ESP braking torque control circuit, function

Normal operation

This is the case when the driving state requires no control mode.

Antilock brake system (ABS) closed-loop operation

From the wheel speed signals the ESP control unit (N30/4) detects that the criteria for ABS control exist. The respective solenoid valves are actuated accordingly on an individual basis.

Acceleration slip regulation (ASR) closed-loop operation

As soon as a drive wheel tends to spin, the ESP control unit (N30/4) actuates the corresponding rear axle solenoid valves.

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Electronic Stability Program (Electronic-Stability-Program) (ESP) closed-loop operation

In the ESP control unit (N30/4) the signals of the wheel speeds, the steering angle, the yaw rate of the vehicle, the lateral acceleration as well as the brake pressures are detected and processed by a drive status regulator. After the evaluation of input signals, the required solenoid valves are actuated in order to regulate the brake pressure in the individual wheel brakes.

Brake Assist (BAS) operation

From the pressure buildup speed, the ESP control unit (N30/4) detects that an emergency stop situation exists and initiates
BAS mode.

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Electronic brake force distribution (EBV) operation

EBV determines the maximum permissible brake pressure for both rear wheel brakes based on the rpm difference between the front and rear axle wheels. Based on the vehicle load, EBV regulates the brake pressure at the rear axle so that overbraking is prevented.

EBV replaces the original pressure/load-dependent rear axle regulator which distributed the brake force between the axles as per predetermined, fixed characteristics.

Control mode with ESP deactivated

ESP drive torque control is switched off up to a speed of 60 km/h by actuating the "ASR OFF" switch (N72/1s5) in the upper control panel control unit (N72/1).

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Start-off assist (AAS)

AAS prevents the vehicle from rolling away unintentionally when starting off (against the desired direction of travel) on inclines.

The brake pressure specified by the driver during the stopping procedure is maintained in the brake system when vehicle standstill is detected.

The brake must be actuated for the function to be activated or maintained. If the brake is released (stop lamp switch no longer actuated), the brake pressure is released after a pressure hold time of t = 2 s. The driver has enough time to actuate the accelerator pedal and start off during this period.

If the brake is released and the brake pedal is also not actuated within the specified time, the function is canceled and the vehicle can roll away.

If the vehicle threatens to roll away when AAS is active, the driver can increase the brake pressure at any time.

The AAS function remains active regardless of the length of time the stop lamp switch is actuated.

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AAS activation

AAS is activated when the vehicle speed < minimum speed for a specified time interval (standstill detection). The following operating conditions are also required at the same time:

 Stop lamp switch activated (brake depressed)

 Engaged gear and road incline that exceeds a specified threshold value

 Parking brake not operated

Maintaining AAS activation

AAS remains active when the brake is actuated (stop lamp switch activated) and when the following operating conditions exist:

 Engaged gear (function aborted in "N" position)

 Parking brake not actuated and

brake pressure greater than a specified minimum pressure

AAS also remains active under the following operating conditions:

 Brake not actuated (stop lamp switch deactivated) and accelerator pedal actuated (pedal value > 4 %)

AAS deactivation

AAS is deactivated when the following operating conditions are simultaneously present:

 Brake not actuated (stop lamp switch deactivated) and time interval of t = 2 s exceeded

 Brake not actuated (stop lamp switch deactivated) and accelerator pedal actuated and engine torque greater than or equal to the required starting torque

 Brake not actuated (stop lamp switch deactivated) and parking brake actuated

 Brake not actuated (stop lamp switch deactivated) and no gear engaged

 Brake not actuated (stop lamp switch deactivated) and engine off

 Brake not actuated (stop lamp switch deactivated) and vehicle speed > minimum speed

 A reverse gear switch and neutral position switch are installed in the transmission to detect the gear. If neither of the two switches are activated, the ESP control unit (N30/4) detects an engaged forward gear.