System Safety

Unintentional system self-steering is classified as safety-critical behavior by the active front steering system. The safe system status (failsafe) is the lowest-energy status of the actuating-unit servomotor. Regardless of whether the safe status is brought about by a power loss or by intentional deactivation by the system, it is essential to ensure that the actuating unit does not engage the steering system. The actuating unit is blocked by a lock, which engages the unit's worm-gear drive. The lock is preloaded by a spring and held against the preload by the voltage supply. An interruption of the voltage supply will thus cause the lock to engage the worm-gear drive of the actuating unit.

The locked superimposing gear ensures that manual steering by the driver via the steering column is still possible. The steering then responds like conventional steering. The purely mechanical gear ratio between the steering wheel and the front wheels is maintained. The electric motor of the active front steering actuating unit is connected with 3 phases.

The Servotronic valve switches at zero current to the fast-driving curve. Power-steering support is reduced accordingly. When the ECO is at zero current, the flow rate is 7 L/min. If the active front steering control unit does not send a valid message on the PT-CAN, the SGM operates after 100 ms with a road speed-dependent substitute curve. The substitute curve ensures sufficient steering properties for the passive active front steering system.

The driver is alerted to system faults by way of a warning lamp, a variable warning lamp and Check Control messages in the instrument cluster.

The Check Control message runs as follows: AFS failure. Steer with care.

The following information appears in the control display: Steering behavior altered. Possible to continue the journey. Steering wheel may be at angle.