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Active Brake Assist, Function - GF30.30-P-0020KJ

MODEL 453.0/3/4 

with CODE 258 (Active Brake Assist System) 

G12094828Courtesy of MERCEDES-BENZ USA

Function requirements, general 

IMPORTANT For vehicles with a internal combustion engine, the ME-SFI [ME] control unit transmits the signal for the engine rpm via interior CAN to the center SAM control unit and from there via the front end CAN to the AWF controller unit with active brake intervention. From this, the AWF controller unit with active brake intervention recognizes that the engine is running. For vehicles with an electric drive (only model 453.091/391/491), the EDCU transmits the message "Drive train operational" via interior CAN to the center SAM control unit and from there via the front end CAN to the AWF controller unit with active brake intervention.

Active Brake Assist (AWF with active brake intervention), general 

The Active Brake Assist warns the driver optically and acoustically about a possible collision (e.g. a collision accident) with an obstacle or with the vehicle immediately ahead. To do this, the AWF controller unit with active brake intervention evaluates the traffic situation in front of the vehicle, the route immediately in front as well as the driver activity and constantly determines the risk potential of a possible collision. Output of appropriate warnings takes place in a vehicle speed range of v = 30 km/h (approx. 19 mph) to approx. v ≈ 140 km/h) (approx. 87 mph). A possible collision can be prevented or at least its consequences can be mitigated. If the driver reacts to the acoustic warning and brakes, he will be assisted in this by the adaptive Brake Assist. This calculates the required brake force amplification according to the situation.

Autonomous partial braking is also initiated if there is no reaction from the driver to the warnings. In this way a possible collision can be prevented or at least its consequences can be mitigated. The center SAM control unit is the central data interface in the AWF system and routes the data between the AWF controller unit with active brake intervention, the ME-SFI [ME] control unit (except model 453.091/391/491) or the EDCU (only model 453.091/391/491) and the instrument cluster for issuing a signal to the driver. It also receives data from the Electronic Stability Program control unit for information collection and sends corresponding data to the Electronic Stability Program control unit for the introduction of an autonomous braking. Autonomous braking is only introduced in the event of a possible collision with dynamic (moving) objects. A stationary object must thus have already been recognized as a dynamic one. For purely static objects, there is no autonomous braking. Environmental influences or sources of error outside the system may temporarily interrupt the Active Brake Assistant. Possible causes are:

The following representation shows the detection range of the AWF controller unit with active brake intervention.

IMPORTANT More detailed information on the function sequence for braking assistance and autonomous braking is provided in the separate subfunction "BAS PLUS function braking".

Schematic diagram of radar detection range 

G12094829Courtesy of MERCEDES-BENZ USA

Active Brake Assist warning and indicator lamps (shown with instrument cluster with code K35 (IC with monochrome display) on vehicle with engine 281) 

G12094830Courtesy of MERCEDES-BENZ USA

The Active Brake Assist function includes the following subfunctions: 

Self-test function sequence 

After switching on the ignition (circuit 15 ON), the Active Brake Assist performs a self-test. During this self-test the assembly parts of the system are checked for proper operation and the radar sensor for soiling. The distance warning function indicator lamp (A1e23) and the AWF warning lamp are actuated for the self-test. After successful checking of the system, these go out again and the system is operational. If the system detects an error, the distance warning function indicator lamp is actuated. For vehicles with code K36 (IC with a colored 8.9 cm (3.5") display), the fault message "Active Brake Assist nonfunctional" is issued for temporary faults or the fault message "Fault - Electronics Service Required" for other malfunctions in the multifunction display of the IC to the driver. In addition to the respective fault message the crossed-through symbol of the AWF also appears in the multifunction display.

IMPORTANT The circuit status circuit 15 ON and therefore also the power supply is received by the center SAM control unit and the AWF controller unit with active brake intervention via the vehicle interior fuse and relay module. To do this, the ignition lock switches on a relay in the vehicle interior fuse and relay module which switches through the circuit "15g" to the AWF controller unit with active brake intervention and to the center SAM control unit. Detailed information about the display message for the AWF and possible cause which generate a fault message can be taken from the operator's manual.

Function sequence for collision warning 

The collision warning function sequence is subdivided into:

Reading in the input signals 

Apart from data from the radar sensor, the AWF controller unit with active brake intervention also evaluates the following influencing factors:

Direction of travel and vehicle speed:

The direction of travel is defined using the wheel rotation direction. The vehicle speed is calculated from the wheel speed from the front axle rpm sensors and the rear axle rpm sensors. The Electronic Stability Program control unit transmits the respective information via the interior CAN to the center SAM control unit. This makes the information available via the front end CAN of the AWF controller unit with active brake intervention.

Acceleration/yaw rate:

The acceleration/yaw rate is detected by the Electronic Stability Program control unit. The Electronic Stability Program control unit transmits this information via the interior CAN to the center SAM control unit. This makes the information available via the front end CAN of the AWF controller unit with active brake intervention.

