Integral Active Steering

Integral Active Steering

Active steering (AL) and rear axle slip angle control (HSR)
The combination of active steering (AL) and rear axle slip angle control (HSR) is referred to as Integral Active Steering: optional equipment 2VH.
Integral Active Steering can influence the steering angle set by the driver without affecting the steering wheel angle. This means that for the first time the steering angle is specified at the front and rear axle independently of the driver.
This leads to the following advantages:
- The variable steering gear ratio optimized for every driving speed means that on the whole the vehicle can be driven with a more sport-oriented style and more accurately. The amount of effort required by the driver is reduced, thus making an additional contribution to active safety.
- At lower driving speed, very agile driving characteristics set in. The turning circle of the vehicle is reduced.
- At higher driving speed, effortless, comfortable driving characteristics set in. The vehicle seems to be directly "linked".

This document describes the function of the Integral Active Steering with rear axle slip angle control. The active steering function on the front axle is described in detail in the vehicle engineering diagnosis 320103001.

Brief component description
The following components are described:
- Active front steering control unit
- Planetary gearbox with overriding function with servomotor and servomotor lock
- HSR control unit
- Actuator for rear axle slip angle control
- ICM control unit

Active front steering control unit
The AL control unit is fitted in a protective housing on the floor of the vehicle in the left-hand footwell. The purpose of the AL control unit is to activate the servomotor of the active steering. In addition, the AL control unit processes the rotor position sensor signals. A temperature sensor in the AL control unit monitors the output stage for the servomotor.
Integrated Chassis Management (ICM) is the higher-level control unit for the suspension control systems.
The control unit for the active steering is connected across 4 plug connections to the vehicle electrical system. The AL control unit is equipment attached to the bus on the FlexRay.







Planetary gearbox with override function with servomotor and servomotor lock
The overriding gear of the active steering is located on the steering box. The overriding gear is installed in the split steering shaft. The overriding gear is a planetary gear train with 2 inputs and one output. The first input is formed by the lower steering shaft of the steering column. The second input shaft is a worm gear on the planetary gear train. The servomotor (actuated by the AL control unit) drives the worm gear of the planetary gear train. The gear ratio from the worm to the worm gear is 20.5:1. For possible failures, the worm gear pair is designed as self-inhibiting. The worm gear pair overrides the steering angle of the direct drive-through from the steering shaft. The total steering angle at the steering pinion comprises:
- Steering angle, applied at the steering wheel
- Steering angle applied by the electric servomotor







The electric servomotor on the planetary gearbox with override function receives its voltage supply from three phases (= wires) (U, V, W). In the event of a short circuit, the electric servomotor can turn a maximum of 120°. This prevents inadvertent start-up movements of the servomotor in the event of a short circuit. A temperature sensor in the control unit for active steering (AL) monitors the output stage for the servomotor.
- in the event of faults to prevent it from rotating due to external forces
- when the vehicle is at a standstill and the is engine off (not when the vehicle is at a standstill and the engine is running)
If the engine is switched off by the automatic engine start-stop function, the electric servomotor lock remains applied.

In the event of blocking, the electric servomotor lock engages in the corresponding gearing on the worm. The lock is spring-loaded, with the force of the spring countered by the preload. Any interruption of the voltage supply consequently causes the lock to engage. The driver can still control the vehicle by means of the steering wheel, even if the planetary gearbox with override function is locked in this way. Under these circumstances, the steering performs in much the same way as conventional steering systems. The purely mechanical transfer between the steering wheel and front wheels is always maintained.

HSR control unit
The control unit for the rear axle slip angle control (HSR) is in the luggage compartment well. The HSR control unit activates the servomotor in the actuator of the rear axle slip angle control. A temperature sensor in the HSR control unit monitors the output stage for the servomotor.







A rotor position sensor and a steering tie rod position sensor are installed in the actuator. The signals of these sensors are evaluated by the HSR control unit. The HSR control unit is connected on the FlexRay via the control unit for the Active Steering (AL).







Actuator for rear axle slip angle control
The following components are installed in the actuator:
- Servomotor
- Rotor position sensor
- Steering tie rod position sensor







Servomotor
The electromechanical actuator consists of a servomotor that moves both track rods via a spindle gear drive. The track rods are connected to the track-rod arms. The actuator is configured for a maximum lifting movement of 8 mm, which leads to an angle of maximum 3° on the wheel. The spindle gear drive of the rear-wheel steering is designed as self-inhibiting. Following a system failure, the vehicle has the same characteristics as if designed without rear-wheel steering.
The servomotor is a brushless three-phase motor. The servomotor on the planetary gearbox with overriding function is supplied over 3 phases (= lines) with voltage (U, V, W). In the event of a short circuit, the servomotor can continue rotating due to external forces (no servomotor lock present). The consequence is an inconsistent steering wheel angle of up to 45° that can be perceived by the driver.







Rotor position sensor
The rotor position sensor consists of a magnetoresistive element and a permanent magnet. The permanent magnet is located on the front of the rotor shaft of the servomotor. The magnetoresistive element measures the direction of the magnetic field horizontally and vertically. The rotor position sensor has a measuring range of 180°. The rotor position sensor supplies 2 voltage signals. A rotation through 360° is based on 2 signal sequences. The two voltage signals are used to calculate the position of the rotor. The number of half turns is counted by the HSR control unit, which stores this number in its memory when the ignition is switched off.

