Brief Description Of Components

The sensors incorporated into the Active Front Steering system are:

• Motor-position sensor 

  The motor-position sensor registers the rotor position of the electric servomotor. The rotor position is communicated to the control unit for Active Steering (AL control unit).

  NOTE: Digital motor-position sensor for 3rd generation Active Steering from X5. The 3rd generation Active Steering has a digital motor-position sensor.

  The 3rd generation Active Steering includes a digital motor-position sensor. The motor-position sensor is on the output side of the electric servomotor (on the planetary gearbox with override function). Operating on what is known as the magneto resistive principle, the motor-position sensor registers the rotor position of the electric servomotor (= angle of rotation).

  The motor-position sensor consists of a magneto resistive element and a permanent magnet. The permanent magnet is on the end face of the rotor shaft of the electric servomotor. The magneto resistive element measures the magnetic field in the horizontal and vertical directions.

  The motor-position sensor has a measuring range of 180°. The motor-position sensor supplies 2 voltage signals. A rotation through 360° is based on 2 signal periods. The two voltage signals are used to calculate the rotary position of the motor. The number of half turns is counted by the AL control unit, which stores this number in its memory when the ignition is switched off.

• Cumulative steering angle sensor and steering angle sensor 

  The cumulative steering angle sensor captures the steering angle that the Active Steering produces at the steering box.

  As far as its function is concerned, the cumulative steering angle sensor corresponds to the steering angle sensor in a conventional steering system. The steering angle sensor in the steering column switch cluster registers the angle to which the driver turns the steering wheel. Both signals are needed by the control unit for Active Steering (AL control unit).

  NOTE: No cumulative steering angle sensor from X5. The 3rd generation Active Steering no longer has a cumulative steering angle sensor.

  Vehicles with 2nd generation Active Steering have a cumulative steering angle sensor. The cumulative steering angle sensor is flange-mounted to the output end of the steering box.

  The cumulative steering angle sensor consists of a magneto resistive element and a permanent magnet. The permanent magnet is mounted on the end of the steering pinion. The cumulative steering angle sensor is connected to the AL control unit and the DSC control unit by the F-CAN (chassis CAN = sub-bus of the PT-CAN). The angles measured by the cumulative steering angle sensor are signalized by the AL control unit. The DSC control unit uses calculated values delivered by the AL control unit through the F-CAN. Moreover, the DSC sensors and the steering angle sensor in the steering column switch cluster (SZL) are also connected to the F-CAN.

  The cumulative steering angle sensor measures the angle of rotation of the steering pinion. The angle measured at the steering pinion is the cumulative steering angle (total steering angle) consisting of:

  • Steering angle, applied at the steering wheel
  • Steering angle, applied by the electric servomotor at the planetary gearbox with override function

  The cumulative steering angle sensor has a measuring range of 180°. The half turns are counted in the AL control unit and in the DSC control unit. The signal from the cumulative steering angle sensor is sent on the F-CAN (every 10 milliseconds). This signal is received by the AL control unit and the DSC control unit. The DSC control unit broadcasts the cumulative steering angle to the other bus nodes on the PT-CAN. The cumulative steering angle is the definitive signal for all the control units that need the steering angle sensor signal.

  The cumulative steering angle sensor is discontinued from the E70.

  The Active Steering control unit computes a "virtual" cumulative steering angle. This virtual sensor supplies the same information as the cumulative steering angle sensor. The virtual cumulative steering angle describes the exact position of the steering rack. Cumulative steering angle 0° = exact middle of steering rack.

  The control unit Active Steering derives the cumulative steering angle from the following signals:

  • Steering angle sensor signal
  • Motor-position sensor signal
  NOTE: Commissioning if a signal is missing. If the signal from the motor-position sensor is lost, the position of the steering rack is unknown and must be recalibrated. For this reason, Active Steering must be commissioned as new!

  The motor-position sensor registers the position of the electric servomotor.

