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Adaptive Brake (ABR), Function - GF42.47-P-0001KJ

MODEL 453.0/3/4 

Block diagram 

G12096870Courtesy of MERCEDES-BENZ USA

Function requirements, general 

ADAPTIVE BRAKE, general 

ADAPTIVE BRAKE assists the driver in dangerous situations which occur suddenly by means of targeted braking, and thus serves active safety. After switching on the ignition, the system runs a self-test, in which the Electronic Stability Program warning lamp and the anti-lock braking system indicator lamp can be actuated through the instrument cluster. In model 453.091/391/491 the brake system vacuum supply indicator lamp is also actuated during the self-test. This goes off again if the self-test is completed successfully.

To record the driving situation at any given moment, the Electronic Stability Program control unit evaluates the data from the following components:

The system is made up of the following subfunctions:

Function sequence for ESP® 

ESP® prevents the vehicle from breaking away when oversteering or understeering. It ensures that the vehicle does not deviate from the course specified by the driver (within physical limits).

Brake forces are produced selectively at the individual wheels to correct any deviations.

Furthermore, reduction of the drive torque takes place in order to increase driving stability.

The Electronic Stability Program control unit processes the following measured quantities to determine the vehicle behavior:

Differentiation is made between the following intervention types:

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 understeers, the maximum possible lateral force at the front axle has been exceeded. In other words, the vehicle pushes via the front axle to the outer edge of the curve.

The resulting instability is counteracted by means of a reduction in drive torque and the following brake interventions at up to 3 wheels:

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 Electronic Stability Program control unit sends a signal over the interior CAN (CAN-B) to reduce drive torque to the ME-SFI [ME] control unit (except model 453.091/391/491) or to the electric drive control unit (model 453.091/391/491), which then reduces the engine output accordingly.

On vehicles with code 429 (twinamic) any pending shift operation is suppressed during the control intervention.

To do so, the Electronic Stability Program control unit sends a signal over the interior CAN to the ME-SFI [ME] control unit, which forwards the signal over the drive train CAN to the dual clutch transmission control unit. The dual clutch transmission control unit then suppresses the shift operation.

EBD function sequence 

The EBD [EBV] function provides assistance when the driver applies medium force to the brake pedal.

EBD [EBV] prevents overbraking of the rear axle and increases vehicle stability when braking in a curve by reducing the pressure at the rear wheel on the inside of the curve or increasing it at the front wheel on the outside of the curve as required. The sideslip angle (angle between vehicle longitudinal axis and direction of movement of the vehicle's center of gravity) is calculated using the yaw rate (speed of vehicle rotation about vertical axis).

The yaw rate, the lateral acceleration and the turning angle of the front wheels (calculated from the steering wheel angle) can be used to determine the lateral forces on the wheels. The longitudinal forces at the wheels are determined using the engine torque, transmission stage (with code 429 (twinamic)) and the brake pressure at each wheel.

If the measured yaw rate does not match the specified value or if the determined side-slip angle is too large, the Electronic Stability Program control unit generates a signal for brake force buildup or reduction for the relevant wheel. The resulting forces stabilize the vehicle.

ABS function sequence 

The ABS prevents the wheels from blocking when braking and as a result maintains the steerability and driving stability during vehicle deceleration. If a locking wheel is detected by the Electronic Stability Program control unit on the basis of the signals from the rpm sensors, the brake pressure is reduced at the appropriate wheel until the wheel begins to turn again.

Function sequence for ASR, ETS 

ASR and ETS prevent the drive wheels from spinning when driving. ASR and ETS also serve to provide improved directional stability and road adhesion for increased traction potential over the entire vehicle speed range. Spinning of the drive wheels is detected by the Electronic Stability Program control unit using the signals from the rpm sensors. Wheel spinning is countered by reduction of the drive torque.