Steering tie rod position sensor
The steering tie rod position sensor is a non-contact position sensor (PLCD = Permanent-magnetic Linear Contactless Displacement). The steering tie rod position sensor consists essentially of a special core of soft magnetic material. This core is wrapped in a coil (primary coil) along its entire length. The core has another short evaluation coil on each end. A permanent magnet approximated to the sensor leads to local magnetic saturation. This enables a position to be determined.

ICM control unit
New: Sensors that used to be installed separately in the DSC sensor are now fitted in the ICM control unit. The ICM control unit uses these sensors to calculate variables that are important for the dynamic handling characteristics of the vehicle at that time:
- Longitudinal acceleration and vehicle inclination in longitudinal direction
- Lateral acceleration and vehicle inclination in transverse direction
- Yaw rate

The ICM control unit is available as a basic version and a high version. The high version differs in that it has:
- A larger microcontroller (required for calculation of the control operation of Integral Active Steering and Active Cruise Control)
- A redundant sensor system for lateral acceleration and yaw rate (safety requirement for Integral Active Steering).







The ICM control unit is located on the transmission tunnel in the vicinity of the vehicle centre of gravity.

System functions
The following system functions are described:
- Integral Active Steering system network (AL with HSR)
- Distributed functions
- Servotronic
- Driving stabilization
- Snow chain detection (only BMW-approved snow chains)

Integral Active Steering system network
The following graphics show the system network for the Integral Active Steering.







NOTICE: Forced connection active steering and rear axle slip angle control (HSR).

The rear axle slip angle control can not be ordered as optional equipment on its own.

Distributed functions
The Integrated Chassis Management (ICM) is the higher-level control unit in which the setpoint values of the Integral Active Steering are calculated. The ICM uses the current driving status and steering angle of the driver to calculate the setpoint values for the variable steering box ratio and yaw-rate control. Once these values have been prioritized, the ICM provides resulting setpoint values for the AL and HSR control units. This is a specified angle that is set at the front wheels and rear wheels. Both control units receive the setpoint value and trigger the actuator in such a way that the setpoint value is implemented correctly. Both control units are thus exclusively control units for actuators.
The control units are connected on the FlexRay. The FlexRay is routed through the AL control unit to the HSR control unit.

Servotronic
Both the base steering and Integral Active Steering have Servotronic. The speed-dependent steering servo is implemented by the Servotronic valve on the steering box. The Servotronic valve is always controlled by the ICM control unit. This means that the ICM control unit also contains the logic of the Servotronic function.
The ICM control unit also activates the ECO valve. This valve enables electronic adjustment of the volumetric flow of the power steering pump.

NOTICE: Other functional descriptions are important.

For details of the driving stability control systems, consult the functional descriptions in the diagnosis system.

Driving stabilization
If the vehicle tends to oversteer, for instance, when cornering, the yaw-rate control can correct the steering angle on the front wheels. This stabilizes the vehicle (with the emphasis on convenience and comfort). In this driving situation, therefore, the Active Steering supports Dynamic Stability Control (DSC). DSC does not intervene unless the stabilizing effect of the steering does not suffice to counteract the tendency to yaw.
With conventional systems, the driver has to actively steer the vehicle in a straight line if the brakes are applied on a road surface with non-uniform coefficients of friction. In such situations, the Active Steering performs this active steering intervention, so stabilizing the vehicle. Compared to pure ABS control operation, Active Steering with yaw moment compensation shortens the stopping distance.

Snow chain detection (only BMW-approved snow chains)
It must not be possible for customers to switch active steering systems on or off. In the case of Integral Active Steering, active steering intervention on the rear axle and rear wheels is not possible with snow chains fitted. When snow chains are fitted, the rear-wheel steering is deactivated to ensure free wheel travel in all cases. If snow chains are used, "Snow chains fitted" must be set in the control centre in the settings menu. If the permitted maximum speed with snow chains of approx. 65 km/h is exceeded, the rear-wheel steering is reactivated despite the setting "Snow chains fitted".
Snow chain detection is computed in the HSR control unit. There are two procedures for snow chain detection. Up to approx. 25 km/h, the signals of the ride height sensors (front left and rear left) are used. Up to 50 km/h, the signals of the wheel speed sensors (front left and rear left) are used. The signal shapes (pattern as the chain links move) are used to determine the presence of a snow chain. The individual signals are always combined by the HSR control unit to form one overall criterion. As a fail-safe feature, the system has recourse to the sensor signals front right and rear right.

Snow chain detection becomes active as follows:
- Terminal 15 on
- Driving speed greater than approx. 4 to 6 km/h

The snow chain detection is activated but does not compute:
- Driving speed greater than approx. 65 km/h
- No snow chain detected after a certain time

There is a Check Control message for snow chain detection.







Notes for Service department

General notes
After installation of the new ICM control unit, the diagnosis system must be used to start-up the unit. Here, the following steps are carried out (depending on vehicle equipment):
- Adjustment of the sensor system integrated in the ICM
- Adjustment of the ride height sensors
- Initialization of the Integral Active Steering.

Diagnosis instructions

NOTICE: Follow the instructions for service functions.

The diagnosis system provides the following service functions for the Integral Active Steering:
- HSR function check (articulation test)
- Start-up rotor position sensor HSR
- Rear axle wheel alignment

Path: Service functions > Chassis > Rear axle slip angle control
- Start-up, alignment of active steering

Path: Service functions > Chassis > Active steering
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.