  • > From E70 (other model series to follow)

  The cumulative steering angle sensor on the X5 is discontinued. Active Steering computes a "virtual" cumulative steering angle. The virtual cumulative steering angle is derived from the following signals:

  • Steering angle sensor signal
  • Motor-position sensor signal

  Cumulative steering angle sensor: BMW 1, 3, 5, 6-Series  

  Vehicles with 2nd generation Active Steering have a cumulative steering angle sensor. The cumulative steering angle sensor is flange-mounted to the output end of the steering box.

  NOTE: No cumulative steering angle sensor from X5. The 3rd generation Active Steering no longer has a cumulative steering angle sensor. Further model series to follow.

  The cumulative steering angle sensor consists of a magneto resistive element and a permanent magnet. The permanent magnet is mounted on the end of the steering pinion.

  The cumulative steering angle sensor is connected to the AL control unit and the DSC control unit by the F-CAN (chassis CAN = sub-bus of the PT-CAN). The angles measured by the cumulative steering angle sensor are signalized by the AL control unit. The DSC control unit uses calculated values delivered by the AL control unit through the F-CAN. Moreover, the DSC sensors and the steering angle sensor in the steering column switch cluster (SZL) are also connected to the F-CAN.

  The cumulative steering angle sensor measures the angle of rotation of the steering pinion. The angle measured at the steering pinion is the cumulative steering angle (total steering angle) consisting of: Steering angle, applied at the steering wheel and steering angle, applied by the electric servomotor at the planetary gearbox with override function

  The cumulative steering angle sensor has a measuring range of 180°. The half turns are counted in the AL control unit and in the DSC control unit. The signal from the cumulative steering angle sensor is sent on the F-CAN (every 10 milliseconds). This signal is received by the AL control unit and the DSC control unit. The DSC control unit broadcasts the cumulative steering angle to the other bus nodes on the PT-CAN. The cumulative steering angle is the definitive signal for all the control units that need the steering angle sensor signal.

  The cumulative steering angle sensor is discontinued from the E70.

  The Active Steering control unit computes a "virtual" cumulative steering angle. This virtual sensor supplies the same information as the cumulative steering angle sensor. The virtual cumulative steering angle describes the exact position of the steering rack. Cumulative steering angle 0° = exact middle of steering rack.

  NOTE: Commissioning if a signal is missing. If the signal from the motor-position sensor is lost, the position of the steering rack is unknown and must be recalibrated. For this reason, Active Steering must be commissioned as new!

  The control unit Active Steering derives the cumulative steering angle from the following signals: Steering angle sensor signal - Motor-position sensor signal. The motor-position sensor registers the position of the electric servomotor.

  The steering angle sensor on the BMW 1 and 3-Series is an optical sensor.

  From 09/2005, the BMW 5 and 6-Series also have an optical steering angle sensor.

  The steering-angle sensor is installed in the steering column switch cluster (SZL).

  The steering angle sensor is a contact-free optical system for measuring angles. The steering angle sensor is secured to the circuit board with the electronic evaluation unit.

  NOTE: Optical steering angle sensor also on BMW 1 and 3-Series From 09/2005, these model series also have an optical steering angle sensor.

  The steering angle sensor comprises of the Encoded disc and Optical sensor

  The encoded disc is connected to the steering wheel by the coil spring cassette. When the steering wheel is moved, the encoded disc moves within the optical sensor. There are different line patterns on the encoded disc for purposes of evaluation.

  The optical sensor is built as follows: LED and fibre-optical conductor projects the light onto the encoded disc through the optical conductor and the Line camera converts the optical signals (emitted by the LED) that penetrate the encoded disc into electrical signals.

  Light from the LED is projected onto the encoded disc through the optical conductor. Depending on the position of the steering wheel, more or less light will penetrate the encoded disc and reach the line camera. This optical signal is picked up by the line camera and converted into an electrical signal. Signal path: LED -> Optical conductor -> Encoded disc -> Line camera -> Analogue signal -> Electronic evaluation unit.

• DSC sensor 
  • > E60, E61, E63, E64 up to 09/2005:

  2 DSC sensors transmit redundant signals capturing the rate of yaw (rotation about the vertical axis) and lateral acceleration to the AL control unit via the chassis CAN (F-CAN).

