BMW 7 Series: DSC Functions In Detail

ADB Active Even When DSC is Off

The DSC function ADB has been around for a long while on a wide variety of BMW vehicles and especially in the form of the ADB-X version on the xDrive models.

If one of the wheels of a driven axle is spinning, it cannot transmit any driving force (torque) to the road. And because the differential distributes the torque equally between the two wheels, the other wheel on the axle can not transmit any driving force either.

ADB brakes the spinning wheel so that the driving torque and braking force are in equilibrium on that wheel.

Then, by virtue of the differential, an equal amount of driving torque is applied to the wheel that is not spinning.

And because that wheel is offered grip by the road surface, a driving force can be transmitted that results in forward motion of the vehicle.

Thus ADB increases traction on slippery surfaces and has a similar effect to a differential lock.

• On the xDrive models, ADB-X remains active even when the DSC is switch off in order to achieve improved traction, especially when driving off road.
• On vehicles with rear-wheel drive , that individual braking of the driving wheels was previously only active when the DSC was active or in "DTC" mode.
• On the new F01/F02 (and the top 1 Series and 3 Series models) that ADB subfunction remains active even when the DSC is switched off.

That function is called "Electronic Differential Lock Control" and uses the DSC to emulate the effect of a differential lock on the driving wheels. In contrast with previous arrangements, such brake modulation for the purposes of increasing drive transmission takes places even if the driver has switched off the DSC.

Brake Modulation for Increasing Agility

The highly advanced hardware of the DSC F0x enables brake modulation to be carried out without unpleasant feedback for the driver. On the F01/F02, that capability is utilized to influence the vehicle's self-steering characteristics is a specifically targeted way.

Fig. 57: View Of Brake Modulation For Increasing Agility

INDEX REFERENCE CHART

1. Course of an understeering vehicle
2. Course of a vehicle with neutral handling
3. Individual modulation of brakes to prevent understeer
4. Increased engine torque to compensate for braking force

13. Yaw force acting on the vehicle as a result of individual modulation of brakes

If the vehicle begins to understeer, e. g. when cornering quickly, the central dynamic handling controller on the ICM detects the very first indications that it is starting to happen. A required setting is transmitted to the DSC requiring it to apply the brake on the rear wheel on the inside of the bend. The DSC sensitively applies the required setting and without generating a level of noise perceptible by the driver. The uneven braking effect thus produced, creates a yaw force acting around the vehicle's center of gravity. As a result, the vehicle turns towards the inside of the bend, doesn't understeer and corners with absolutely neutral handling.

This type of brake modulation increases road safety because it prevents the vehicle drifting towards the outside of the bend. The disadvantage, however, is that the vehicle is slowed slightly by the application of the brake and thus a degree of momentum is lost.

Therefore, in typical BMW fashion, the solution is taken a step further. Whenever handling stability considerations allow, the engine torque is increased simultaneously with brake application. The higher engine torque is transmitted to the road by the wheel on the outside of the bend that is not being braked. The control strategy ensures that the increase exactly matches the retardation by the brake application.

While that DSC function is active, there is no display of any kind on the instrument cluster.

In that way, highly advanced components (DSC) and intelligent control strategies are combined to produce an overall effect that substantially improves agility without impairing the handling stability of the vehicle.

Automatic Hold

This function has been around since the E65, on which it was called "Auto-P". The Automatic Hold function was also used on the E70/E71.

Although the Automatic Hold function is computed on the DSC control unit, it can not be put into effect without an electromechanical parking brake (EMF). The EMF is always required whenever the DSC hydraulic modulator is unable to permanently hold the vehicle stationary. In particular, that is the case when the engine is switched off.

When the Automatic Hold function is active, the driver first of all brakes the vehicle to a standstill. It is then held stationary by the DSC hydraulic modulator. That is achieved by maintaining the final brake pressure applied by the driver. If the vehicle starts to roll on an incline, the DSC hydraulic modulator actively generates brake pressure.

