GF42.45-P-0001RC Electronic Stability Program (ESP) Function

GF42.45-P-0001RC Electronic stability program (ESP) function

General function requirements
^ Circuit 61 ON

Electronic Stability Program (ESP) system, general
ESP is an active safety system for improving vehicle stability in all driving situations. The vehicle is stabilized and kept safely on course when cornering, braking or coasting without power by individual brake system interventions at one or more wheels on the front axle and/or rear axle. ESP supplements the functions of the antilock brake system (ABS), acceleration slip regulation (ASR) and engine braking regulation (EBR [MSR]).

The system is made up of the following component functions:
^ ESP function sequence
^ ABS function sequence
^ Brake Assist (BAS) function sequence
^ ASR function sequence
^ EBR [MSR] function sequence
^ Electronic brake force distribution (EBD) function sequence
^ Passive ASR and ESP switching function sequence
^ System fault display function sequence

The master control unit for all of the specified functions is the ESP and BAS control unit (N47-5).

Function sequence, ESP
ESP prevents breakaway when the vehicle oversteers or understeers. Within physical limits it ensures that the vehicle does not deviate from the course specified by the driver. The ESP and BAS control unit evaluates the signals from the following components to determine the current driving situation:

- Left front rpm sensor (L6/1)
- Right front rpm sensor (L6/2)
- Left rear rpm sensor (L6/3)
- Right rear rpm sensor (L6/4)
- BAS diaphragm travel sensor (A7/7b1)
- BAS release switch (A7/7s1)
- ESP pressure sensor 1 (B34/1)
- Yaw rate, lateral and longitudinal acceleration sensor (B24/15)
- Steering angle sensor (N49)

Brake forces are produced selectively at the individual wheels to correct the vehicle behavior.

Furthermore, reduction of the drive torque takes place in order to increase directional stability and road adhesion. A distinction is made between under steering and over steering vehicle behavior. The ESP and BAS control unit detects the vehicle behavior using the data from the steering angle sensor and yaw rate, lateral and longitudinal acceleration sensor.

Intervention in the case of oversteer:

If the vehicle begins to oversteer, brake pressure is built up at the outer front wheel. The resulting reduction in lateral force at the outer front wheel generates a yawing moment which counteracts the tendency of the vehicle to rotate inward. The vehicle speed decreases as a result of the brake force at the front wheel, which also enhances stability.

Intervention in the case of understeer:

If the vehicle starts to understeer, the maximum possible lateral force at the front axle has been exceeded.

This means that the vehicle pushes itself over the front axle and toward the outer edge of the corner. If at this point the driver depresses the accelerator pedal, the drive torque is first of all reduced. If this is not enough or the accelerator pedal is not depressed, the incipient instability is prevented by brake application at up to three wheels:

Stage 1: Brake pressure reduction at inner rear wheel

Stage 2: Stage 1 plus brake pressure buildup at outer rear wheel

Stage 3: Stage 2 plus brake pressure buildup at inner front wheel

Depending on the brake force, a torque is generated which causes the vehicle to rotate inward with a simultaneous reduction in speed. This has a considerable stabilizing effect.

The ESP and BAS control unit transmits via engine compartment CAN a signal for drive torque reduction to the ME-SFI [ME] control unit (N3/10) which reduces the engine output accordingly.

A pending shift operation is suppressed for the duration of control intervention. To achieve this, the ESP and BAS control unit transmits a signal via engine compartment CAN to the ETC control unit (N15/3) (with transmission 722.6) or to the fully integrated transmission control unit (VGS) (Y3/8n4) (with transmission 722.9). The ETC control unit or fully integrated transmission control unit suppresses the shift operation.

ABS function sequence
ABS prevents the wheels locking when braking and thus maintains the steerability and directional stability and road adhesion during vehicle deceleration. If a locking wheel is detected by the ESP and BAS control unit on the basis of the signals from the rpm sensors, the brake pressure is reduced at the appropriate brake cylinder until the wheel begins to turn again.

BAS function sequence
BAS detects emergency braking situations from rapid actuation of the brake pedal and, if necessary, increases the brake pressure in order to achieve maximum possible deceleration. The ESP and BAS control unit evaluates the increase in pressure in the brake system and initiates an emergency stop if a certain triggering threshold is exceeded.

Function of ASR
ASR prevents the drive wheels from spinning while driving. In addition, it improves driving stability with an increased traction potential over the entire vehicle speed range. The ESP and BAS control unit records the spinning of the drive wheels via the signals from the rpm sensors. Wheel spinning is countered by reduction of the drive torque. The ESP and BAS control unit transmits via engine compartment CAN a signal for drive torque reduction to the ME-SFI [ME] control unit, which reduces the engine output accordingly.

The system constantly checks whether the drive torque specified by the driver via the accelerator pedal sensor (B37) can be allowed again e.g. due to improved road adhesion. The drive torque is transmitted to the opposite, stable drive wheel by means of intervention by the brake system on the spinning wheel.

EBR [MSR] function sequence
EBR [MSR] reduces brake slip at the drive wheels in deceleration mode and improves driving stability.

If slip occurs at the drive wheels when the throttle is released, this is detected by the ESP and BAS control unit. The ESP and BAS control unit transmits a corresponding signal to increase the engine torque to the ME-SFI [ME] control unit via engine compartment CAN. Wheel slip at the drive wheels is reduced by increasing drive torque and as a result the lateral stability of the vehicle is improved. This process takes place without informing the driver.

EBD function
EBD allows the vehicle brake system to be configured according to requirements. The stability of the rear wheels is monitored and they are used more heavily overall to brake the vehicle. This allows greater potential brake force at the front wheels, which can be used by vehicles with heavier front axle loads. If the rear wheel/front wheel slip ratio exceeds a specified limit value during a brake application, the ESP and BAS control unit prevents any further increase in brake pressure at the rear wheel.

Passive ASR and ESP switching function sequence
The ASR and ESP functions can be switched passive via the ESP OFF switch (N72/1s1) in the upper control panel control unit (N72/1). The upper control panel control unit transmits the status of the ESP OFF switch via interior CAN to the central gateway control unit (N93), which forwards the information to the ESP and BAS control unit via engine compartment CAN.

If the system is switched passive, the ESP warning lamp (A1e41) lights up in the instrument cluster (A1).

When the system is switched passive, the control thresholds are raised and EBR [MSR] is switched off.

ABS cannot be deactivated.

ESP is always active during a brake application.

System fault display function
The driver is informed of the system status and any faults by means of the following:

- ABS indicator lamp (A1e17)
- ESP warning lamp
- Message via upper multifunction display (A1p13) in instrument cluster

In the event of failure of the ESP and BAS control unit, a basic brake function without ABS is always available.