  A vehicle equipped with Active Front Steering features a second DSC sensor integrated along with the standard DSC sensor as a redundant sensor for safety reasons. The standard DSC sensor is underneath the driver's seat. The second DSC sensor is located underneath the front-passenger seat.

  NOTE: Only one DSC sensor from 09/2005. The 2nd DSC sensor is discontinued from 09/2005. 1 DSC sensor supplies redundant signals. This DSC sensor is located under the front-passenger seat.

  The DSC sensors are a combination of lateral acceleration and rate-of-yaw sensors. The DSC sensor consists of:

  • Housing with connector
  • Damper to prevent mechanical overstressing
  • Sensor element: The sensor element consists of 2 piezoelectric acceleration sensors. A spring-mounted weight is hung in the measuring cell of the acceleration sensor.
  • Housing cover

  In terms of engineering design, the two DSC sensors are virtually identical. DSC sensor 2 has an extra terminating resistor. The two DSC sensors have different part numbers. In addition, each of the two DSC sensors has its own identification on the CAN bus (CAN message). This excludes the possibility of confusing the DSC sensors.

  Each of the two DSC sensors supplies a rate-of-yaw signal and an acceleration signal. Any accelerated motion accelerates the spring-mounted suspended mass. The force needed to achieve this is generated by mechanical tension in the piezoelectric material. This results in a shift in the electric charge. Electrodes are used to detect this shift, which is output as an electric signal for processing.

  The second DSC sensor implements redundancy in terms of registration of the signals for lateral acceleration and rate of yaw. The fact that two DSC sensors are used means that plausibility can be monitored. The two DSC sensors are triggered by the DSC control unit (excited every 10 milliseconds). Each time they are triggered, the two DSC sensors send their signals to the F-CAN.

  • > E70, E81, E87, E90, E91, E93, E93

  Only 1 DSC sensor is needed for the Active Steering. This DSC sensor transmits redundant signals capturing the rate of yaw and the lateral acceleration.

  • > E60, E61, E63, E64 from 09/2005

  Only 1 DSC sensor is needed for the Active Steering.

  This DSC sensor transmits redundant signals capturing the rate of yaw and the lateral acceleration.

The control units networked for the purposes of AFS are as follows:

• AL: Active Steering control unit 

  The AL control unit computes the nominal values for the electric servomotor of the planetary gearbox with override function.

  BMW 5-Series and 6-Series - The AL control unit is located in the passenger-side footwell. The AL control unit is bolted to the floor pan (see illustration for DSC sensors). The AL control unit is protected by a housing to prevent it from being stepped.

  BMW 1-Series, 3-Series, X5 - The AL control unit is not located inside the vehicle. The AL control unit is mounted on the side frame behind the front left side panel.

  The AL control unit housing has a stud for connecting the shield for the 3 phases (U, V, W) of the electric servomotor. The AL control unit incorporates 2 processors.

  The AL control unit is connected by the chassis CAN (sub-bus of the F-CAN) and the PT-CAN by the DSC control unit. 2 connections on the AL control unit are the interface to the on-board network.

  The AL control unit is integrated into the on-board network by the powertrain CAN and the chassis CAN. The function algorithms for calculating the nominal values for controlling the electric servomotor are stored in the AL control unit. In addition to the actual system functions, the control unit has the following functions:

  • Control of the power-steering pump
  • Pre-drive-check: The AL control unit is initialized after the ignition is switched on (pre-drive-check). The electric servomotor is not actuated while initialization is in progress. The sensor signals are checked and, if necessary, calibrated.
  • Safety monitoring functions: If errors are detected either the status changes directly to "Error" or rate-of-yaw control is deactivated. When system status is "Error", activation of the electric servomotor is disabled. System status changes to "Drive" once initialization has completed successfully.
  • Sensor plausibility check
  • Actuator monitoring
  • Vehicle authentication: Check with the Car Access System (CAS control unit) to ensure that drive authorization has been verified.

