BMW F12 2011 Technical Training Manual page 42

F12￿Chassis￿Dynamics
5.￿Steering
Index
1
2
3
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.
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Explanation
Active￿Steering￿lock
HSR￿actuator
Active￿steering￿servomotor￿with￿motor￿angular￿position￿sensor