AS System Components E70
5
E70 Lateral Dynamics Systems
Inde
x
Explanation Inde
x
Explanation
1
DSC Sensor
5
Fluid r
eservoir
2
Steering Column Switch Cluster
6
Steering gear
3
AS control unit
7
Hydraulic pump
4
AS actuating unit
8
P
ower steering cooler
A
S System Overview
6
E70 Lateral Dynamics Systems
L
egend for AS System Overview
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E70 Lateral Dynamics Systems
Index Explanation Index Explanation
1
Wheel speed sensor, front left
9
Wheel speed sensor, front right
2
Solenoid lock
10
Junction box
3
A
S control unit
11
D
SC sensor
4
AS actuating unit
12
Wheel speed sensor, rear left
5
EVV (Electronically Controlled Bypass Valve)
13
Wheel speed sensor, rear right
6
Digital Motor Electronics
14
Brake light switch
7
Dynamic Stability Control
15
Steering column switch cluster
8
Servotronic valve
16
Instrument cluster
Active Steering Features
Previous Active Steering Systems at BMW
The dynamic steering system known as Active Steering was used for the first time in the
world by BMW in the E60. The "variable steering-transmission ratio" is adjusted to the
optimum ratio for all road speeds. During this process and depending on the road speed,
an electric motor uses a worm drive to mesh with a special planetary gearbox with over-
ride function and gear into the steering chain.
This allows the steering system to generate a greater or reduced steering angle at the
front wheels depending on the driving situation. Since it was implemented in the E90,
Active Steering is also able to intervene in critical dynamic driving situations (braking with
different friction coefficients on the left and right) by correcting the steering angle if the
vehicle starts to become unstable using so-called yaw moment compensation.
This stabilizes the vehicle more quickly than the driver could. Active Steering thus
reduces intervention in the lower DSC control range and thereby provides optimum:
• Agility
• Comfort
• Active safety
Increased Agility
Due to the direct transmission ratio, the vehicle is perceived as having a greater agility and
handling performance up into the mid driving speed range (approximately 100 km/h).
The driver also has far greater control when swerving, for example - this combined with
considerably enhanced steering precision and reduced steering effort. Direct contact
with the road via the steering wheel is maintained throughout.
Increased Convenience
Some previous steering systems have required more than 3 full steering-wheel turns to
achieve a full wheel lock from the far left over to far right. Active Steering reduces this at
low speeds to less than 2 steering-wheel turns from lock to lock.
The benefit: less steering effort is required when turning in city traffic or maneuvering in
narrow parking spaces. On twisting roads in mountainous regions, for example, the
reduced steering wheel angle also ensures that hands always remain in the optimum
position on the steering wheel, which means that crossing of hands, or even arms in
some cases, is no longer necessary. The multifunction buttons on the steering wheel or
the gearshift paddles for the sequential manual transmission (SMG) therefore always
remain conveniently within reach during every road situation.
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E70 Lateral Dynamics Systems
Increased Active Safety
At higher speeds, the transmission ratio of the Active Steering is more indirect. This
means that a rotational movement of the steering wheel generates a smaller steering
angle than at lower speeds.
This results in more comfortable directional stability. The effects of "snatching" the
steering wheel, e.g. in a frightening situation, are considerably lessened by this. An assist-
ing intervention by the Active Steering counteracts yawing forces and rotational move-
ments of the vehicle around its vertical axis when they first occur by correcting the steer-
ing angle of the wheel. Yawing forces (without active brake intervention) occur when the
vehicle changes lane or swerves or when the load changes under cornering.
Active Steering electronically controls the stabilization function at all speeds without the
driver being aware of this. It also assists the driver by actively counter-steering when
there are different friction coefficients under braking.
Active Steering System Changes (as compared to E6X/E9X)
For the first time, the customer can choose between standard power steering and the
innovative Active Steering system in a BMW all-wheel-drive vehicle.
Details in which the Active Steering in the E70 differs from those which are familiar in
other series:
• No physical cumulative steering-angle sensor
• Virtual cumulative steering angle provides the same information as cumulative
steering-angle sensor
• Virtual cumulative steering angle describes the exact position of the rack and pinion
• Virtual cumulative steering angle as reference for adjustment of track alignment (vir-
tual steering angle "0" = rack and pinion exactly centered
• Virtual cumulative steering angle is calculated from the driver steering angle and
motor position angle
• If the motor position angle is lost or discarded, the position of the rack and pinion is
unknown and must be relearned, i.e. initialize the Active Steering.
• Instead of yaw moment compensation, the E70 has yaw-rate control plus (GRR+)
• Initialization has changed
• There is no longer an ECO valve on the hydraulic pipe. It has been replaced by a
new valve: Electronically Controlled Bypass Valve (EVV)
• The wheel alignment check with KDS has a ne
w feature that must be taken into
consideration.
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E70 Lateral Dynamics Systems
Special Functions
Active Steering Transmission Ratio
The Active Steering varies the steering-gear ratio as a function of the road speed and the
steering angle requested by the driver. The steering is designed to be indirect at high
speeds. At lower speeds, the steering is more direct.
The ease of handling the vehicle at low speeds or when parking is greatly increased by
the Active Steering actuating unit. It is no longer necessary to grapple with the steering
wheel.
