BMW F12 2011, Техническое учебное пособие, Страница: 42 / 68

F12�Chassis�Dynamics
5.�Steering
38
Index Explanation
1 Active�Steering�lock
2 HSR�actuator
3 Active�steering�servomotor�with�motor�angular�position�sensor
5.2.1.�Functional�ranges
The�Integral�Active�Steering�further�improves�driving�dynamics�through�additional�control�interventions
at�the�front�and�rear�axle.�The�vehicle�speed,�lateral�acceleration�and�yaw�rate�calculated�from�mea-
sured�variables�are�compared�with�a�set-point�value.�The�difference�between�the�set-point�and�actual
value�is�converted�into�a�reference�yaw�moment.�The�resulting�control�interventions�are�distributed�in
an�optimum�ratio�between�the�active�steering�AL�or�rear�axle�slip�angle�control�(HSR).
The�EPS�is�supplemented�by�a�planetary�gearbox�with�override�function�in�the�steering�gear�input�and
an�active�steering�servomotor.�The�active�steering�servomotor�can�modify�the�steering�angle�specified
by�the�driver�with�reference�to�default�values�from�the�active�steering�control�unit.
To�do�this,�the�active�steering�control�unit�evaluates�the�sensor�signals,�which�include:
• Steering�angle
• Wheel�speed
• Yaw�rate
• Lateral�acceleration.
In�the�event�that�the�system�fails,�the�active�steering�servomotor�is�braked�and�disabled�by�a�lock.�The
full�range�of�vehicle�maneuverability�is�retained�as�a�permanent�mechanical�connection�between�the
steering�wheel�and�the�front�wheels�exists.
Low�speed�range
The�variable�steering�ratio�of�the�active�steering�component�reduces�the�steering�effort�required�to
move�the�front�wheels�from�lock�to�lock�to�roughly�two�steering�wheel�turns.
At�low�speed�ranges�of�up�to�roughly�60�km/h,�the�variable�steering�ratio�of�the�front�axle�is�coupled
with�an�opposing�steering�angle�at�the�rear�axle.�This�increases�the�maneuverability�of�the�vehicle.
High�speed�range
As�the�driving�speed�increases,�the�degree�of�steering�angle�amplification�by�the�Active�Steering�com-
ponent�is�reduced�and�the�steering-gear�ratio�becomes�less�direct.
At�the�same�time,�the�steering�strategy�adopted�by�the�Integrated�Active�Steering�changes.�While�at
low�speeds�the�rear�wheels�are�steered�in�an�opposing�direction�to�the�front�axle,�at�higher�speeds�the
steering�angle�is�in�the�same�direction.
As�a�result�the�instantaneous�center�of�rotation�shifts�backwards.�This�corresponds�to�a�vehicle�with�a
longer�wheelbase�which�enhances�its�directional�stability.�The�radius�of�the�curve�becomes�longer.
Active�steering�adds�a�further�angle�to�the�wheels�at�the�front�axle�which�means�that�the�curve�radius
and�required�steering�angle�remain�at�the�accustomed�level.