Active Front Steering Actuating Unit
The active front steering actuating unit is located on the steering gear. It is integrated in the split steering column between the Servotronic valve and the rack. This actuating unit comprises a brushless synchronous DC motor and a planetary gear. The core component of the active front steering actuating unit is a planetary gear with 2 inputs. One input is from the driver via the steering spindle, the other is from the ring gear via the actuator motor. See Fig 1.
One input shaft is connected via the Servotronic valve to the lower steering spindle. This input shaft is connected to the "sun" gear of the planetary set. The second input comes from the ring gear. The ring gear is driven by the electric motor via a self-locking worm gear drive as a step-down stage. The worm gear drive actuates the ring gear which superimposes the steering angle of the front wheels specified by the driver. See Fig 2.
An electromagnetically controlled safety interlock is fitted. The safety interlock is spring loaded and engages the worm gear drive on the actuator motor when no voltage is applied.
The safety interlock is released at a current of approx.1.8 A. The rotor position of the electric motor is recorded by the motor position sensor on the motor itself. The electric motor is powered by way of 3 phases. The 3 phases are energized alternately by the electronics of the active front steering control unit. The shielding is furnished by the ground connection of the electric motor housing to the body. However, the actuating forces for the steering angle are not applied by the electric motor but rather by the power steering system. See Fig 3.