Just two turns of the steering wheel when the vehicle is stationary are enough to move
the steering wheel from one lock to the other. At high road speeds (> 120 km/h), the
Active Steering allows a more indirect transmission ratio than in conventional steering
systems. The electric motor operates against the steering wheel angle at high speeds.
Together with the elevated steering torque level (Servotronic), this has the effect of con-
siderably improving the vehicle's directional stability.
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E70 Lateral Dynamics Systems
Index Explanation Index Explanation
1
Active steering design
3
Road speed
2
Conventional design
4
Ratio
Yaw Rate Control
If the vehicle is threatening to oversteer, the Active Steering stabilizes the vehicle with a
slight correction to the angle of attack of the front wheels.
Example: The driver initiates a rapid lane change. This change in direction causes a
certain tendency to oversteer in the vehicle.
As the control thresholds for a DSC stabilization intervention are not yet reached in this
case, the angle of attack of the wheels is slightly adjusted (maximum ± 4°) by the Active
Steering. The vehicle is stabilized by this active damping, but the driver does not feel it
through the steering wheel.
Yaw-rate Control Plus (GRR+)
The "yaw-rate control plus" function assists the driver when he is braking on roads with
varying friction coefficients, in a similar way to the yaw moment compensation. This func-
tion represents a significant safety feature.
In the case of braking on different surface friction coefficients (asphalt/ice or snow), a
moment builds up about the z or vertical axis (yaw moment) rendering the vehicle unsta-
ble. In
this special case, the Active Steering control unit calculates the steering angle
(maximum ±4°) required for the front wheels to keep the vehicle stable using the actual
yaw rate and the longitudinal and lateral acceleration.
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E70 Lateral Dynamics Systems
This active counter-steering creates counter-torque about the z or vertical axis (red arrow)
which compensates for the earlier acting yawing force (yellow arrow).
In this way, the vehicle is stabilized by clever interaction between the DSC braking and
the AS function, resulting in a new safety aspect, unique in this class.
Special Feature
This function shortens the braking distance because the yaw-rate control plus allows
higher braking pressure on the high friction coefficient side.
Difference to Yaw Moment Compensation:
With yaw moment compensation, an important calculation principle for automatic early
counter-steering was the difference between the front wheel braking pressures that were
detected by the DSC and based on which the DSC calculated the angle for attack of the
front wheels and transmitted to the AS control unit.
This shared functionality of the DSC and AS control unit is now fully integrated into the
AS control unit with yaw-rate control plus and, due to the high communication rate,
pr
ecise interaction between the two units has now been achieved.
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E70 Lateral Dynamics Systems
Power Steering Assistance
Servotronic
Power steering assistance is implemented by conventional hydraulic rack and pinion
power steering.
The magnitude and direction of the auxiliary hydraulic forces acting on the rack depend
on the rotational angle of the torsion rod. The torsion rod is located between the steering
shaft and pinion.
Servotronic, the speed-dependent power steering assistance, is only available as an
option with Active Steering.
The output stage for Servotronic and its software are located within the AS control unit.
The output stage for actuating the EVV is also within the Active Steering control unit.
Tandem Pump
The hydraulic pumps fitted in the E70 were developed according to a modular design
principle. Depending on the engine and equipment specification, an appropriate hydraulic
pump is located in the same installation space on the engine.
If only the optional Active Steering is fitted, there is a mono-pump that consists only of a
vane section.
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E70 Lateral Dynamics Systems
Index Explanation Index Explanation
A
Radial piston section of pump
3
Steering pressure connection
B
Steering pressure connection
4
Accumulator
1
Suction connection
5
ARS Pressure connection
2
Proportional valve (EVV)
6
Input flange
System Safety
Unintentional "self steering" by the system would be a safety critical feature of Active
Steering and is prevented by the following measures:
The safe system status (fail-safe) is when the actuator motor of the actuating unit is with-
out power. Regardless of whether the system enters fail-safe mode as a result of a loss
of power or an intentional system shutdown, it must be ensured that there is no possibili-
ty of the actuating unit intervening in the steering.
The actuating unit is arrested by a lock that engages in the worm gear of the actuating
unit. The lock is pre-tensioned by a spring and held against this pretension by the power
supply. The actuating unit is locked by an interruption in the power supply.
The arrested superimposed gear unit ensures that it is still possible for steering wheel
movements input by the driver to be transferred along the steering column.
The steering then responds in the same way as conventional steering. The purely
mechanical transmission between the steering wheel and the front wheels is retained.
The electric motor of the Active Steering actuating unit is connected to 3 phases. A short
circuit to earth will therefore prevent the electric motor from completing a full turn, the
maximum possible being only 120° (360°: 3).
The Servotronic valve switches under no electrical load to the fast driving characteristic
curve. The power steering assistance is reduced accordingly. When the EVV is de-ener-
gized, the volumetric flow is 7 l/min.
Steerability is fully guaranteed in this situation. Fault statuses in the system are indicated
to the driver via an indicator lamp system and Check Control messages in the instrument
cluster.
The Check Control message may read:
"Active Steering failure! Steer with care."
The following or similar message will appear
in the Control Display:
"Steering behavior altered! Possible to
continue the journey. Steering wheel may be
at an angle. Have the problem checked by the
nearest BMW Service."
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E70 Lateral Dynamics Systems
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