To do so, the Electronic Stability Program control unit sends a signal over the interior CAN to reduce drive torque to the ME-SFI [ME] control unit (except model 453.091/391/491) or to the electric drive control unit (model 453.091/391/491), which then reduces the engine output accordingly. A check is continuously performed to establish whether the drive torque specified by the driver via the accelerator pedal sensor can be permitted again, e.g. due to an improvement in road surface adhesion. The drive torque is transmitted to the opposite, stable drive wheel by means of intervention by the brake system on the spinning wheel.

BAS function sequence 

The BAS detects emergency braking situations based on rapid actuation of the brake pedal and, if necessary, increases the brake pressure in order to achieve maximum deceleration. To this end, the Electronic Stability Program control unit evaluates the degree of pressure rise in the brake system and the data from the brake pressure sensor for the pedal speed, whereupon it then initiates emergency braking when a specific activation threshold is exceeded. On vehicles with code 258 (Active Brake Assist) the activation threshold for the BAS is lowered, if the distance warning function controller unit with active brake application also detects a collision-critical situation. Therefore, in a collision-critical situation the driver is also supported by the system, if he fails to brake strongly or fast enough.

Function sequence for autonomous brake application (vehicles with code 258 (Active Brake Assist)) 

If the driver does not respond to a collision-critical warning issued by the Brake Assist (distance warning function with active brake application), the distance warning function controller unit with active brake application initiates an autonomous brake application through the Electronic Stability Program control unit and a reduction of the engine torque. The Electronic Stability Program control unit calculates the necessary braking torque (average deceleration) taking the following variables into consideration:

The distance warning function controller unit with active brake application sends the Active Brake Assist status (system status), stationary or vehicle ahead, relative speed in m/s as well as the distance to the vehicle driving ahead or stationary objects and a request to introduce the braking torque in diverse CAN messages over the front- end CAN to the center SAM control unit. The center SAM control unit then sends these CAN messages over the interior CAN to introduce the braking torque to the Electronic Stability Program control unit. The Electronic Stability Program control unit evaluates the incoming requests and checks them in terms of other controlled variables and the driving dynamics circumstances. After incorporating all variables, it then actuates the traction system hydraulic unit directly and sends the request to reduce engine torque over the interior CAN to the electric drive control unit (model 453.091/391/491) or to the ME-SFI [ME] control unit (except model 453.091/391/491).

The Active Brake Assist for model 453 can respond in a vehicle speed range of v ≥ 30 km/h to v 140 km/h to moving obstacles or obstacles in front. The Brake Assist does not respond to stationary objects.

IMPORTANT If a collision is unavoidable, occupant protection measures are taken in addition to intervention by the brake system. Detailed information on this is available in the separate function description for the "Supplemental restraint system function". Detailed information on autonomous brake application is available in the separate function description for the BAS brake application function.

Hill Start Assist function sequence 

When starting off, the Hill-Start Assist prevents the vehicle from rolling back opposite the travel direction of the engaged gear range during the time it takes for the driver to change from the brake pedal to the accelerator pedal.

The function is released automatically if, when the vehicle is stationary, an uphill or downhill gradient which would cause the vehicle to roll opposite the travel direction of the engaged gear range is detected by the Electronic Stability Program control unit. The center SAM control unit reads in the signal of the brake light switch directly and by doing so detects the status of the brake pedal, which it then sends over the interior CAN.

The instrument cluster reads in the signal of the parking brake indicator switch and sends the status of the parking brake over the interior CAN to the Electronic Stability Program control unit.

The brake pressure applied by the driver is maintained in the brake system by the traction system hydraulic unit.

Following release of the brake pedal and detection of a start-off request, reduction of the brake pressure is controlled by means of torque balancing system. This system takes into consideration the incline output torque, the total braking torque and the available drive torque.

When the start-off torque is high enough, the Hill-Start Assist is deactivated and the vehicle starts off without rolling in the undesired direction of travel.

If the driver does not depress the brake pedal, the pressure in the brake system is reduced after t = 1 s.

The Hill-Start Assist is terminated immediately if:

The Hill-Start Assist does not function if:

Function sequence for precharging brake cylinders according to accelerator pedal actuation 

Upon detection of a possible emergency braking situation, the brake pads are brought into contact with the brake disks. The clearance between the brake lining and brake disk is thus compensated before the driver operates the brake. The response time of the brake is improved and a shorter braking distance is achieved.