  The AL control unit uses the various input signals to compute the signals for actuating the electric servomotor. Input signals are:

  • Wheel speeds (wheel-speed sensors via DSC)
  • Yaw rate and lateral acceleration (from the DSC sensor or sensors)
  • Steering wheel angle (from the steering angle sensor)
  • Cumulative steering angle (from the cumulative steering angle sensor)
  • Not 3rd generation Active Steering from E70: The Active Steering control unit computes a "virtual" cumulative steering angle from the following signals: Signal from steering angle sensor and Signal from motor-position sensor.
  • Position of the electric servomotor (from the motor-position sensor)

• SZL: Steering Column Switch 

  The Steering Column Switch provides the steering angle.

• SGM: Safety and gateway module 
  • > E60, E61, E63, E64 up to 09/2005

  The safety and gateway module (SGM) drives the ECO valve in the power-steering pump and the Servotronic valve.

  Moreover, the SGM forms the interface between the PT-CAN and the K-CAN (signals for instrument cluster).

• KGM: Body gateway module 
  • > E60, E61, E63, E64 from 09/2005

  The body gateway module (KGM) forms the interface between the PT-CAN and the K-CAN (signals for instrument cluster).

  With option 216 "Servotronic", the KGM actuates the Servotronic valve.

• JBE: Junction box electronics 
  • > E70, E81, E87, E90, E91, E92, E93

  The junction box electronics (JBE) forms the interface between the PT-CAN and the K-CAN (signals for instrument cluster).

  Moreover, the AL control unit receives its power supply from the distributor in the junction box.

• DSC: Dynamic Stability Control 

  The DSC control unit and the AL control unit are interconnected by the F-CAN (chassis CAN). The signals supplied by the DSC control unit include the road speed signal.

• DME or DDE: digital engine electronics or digital diesel electronics 

  The engine control sends the signal indicating that the engine is running to the AL control unit via the PTCAN bus. The AL control unit notifies the engine control of the approximate drive torque of the power steering pump.

• CAS: Car access system 

  The vehicle is authenticated by the AL control unit and the CAS control unit via the K-CAN/PT-CAN (vehicle identification numbers compared).

  Moreover, the CAS control unit transmits the wake-up signal for the PT-CAN.

The following actuators are actuated by the Active Steering:

• Planetary gearbox with override function and electric servomotor 

  The planetary gearbox with override function uses the electric servomotor to generate the resulting cumulative steering angle at the front wheels.

  The planetary gearbox with override function is mounted on the steering box. The planetary gearbox with override function is built into the split steering spindle.

  The planetary gearbox with override function consists of the planetary gearbox, worm gear, electric servomotor and electric servomotor lock. The electric servomotor lock is a safety element, it locks the worm gear in the event of a fault.

  A zero-play planetary gearbox with override function is the core element of the Active Front Steering system. The gearbox is of the planetary type with two input shafts and one output shaft. The first input shaft is the lower steering spindle of the steering column. The second input shaft is a worm gear on the planetary gearbox.

  The electric servomotor (actuated by the AL control unit) drives the worm gear of the planetary gearbox. The ratio for transmission of movement of the worm to the worm gearwheel is 20.5:1. The worm gear is self-inhibiting in the event of a malfunction. The worm gear superimposes additional movement on the steering lock that results from direct input from the steering spindle. The cumulative steering angle at the steering pinion comprises:

  • Steering angle, applied at the steering wheel
  • Steering angle applied by the electric servomotor

  The amount of override over and above the steering lock applied by the driver depends on dynamic variables, the most important of which is the vehicle's speed on the road.

  At low speeds (such as when the vehicle is being maneuvered into a parking slot, for example), the Active Front Steering applies considerable override over the steering angle imposed directly by the angle of rotation of the steering spindle. Steering becomes very direct. At high speeds the electric servomotor works in the opposite direction to the driver's steering angle. The overall steering linkage becomes more indirect.

  The electric servomotor on the planetary gearbox with override function receives its 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 the electric servomotor making unintentional starting movements in the event of a short circuit.

  There is a temperature sensor in the electric servomotor. This temperature sensor monitors heating in the electric servomotor.

