The BMW active steering system consists of a power assisted rack and pinion steering gear, a double planetary gear system in the steering column, and an electric actuating motor.
The double planetary gear system incorporates an input sun gear connected to the driver’s steering wheel, two planetary gears, an output sun gear connected to the steering pinion, and a rotating housing. If the housing is held still, the driver’s steer inputs are translated from the input sun gear, thought the planetary gears (sharing a common lay shaft) to the output sun gear with a net 1:1 ratio.
if the planetary gear housing is held still, conventional steering operation is retained. This characteristic provides a means of steering the vehicle in the event of an active steering system failure.
The planetary gear housing features external gear teeth and can be rotated by a motor and worm gear. Alteration of the housing position allows rotation of the output sun gear independently of the input sun gear position. if the driver were to hold the steering wheel (and therefore the input sun gear) stationary, driving the planetary gear housing will result in the rotation of the steering pinion (and therefore a steering action at the front wheels).
By adjusting the position of the planetary gear housing, the active steering ECU can synthesize a variable steering ratio or generate steering actions independently of the driver.
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