Active steering

Active steering describes a steering system for a vehicle in which the relationship between the driver’s steer inputs and the angle of the steered road wheels may be continuously and intelligently altered. Whilst active steering systems may be found in agricultural equipment and heavy plant, this article concentrates on the application of active steering in BMW passenger cars.

Active steering describes a type of power electric variable gear ratio power steering technology introduced by BMW in 2003 first appearing on the redesigned 5-series which varies the degree that the wheels turn in response to the steering wheel. At lower speeds, this technology reduces the amount that the steering wheel must be turned – improving performance in situations such as parking and other urban area traffic maneuvers. At higher speeds, the performance is such that the normal increased responsiveness from speed is avoided and it provides improved directional stability.

The BMW active steering system utilises a double planetary gear system located at the base of the steering column to facilitate driver independent steering of the front wheels. The system is intended to offer two principal benefits; a variable steering ratio and stability enhancing corrective steering actions.

The BMW active steering system consists principally 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. I.e. 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. I.e. 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 manipulating 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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Wikipedia