BMW 7 Series: Active Roll Stabilization (ARS)

General Information

Active Roll Stabilization was first fitted on the 7 Series predecessor, the E65/E66, and has been used in similar form since on the E6x and E7x models.

This section only describes the essential details and modifications of the ARS on the F01/F02.

As Vertical Dynamics Control (VDC) is fitted as standard on the F01/F02, the ARS optional extra (now also available in the US markets) is always offered as a combination.

In customer communications, ARS continues to be marketed under the name "Dynamic Drive" on the F01/F02.

System Dynamics

VDC and ARS have to respond with the appropriate speed in the event of rapid lane changes, rapid cornering or rapid changes of direction on winding country roads.

The ARS systems on previous model series had a separate control unit with the appropriate output stages for controlling the ARS valve manifold. The system architecture on the F01/F02 now features two different vertical dynamics management (VDM) control units:

• If ARS is not fitted, the VDM has no output stages for ARS valve manifold control
• If ARS is fitted, the VDM has output stages for ARS valve manifold control

The system dynamics of ARS and VDC are determined by the duration of the following stages:

SYSTEM DYNAMICS OF ARS AND VDC

Operating States

Straight-ahead travel

When the engine is started, the pump delivers hydraulic fluid to the system and a back pressure builds up. The pressure difference of approximately 1 bar which exists between the chambers of the control motor is very small and has no effect on the anti-roll bar.

The pressure valves for the front-axle anti-roll bar (PVV) and rear-axle anti-roll bar (PVH) are not supplied with current and are therefore open. The hydraulic fluid can flow back into the fluid reservoir directly. The pump's new intake restrictor valve is energized so that the circulation pressure can be substantially lowered when driving in a straight line in order to reduce CO2 emission. This condition remains unchanged as long as the vehicle is travelling straight ahead. The system function is displayed continuously up to 10 mph. The full stabilization capacity is available from 10 mph upwards.

Cornering

As the vehicle enters a bend, the signals from the lateral acceleration sensor in the ICM are sent to the VDM control unit. The control unit then sends a pulse-width modulated (PWM) signal to the pressure valves for the front and rear-suspension hydraulic motors and simultaneously stops restriction of pump intake by switching off the power to the intake restrictor valve. The greater the lateral acceleration, the higher is the PWM signal current for the pressure valves. The stronger the current supplied to the valve, the more the valve closes and the higher the pressure which builds up in the anti-roll bars.

The pressures at the anti-roll bars are detected by pressure sensors (10, 11) and signalled to the control unit. The direction valve (9) is activated by the control unit to create a pressure build-up which corresponds to the progression of the bend (left or right-hand bend). A sensor (8) detects the switching position of the direction valve.

Fig. 110: Hydraulic Circuit Diagram, Normal Function - Failsafe Valve Supplied With Current

INDEX REFERENCE CHART

1. Front hydraulic motor (SMV)
2. Rear hydraulic motor (SMH)
3. Front-suspension hydraulic circuit 1 (V1)
4. Front-suspension hydraulic circuit 2 (V2)
5. Rear-suspension hydraulic circuit 1 (V1)
6. Rear-suspension hydraulic circuit 1 (V1)
7. Failsafe valve (FS)
8. Switch-position detector (SSE)
9. Directional control valve (RV)
10. Rear-suspension pressure sensor (DSH)
11. Front-suspension pressure sensor (DSV)
12. Front-suspension pressure valve (PVV)
13. Rear-suspension pressure valve (PVH)
14. Tandem pump (P)
15. Fluid reservoir (HB)
16. Fluid level sensor

Safety Concept

General information

The safety concept prevents malfunctioning of the system by monitoring signals and responds in a defined manner to faults caused by external problems on interfacing units or systems. System monitoring essentially comprises of the following monitoring functions:

• Monitoring of the power supply voltage
• Monitoring of the electrical circuits for the valves and sensors within the system
• Monitoring of FlexRay bus communication and checking signal plausibility
• Monitoring hydraulic function when the vehicle is in motion and predrive test

If a fault is detected, a defined response is initiated according to the significance of the fault (function impairment). The VDM control unit records the fault in the fault memory and displays the response on the instrument cluster.

Limited function (fall-back level)

If a system fault has been detected which allows continued operation of the system with limited functionality, this is indicated together with a warning message

Fig. 111: Identifying ARS Indicator Lamp (Yellow)

Serious fault (fall-back level)

If a serious fault is detected, the ARS system is set to "failsafe mode" and this is indicated on the instrument cluster. A CC message advises the driver to take corners more slowly.

Fig. 112: Identifying ARS Indicator Lamp (Yellow)

If the failsafe condition is activated, the failsafe valve is closed by means of a spring. The hydraulic fluid in the front anti-roll bar is sealed in, thereby ensuring an adequate anti-roll effect and an understeer effect equivalent to that of a conventional suspension and steering system. The failsafe situation is shown by the hydraulic circuit diagram below.

Fig. 113: Failsafe Function Or Neutral Position Hydraulic Circuit Diagram

INDEX REFERENCE CHART

1. Front hydraulic motor (SMV)
2. Rear hydraulic motor (SMH)
3. Front-suspension hydraulic circuit 1 (V1)
4. Front-suspension hydraulic circuit 2 (V2)
5. Rear-suspension hydraulic circuit 1 (V1)
6. Rear-suspension hydraulic circuit 1 (V1)
7. Failsafe valve (FS)
8. Switch-position detector (SSE)
9. Directional control valve (RV)
10. Rear-suspension pressure sensor (DSH)
11. Front-suspension pressure sensor (DSV)
12. Front-suspension pressure valve (PVV)
13. Rear-suspension pressure valve (PVH)
14. Tandem pump (P)
15. Fluid reservoir (HB)
16. Fluid level sensor

Fluid loss due to external leakage

The hydraulic circuits of the ARS system and the steering system are linked to one another by virtue of a shared fluid reservoir. The VDM control unit monitors the fluid level in the reservoir by means of a fluid level sensor.

Loss of fluid due to external leakage in the hydraulic circuits of the ARS or steering systems leads to a drop in the fluid level in the shared fluid reservoir. Fluid loss can result in total failure of the ARS system and impairment of the steering system. If the fluid level sensor trips, the ARS system is set to failsafe mode and a fault is registered on the VDM control unit.

Fig. 114: Identifying ARS Indicator Lamp (Red)

Simultaneously, a CC message is issued warning of impairment of ARS system and steering system function.

The driver is instructed to carefully bring the vehicle to a halt and switch off the engine.

Initialization/reset performance

When the VDM control unit is booted up, various checks and initialization routines are executed. They include checks of the electrical circuits for the valves and the sensors within the system, an authentication check involving querying the VIN number on the CAS and testing FlexRay communication. The system is not enabled until the tests have been successfully completed. Occurring faults are stored and displayed.

Predrive test

Every time the engine is started or the vehicle is stopped, an automatic quick test of the hydraulic function of the failsafe valve and front-suspension pressure regulating valve is carried out which lasts only 450 ms and is imperceptible to the driver. That test is only started when the engine is running and the vehicle is stationary, provided no other fault is present. If the predrive test identifies a fault, the appropriate fault response is initiated.

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