Pressing the accelerator causes the brake pressure to be released and the vehicle starts to move again. Automatic holding and releasing of the brakes in that way makes driving in easier in conditions such as urban traffic and stopping at traffic lights or stop-and-go driving in traffic tailbacks.

After the engine is started, the function can be activated until the next time the engine is switched off. To do so, the driver's door must be closed and the driver's seatbelt fastened. The function can, of course, also be manually deactivated before the engine is switched.

The footwell module reads the signal from the door switch. The ACSM control unit analyses the signal from the seat belt buckle contact. The two signals are transmitted to the DSC control unit via the bus systems. One signal that is not analyzed for the Automatic Hold function on the F01/F02 is the driver's seat occupancy signal.

Conversely, the Automatic Hold function is automatically deactivated if the driver's door is opened and the driver's seatbelt unfastened. To prevent the vehicle rolling away in that situation, the EMF parking mode is activated. As long as the engine is running, the parking mode is effected by means of the DSC hydraulic modulator. If the driver switches the engine off, the function is taken over by the EMF actuator unit.

NOTE: Before the vehicle is driven into a car wash, the Automatic Hold function has to be deactivated as otherwise the brakes are applied when the vehicle is stationary and it can not be rolled.

The Automatic Hold function is activated and deactivated by means of the button marked "AUTO H" on the center console. Activation of the function is acknowledged by the function indicator lamp (green LED in the button). Whenever the green LED is lit, the Automatic Hold function is active. The status of the Automatic Hold function is also indicated on the instrument cluster.

The various function statuses and how they are indicated are summarized below.

AUTOMATIC HOLD FUNCTION DESCRIPTION CHART

When Automatic Hold is holding the vehicle stationary, two additional internal DSC subfunctions are activated: roll-away monitoring and slide detection.

The roll-away monitoring function is described in the section "EMF functions".

The slide detection function is designed to intervene if the vehicle starts to slide after stopping, i. e. if all four stationary wheels start to slip. That can happen on a steep slope when the road is slippery, for instance. If the driver were holding the vehicle stationary and became aware of such a situation, he/she would release the brake.

In that way the vehicle can at least be steered as it rolls down the slope.

The slide detection function is based on exactly the same principle. When the vehicle is being held stationary by Automatic Hold, the slide detection function monitors the signals from the wheel-speed sensors. The DSC releases the pressure on one of the brakes in alternation while keeping the others under pressure. If the wheel-speed sensor registers a movement from the wheel on which the brake is released, then obviously the entire vehicle must be moving. That means that the other wheels, on which the brakes are applied, must be sliding while locked. Under those circumstances, the condition "sliding" would be detected.

The response to detection of sliding is progressive release of brake pressure so that the vehicle becomes steerable. The driver is made aware of the critical situation by a Check Control message and an audible warning signal.

Interface for Adaptive Braking Assistance

The function "Adaptive Braking Assistance" implemented as a co-ordinated strategy by the DSC and Adaptive Cruise Control with Stop&Go function is also available on the F01/F02. It is a function that was previously introduced on the E60/E61 LCI.

The interface relates to two functions on the Dynamic Stability Control:

• brake standby and
• dynamic braking control.

"Brake standby" can be activated by a request signal transmitted by the ICM control unit. That happens when a potential collision situation has been detected with the aid of the radar sensors. And, of course, "brake standby" is also activated if the internal DSC criteria familiar from previous models are met (minimum speed, rapid release of accelerator pedal).

The threshold for triggering dynamic braking control can be influenced by ACC Stop&Go. If a potential collision situation is detected, the ICM control unit sends out a signal requesting lowering of the activation threshold. To be precise, the activation threshold is the rate of increase of brake pressure applied by the driver that has to be exceeded in order to dynamic braking control (braking assistance). That makes it easier for the driver to trigger dynamic braking control. This function is the only means by which a driver braking hesitantly can activate dynamic braking control.

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