Emergency braking is detected on the basis of the release gradient of the accelerator pedal. If a rapid release of the accelerator pedal is detected, the system assumes that the driver intends to brake. The function is activated and a brake pressure of p = 2 to 3 bar is requested.

IMPORTANT In order to adapt the calculated triggering threshold of the release gradient to the driver behavior, a learning algorithm is implemented that places the driver in the category "normal driver" or "hectic driver" on the basis of the average rate at which the accelerator pedal is released. Function sequence for precharging in collision-critical warning (vehicles with code 258 (Active Brake Assist)) 

On vehicles with code 258 (Active Brake Assist) when a collision-critical warning is issued, the Electronic Stability Program control unit slightly pressurizes (precharging) the hydraulic system of the brake system, without decelerating the vehicle while doing so. If this is then followed by an autonomous brake application the vehicle may be braked at an earlier point. The distance warning function controller unit with active brake application then sends the corresponding information over the front-end CAN to the center SAM control unit. The center SAM control unit then sends these CAN messages over the interior CAN to the Electronic Stability Program control unit.

Steer Assist function sequence 

Steer Assist is intended to help the driver achieve the optimal steering characteristics in critical situations. Steering assistance is provided with the aid of the electric power steering control unit (N68) and the Electronic Stability Program control unit.

The following functions are performed here:

The current vehicle status is sensed by the Electronic Stability Program control unit. If required, the "Steering assistance request" message is sent over the interior CAN to the electrical power steering control unit (N68), which then actuates the electric power steering actuator motor (A91m1) accordingly.

Function sequence for Crosswind Assist 

Additional function requirements for Crosswind Assist:

The Crosswind Assist function assists the driver in the event of strong crosswinds and minimizes the vehicle's yaw response and the track offset. The Electronic Stability Program control unit detects the movement of the vehicle when it encounters strong gusts of wind. The Electronic Stability Program control unit also processes the steering wheel angle from the steering wheel angle sensor, the steering rack position from the electrical power steering control unit, the wheel speeds and the brake pressures via the chassis CAN.

System fault display function sequence 

The driver is informed about the system status and about faults by the following displays:

The Electronic Stability Program control unit then sends the system status to the instrument cluster over the interior CAN. If the Electronic Stability Program control unit fails or if electronic faults occur, basic braking without ABS is always available.

IMPORTANT The status of the brake fluid level switch and the parking brake indicator switch is read in and evaluated directly by the instrument cluster.

Function sequence for vacuum pump closed-loop control (model 453.091/391/491) 

As electric vehicles do not have a manifold vacuum for the brake booster, the vacuum has to be generated through the electric vacuum pump. The vacuum pump is regulated by the brake booster vacuum pump control unit. The vacuum in the brake booster is then evaluated through the brake booster vacuum sensor. The vacuum sensor is mounted between the brake booster and the vacuum line with integrated check valve. The vacuum line for the vacuum sensor and the check valve form a unit. The vacuum sensor itself consists of two redundant vacuum sensors, to ensure that in the event of a faulty sensor signal a switch can be made to the other sensor signal. The brake booster vacuum pump control unit is connected over the electric drive LIN to the electric drive control unit. In the event of system error (e.g. vacuum too low) the electric drive control unit sends the fault message over the interior CAN to the instrument cluster. This then actuates the brake system vacuum control indicator lamp. For severe faults, the brake fluid and parking brake warning lamp is then actuated. Similarly, for diagnostic purposes the vacuum pump can be actuated through the electric drive control unit and the brake booster vacuum pump control unit.

  Electrical function schematic for Adaptive Brake (ABR) Model 453.0 (except 453.091), 453.3 (except 453.391), 453.4 (except 453.491) PE42.47-P-2050-97BBA 
Model 453.091/391/491 PE42.47-P-2050-97BEA 
  Overview of system components, Adaptive Brake (ABR)   GF42.47-P-9997KJ 
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