System functions
The dynamic stability control system (DSC) controls longitudinal and transverse dynamics by means of engine and brake system intervention.
Dynamic Stability Control (DSC) incorporates the following functions.
- Anti-lock brake system (ABS)
- Electronic brake force distribution (EBV)
- Cornering brake control (CBC)
- Automatic stability control (ASC)
- Dynamic traction control (DTC)
- Electronic brake force distribution (MSR)
- Dynamic brake control (DBC)
- Electronically controlled deceleration (ECD only in conjunction with ACC)
- Trailer stabilizing control (only in conjunction with trailer module for E60 and E61)
Active Steering also influences the vehicle's yaw characteristics. Consequently, the software in DSC is modified accordingly in cars fitted with Active Steering (for example, 2 DSC sensors).
Dynamic Stability Control
Dynamic Stability Control (DSC) detects the current status of the vehicle by evaluating the sensor signals.
This status is compared with the nominal values derived from a computational model. In this way, the system recognizes incipient instabilities.
The vehicle is stabilized as soon as a deviation exceeds the control threshold stored in the DSC control unit.
Stabilization (within the limits imposed by the laws of physics) is achieved by reducing engine power and by braking individual wheels.
DSC interventions override the ABS and ASC functions.
The DSC function can be deactivated by means of the DSC button.
Anti-lock braking system
The anti-lock braking system (ABS) stops the wheels locking during braking.
Advantage: Shorter stopping distances, the car retains its directional stability and remains steerable.
Brake pressure is regulated at all wheels to ensure that each wheel runs in the best possible slip range.
When this happens, slip is controlled so that the maximum possible braking and lateral stability forces can be transmitted.
ABS alone is available for braking if a sensor for DSC fails or if a bus fault occurs (PT-CAN or chassis CAN).
ABS is the surviving safety function in circumstances in which DSC control is no longer possible.
Electronic brake force distribution
Electronic brake force distribution (EBV) is a component of the ABS. EBV regulates the brake force distribution between the front and rear axles, depending on vehicle load.
Advantage: Regardless of the load state of the vehicle, the best possible braking distance is achieved while driving stability is maintained.
If ABS fails, the EBV function is sustained for as long as possible.
The signals from at least two wheel speed sensors are needed for the EBV function.
Cornering brake control
Cornering Brake Control (CBC) is an extension of ABS. CBC increases driving stability when the brakes are applied as the car corners ("cornering logic").
Advantage: Optimum driving stability if brakes are partially applied when cornering.
The shift in wheel loads as the car corners (the onset of this phenomenon requires no more than light application of the brakes) can result in a reduction in handling stability. If required, CBC generates a stabilizing load moment when the brakes are applied lightly outside the ABS intervention range.
Automatic stability control
Automatic stability control (ASC) prevents the wheels spinning during acceleration by intervention in brake and engine operations.
Advantage: More traction and better driving stability.
If, for example, one of the wheels of the drive axle is on a high-grip surface and the other is on a slippery surface, the wheel tending to spin is braked. If necessary, the engine's power output is also reduced.
Dynamic traction control
Dynamic traction control (DTC) offers better traction as a trade-off against a reduction in stability in some circumstances. Consequently, its use should be reserved for exceptional conditions (driving in deep snow, for example).
The DTC function approximates to that of DSC with a slightly modified control strategy. DTC can be activated by deactivating DSC (DSC button). DTC intervenes in the braking actions to imitate the function of a conventional differential lock.
Advantage: Higher traction is available with DTC.
Vehicle stabilization intervention (e.g. reduced power output) is made slightly later than with DSC.
This enhances traction with a slight loss of driving stability. Occasionally, a compromise is needed between driving stability and traction.
This is especially true when accelerating and driving uphill on loose surfaces or in deep snow (= friction values demanding increased slip).
DTC allows DSC to provide a high degree of driving stability while retaining sufficient traction.
Engine drag torque control
The engine drag torque control (MSR) counteracts the tendency of the wheels to lock on smooth surfaces.
The engine's drag torque generated by downshifts or abrupt load changes can lock the driven wheels (especially on surfaces with a low coefficient of friction).
The wheel speed sensors tell MSR as soon as the wheels are about to lock. MSR then briefly reduces the engine's drag torque by opening the throttle slightly.
Advantage: The drive wheels retain their lateral stability in overrun mode.
Dynamic brake control
Dynamic brake control (DBC) assists the driver in emergency braking situations by automatically increasing the brake pressure.
Advantage: Shortest possible stopping distances in emergency-braking situations, because the ABS control threshold is reached at all four wheels.
In emergency-braking situations, drivers often fail to apply sufficient force to the brake pedal. ABS regulation is then not activated.
In the following situations, the return pump increases the brake pressure until ABS regulation is activated:
- When the brake pedal is rapidly depressed with insufficient pedal pressure
- When the brake pedal is depressed slowly and the demand for deceleration is subsequently high, after one wheel reaches the ABS control threshold.
Which wheel locks first depends on load and coefficient of friction of the road surface.
Example of a typical situation:
The traffic slows, making light braking necessary at first, but then demands as short a stopping distance as possible.
Electronically controlled deceleration
The electronically controlled deceleration (ECD) reacts to a demand from the Active Cruise Control (ACC).
When ACC requires deceleration, DSC responds by applying the disc brakes on all four wheels (maximum rate of deceleration 2.5 m/s 2 ).
When the car is on a decent with the speed preset by the driver, ECD automatically applies the brakes in order to keep the car's speed constant at the preset value.
When the brakes are applied automatically in this way, the brake lights are activated in accordance with the requirements of road-safety legislation. The light module does not activate the brake lights unless the vehicle's rate of deceleration is greater than 1m/s 2 . This prevents the brake lights from flickering on and off.
Trailer stabilizing control (only in conjunction with trailer module for E60 and E61)
Trailer stabilizing control detects when a trailer is rocking about its vertical axis. The system functions at a speed of about 65 km/h when the trailer socket is in use.
With the aid of the DSC sensors, Dynamic Stability Control (DSC) monitors the vehicle's yaw behavior. If the trailer's rocking motion exceeds the limit, the engine output is reduced. In addition, DSC brakes all four wheels automatically.
If DSC is deactivated or faulty, then the trailer stabilizing control does not work either.
Tyre defect indicator
The Run Flat Indicator (RPA) is not a function of the Dynamic Stability Control system.
RPA is integrated into the DSC control unit as the four wheel speed signals are required for this function.
By comparing the speed signals for all four wheels, the system detects differences in rolling circumference at the individual wheels. This enables the system to recognize a sudden loss of pressure in the tires.