Active Steering from ZF Lenksysteme

Feb. 2008 -- The BMW X5 is the first SUV/SAV to offer active steering from ZF Lenksysteme, a 50:50 joint venture of Robert Bosch GmbH and ZF Friedrichshafen AG, as an option. The active steering system varies the ratio between steering wheel movement and front wheel angle. At slower speeds a slight turn of the steering wheel has a more pronounced effect on the steering response. The opposite is true at higher speeds.

Technically, this effect is achieved by using a superimposing gear with an electric servo motor that increases or reduces the steering ratio as vehicle speed increases or decreases. This results in a higher or lower pinion angle used to adjust the wheel position and modify the steering effect. System advantages include easier handling, a more comfortable ride and – most of all – the added safety that is by no means restricted to the higher steering ratio at fast speeds. The system stabilizes the vehicle by moving the servo motor even when the driver doesn’t actually turn the steering wheel. This takes place within milliseconds and is hardly noticeable to the driver.

“By automatically adjusting the angle of the front wheels in potentially dangerous or extreme situations, active steering can keep the vehicle from skidding. The system responds much faster than the driver can react,” explains Gerd Reimann, in charge of active steering development at ZF Lenksysteme.

Advantages with networking

This advantage is further enhanced when the active steering system is networked with other driver assistance systems. This requires the integration of sensor systems and control software. Since its market introduction, the active steering system in the BMW 5-series models has been equipped with active yaw rate sensors and control software. Hardly perceptible to the driver, the active steering system instantly compensates for unintentional lateral vehicle movements. Joint engineering partners BMW and ZF Lenksysteme received the German Industry Innovation Award for the steering system in 2003. But before the system was integrated into 6-cylinder BMW 3-series models, the engineers enhanced the system by adding a yaw torque compensation module. This effectively keeps the vehicle from skidding in sudden braking situations on different road surfaces, i.e. when the wheels on the right-hand side are on muddy soft shoulder and the left wheels are on dry asphalt. The networked control electronics instantaneously decide whether it is better to stabilize the vehicle by applying braking force or by counter-steering via the active steering system. The networking options available with active steering integrated into different suspension or drive systems have not yet been fully exploited in practice.

Source: ZF

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