  The electric servomotor lock blocks the electric servomotor if a fault develops and when the vehicle is at a standstill.

  The electric servomotor lock inhibits movement by engaging the teeth of the worm gear. The electromagnetic lock is spring-loaded, with the force of the spring countered by the power supply. Any interruption of the power 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 transmission link between steering wheel and front wheels is always sustained. The electric servomotor lock releases when the supply voltage is approximately 3.2 volts.

• Hydraulic steering with Servotronic valve 

  The conventional hydraulic steering provides the power-steering assistance.

  "Servotronic" (= speed-dependent power-assisted steering) is part of the Active Steering option.

  The individual components of the hydraulically assisted steering system are housed in the engine compartment. The Servotronic valve is on the steering box.

  The hydraulic steering consists of the following components:

  • Power-steering pump with ECO valve
  • Hydraulic fluid expansion tank
  • Power steering cooler
  • Hydraulic lines and hydraulic hoses
  • Steering box with Servotronic valve

  The Servotronic controls the degree of assistance provided by the hydraulic steering as a function of the vehicle's speed. The flow of hydraulic fluid is restricted to a greater or lesser extent depending on how the Servotronic valve is actuated. Restriction of the flow depends on the current actuating the Servotronic valve.

  BMW 5 and 6-Series up to 09/2005 - The safety and gateway module (SGM) actuates the Servotronic valve. In a vehicle fitted with Active Steering, the AL control unit determines the nominal current for the Servotronic valve. In the event of the AL control unit failing, the SGM assumes the default value of the nominal current.

  BMW 1, 3 and 5-Series and the BMW 5 and 6-Series from 09/2005 - The Servotronic valve is actuated directly by the AL control unit. The signals and messages required for the Servotronic are as follows:

  • Road speed from the DSC control unit via the PT-CAN
  • Status of the engine from the DME control unit via the PT-CAN
  • Terminal status of the CAS control unit via the K-CAN

  The Servotronic valve is actuated only when terminal 15 is ON and the engine is running. When the speed signal is present the default value of the nominal current is taken from the characteristic map. The current supply to the Servotronic valve is interrupted by the faults listed below. Under these circumstances, steering assistance is limited to a minimum:

  • Speed signal from the DSC control unit incorrect or no speed signal
  • Terminal status from CAS control unit via K-CAN incorrect or missing
  • Line fault to the Servotronic valve (exception: short-circuit to positive)

  In the event of a short-circuit to positive, the entire on-board network voltage is applied to the Servotronic valve. This means that the Servotronic valve is fully actuated. Under these circumstances steering assistance is increased to maximum.

• ECO valve in the power-steering pump 

  A vehicle fitted with the "Active Steering" option has a controlled-output power-steering pump.

  The power-steering pump is regulated in accordance with demand by the ECO valve. The ECO valve is integrated into the power-steering pump.

  The power-steering pump is a vane-cell pump. The power-steering pump accommodates the ECO valve (ECO = Electronically Controlled Orifice). The ECO valve regulates the volume flow of the hydraulic fluid, thus regulating the pump's hydraulic power.

  Special features on X5: A modular concept was selected for the power-steering pump on the X5. This modular concept was made necessary (same installation space) by the number of different engines and equipment versions. With option 2VA "Adaptive Drive", for example, a tandem pump is fitted. The radial pistons supply the active roll stabilization (ARS). The vane-cell pump supplies the steering.

  Active Front Steering can generate faster wheel-angle rates (wheels move faster in response to the steering wheel) than a conventional hydraulically power-assisted rack-and-pinion steering systems. The power available in the hydraulic system must be correspondingly higher in order to obtain these faster wheel-angle rates. A conventional vane-cell pump of adequate size and capacity would result in unacceptable power losses. Fuel consumption, too, would be increased to an unnecessary extent. This situation can be alleviated with the aid of a variable-output power-steering pump. The ECO valve in the power-steering pump regulates the volume flow in accordance with demand and reduces the static pressure in the hydraulic system. The ECO valve therefore affects the power consumption of the power-steering pump. The fuel consumption and carbon dioxide emissions (CO2) for the vehicle's engine are reduced.

