicle was replaced by a newly developed rear axle concept which integrates the AKC active rear axle kinematics. The basic axle entails a modular further development of a semi-trailing arm rear suspension. As part of this setup, the rear of the two outboard kinematics points of the lower control arm were replaced with an integral link and an added toe link. The toe link defines the track across the wheel hub and enables the toe-in to be adjusted precisely. As an alternative to a suspension strut, which tends to be used on semi-trailing link axles, the integral link in an extreme outboard position enables separate springs and dampers to be used. The rear axle system can not only be combined with AKC but also with an electric or conventional drive module.

Steer-by-wire: steering toward automated driving
The connection of the two steering systems can also be leveraged in the future for partially or fully automated vehicles. This is demonstrated by the ZF prototype by means of an additional control element in the vehicle cockpit's center console. With this, the steering systems at the front and rear axles can be steered completely electronically. Such a steer-by-wire function – still as a demo mode in the prototype – can be used in the future to control partially and fully automated steering maneuvers, for instance overtaking and lane change operations. The connection of the two steering systems in this demonstration vehicle shows the degree of safety that can be achieved even when driving on slippery surfaces to avoid skidding of the vehicle when changing lanes or overtaking.

"The prototype only shows a part of the potential of ZF's competences for passenger cars," says Sommer. "Safety and comfort can be increased even further when we combine our steering, braking, active chassis systems as well as camera and radar systems to offer the next generation of automated driving and driver assist systems."