  When the ECO valve closes, the static pressure in the hydraulic system decreases. The hydraulic fluid delivered by the pump returns through the open pressure-control valve in the bypass to the expansion tank. This means that the power-steering pump does not have to operate against the hydraulic resistance in the hydraulic system (cooler of the power-steering pump, hydraulic hoses). The power-steering pump needs less power.

  When the ECO valve opens, the pressure in the hydraulic system increases. The pressure-control valve closes the return to the expansion tank in the bypass. The pressure limiting valve limits system pressure to 135 bar.

  BMW 5 and 6-Series up to 09/2005 - The safety and gateway module (SGM) actuates the ECO valve in the power-steering pump. In a vehicle fitted with "Active Steering", the AL control unit determines the nominal current for the ECO valve in the power-steering pump. In the event of the AL control unit failing, the SGM assumes the default value of the nominal current.

  BMW 1 and 3-Series, X5 and BMW 5 and 6-Series from 09/2005 - The ECO valve is actuated directly by the AL control unit. Actuation of the ECO valve remains available in the event of the Active Steering failing. A fail safe function via another control unit is not necessary.

  The signals and messages required for control of the power-steering pump are as follows:

  • Status of the engine from the DME control unit via the PT-CAN
  • Terminal status of the CAS control unit via the K-CAN
  • Wheel speeds
  • Steering-angle rate
  • Status of the rate-of-yaw control system

  The ECO valve in the power-steering pump is not actuated unless terminal 15 is On and the engine is running.

• Warning light and Check Control 

  The dedicated-function warning light in the instrument cluster lights up to indicate a fault in the Active Steering.

  At the same time a Check-Control message is shown in the LCD display. The text for the Check-Control message can be called up in the Central Information Display (CID).

  The following displays are present on the instrument cluster for the Active Steering:

  • Dedicated-function warning and indicator light (required by law)
  • LCD display showing the symbol for the Check Control

  The driver receives indication of faults in the Active Front Steering system by means of the following:

  • Via the dedicated-function warning and indicator light
  • Check-Control message via symbol (yellow)
  • Text of the Check Control message in the Central Information Display (with additional informative text)

The steering-column adjustment consists of the following components:

• Switch for steering-column adjustment 

  This switch enables the driver to adjust the tilt and reach setting of the steering column. The switch for steering-column adjustment is on the left-hand side of the steering column.

• Steering-column adjustment drive unit 

  Two motors are built into the drive unit. 1 motor drives the tilt and reach adjustment of the steering column. The second motor changes the direction of adjustment via the adjustment gears.

  The adjustment movement and position recognition for both tilt and reach adjustment are recorded in the drive unit for the steering-column memory. There are 2 Hall sensors and 2 magnetic rings (16-pin) integrated in the drive unit.

• Steering column switch cluster 

  The switch for steering-column adjustment sends its signal to the steering column switch cluster (SZL). The steering column switch cluster forwards this signal to the center console switch cluster (SZM).

  • >up to 09/2005

  Signal path: Switch for steering-column adjustment -> Steering column switch cluster (SZL) -> byteflight  - > Safety and gateway module (SGM) -> K-CAN -> Center console switch cluster (SZM).

  • >from 09/2005

  Signal path: Switch for steering-column adjustment -> Steering column switch cluster (SZL) -> Body gateway module (KGM) -> K-CAN -> Center console switch cluster (SZM).

• Center console switch cluster 

  The center console switch cluster (SZM) actuates the steering-column adjustment. The center console switch cluster also stores the positions for the steering-column memory.

• Door contact in the driver's door 

  The center console switch cluster needs the signal from the door contact. The signal for easy entry and exit is used in conjunction with the terminal status.

  • >up to 09/2005

  Signal path: Driver's door contact switch -> Driver's door module (TMFA) -> byteflight  -> Safety and gateway module (SGM) -> K-CAN -> Center console switch cluster (SZM).

  • > from 09/2005

  Signal path: Driver's door contact switch-> Body gateway module (KGM) -> K-CAN -> Center console switch cluster (SZM).