Steering wheel angle and steering wheel velocity:

The steering wheel angle sensor detects/records the steering wheel angle. The Electronic Stability Program control unit receives the steering wheel angle sensor signals via the chassis CAN and sends information about the steering wheel angle and steering wheel velocity to the center SAM control unit via the interior CAN. This makes the information available about the steering wheel angle and steering wheel velocity via the front end CAN of the AWF controller unit with active brake intervention. The data from the steering wheel angle sensor serve to test the plausibility of the yaw rate detected by the Electronic Stability Program control unit.

Status of the reverse gear (for vehicles with code 410 (5-speed manual transmission)):

The signal from the reversing lamp activation/idle recognition switch is read on by the center SAM control unit via a direct line. The center SAM control unit makes the information available via the front end CAN of the AWF controller unit with active brake intervention.

Gear range (for vehicles with DKG) (code 429 (twinamic)):

For vehicles with a 6-speed DKG the gear stage is sent from the DKG control unit via the drive train CAN to the ME-SFI [ME] control unit. The ME-SFI [ME] control unit sends this information over the interior CAN to the center SAM control unit. The center SAM control unit then sends the information regarding gear stage via the front end CAN to the AWF controller unit with active brake intervention.

Gear range (model 453.091/391/491):

For vehicles with electric drive the engaged gear range is provided by the electronic selector lever module control unit via the electric drive CAN to the EDCU. This sends the gear range over the interior CAN to the center SAM control unit. The center SAM control unit makes the information available via the front end CAN of the AWF controller unit with active brake intervention. The EDCU also reads in driving program recognition sensor over a direct line to recognize the gear range.

Driver activity 

Accelerator pedal position and engine torque (except model 453.091/391/491):

The ME-SFI [ME] control unit transmits information concerning the accelerator pedal position of the accelerator pedal sensor and concerning the current engine torque via the interior CAN to the center SAM control unit. This makes the information available via the front end CAN of the AWF controller unit with active brake intervention.

Accelerator pedal position and engine torque (except model 453.091/391/491):

The power electronics control unit transmits information about the current engine torque via the electric drive CAN to the EDCU. The EDCU transmits the information concerning the current engine torque and the accelerator pedal position of the accelerator pedal sensor via the interior CAN to the center SAM control unit. The center SAM control unit makes the information available via the front end CAN of the AWF controller unit with active brake intervention.

Brake pedal position:

The center SAM control unit reads in the information about the position of the brake pedal over a direct line from the brake lights switch. The center SAM control unit transmits this information from the brake lights switch concerning the position of the brake pedal via the interior CAN to various control units, it also makes the information available via the front end CAN of the AWF controller unit with active brake intervention. For vehicles with an electric drive the signal of the brake lights switch is also read in over a direct line by the EDCU.

Setting the turn signal (turn signaling):

The center SAM control unit which functions as a signaling master transmits information about turn signaling via the front end CAN of the AWF controller unit with active brake intervention.

Evaluation and issuing of a collision warning 

The AWF controller unit with active brake intervention calculates the distance to the obstacle. With the aid of the information made available via the front end CAN the time until a possible collision is calculated.

Where necessary the AWF controller unit with active brake intervention sends the request to output a warning in a CAN message via the front end CAN to the center SAM control unit. The center SAM control unit transmits this CAN message to output a warning via the interior CAN to the IC. The AWF differentiates here between a static warning and a dynamic (collision-critical) warning. For brake force amplification and to introduce an autonomous braking, the AWF controller unit with active brake intervention sends the system status, stationary objects or those moving immediately ahead, relative speed in m/s2 , as well as the distance to the objects moving immediately ahead or stationary objects, in various CAN messages via the front end CAN to the center SAM control unit. The center SAM control unit transmits these CAN messages via the interior CAN to the Electronic Stability Program control unit.

Static warning 

Output of the static warning takes place purely optically and then only after from a vehicle speed of v > 30 km/h (about 19 mph). If, in flowing traffic, the temporal interval of t = 0.8 s is no longer given to the vehicle driving immediately in front, and remains for a time of t > 3 s in this critical interval (e.g. at a speed of v = 100 km/h (about 62 mph) interval about s = 22.2 m (about 72.83 ft)), the AWF controller unit with active brake intervention sends the request to output a warning via the front end CAN to the center SAM control unit, which sends this via the interior CAN to the IC. The instrument cluster reacts by actuating the AWF warning lamp. The minimum period of a static warning is t = 0.6 s. After a static warning no further static warning is issued for a time threshold of t = 5 s in as far as the distance does not go so far down that a dynamic or collision-critical warning is issued.

Dynamic (collision-critical) warning 

Output of the dynamic warning (collision-critical) takes place purely optically and acoustically only from a vehicle speed of v > 30 km/h (about 19 mph). If an obstacle within the detection range of the radar sensor is classified as accident-critical. This depends on the relative speed to the vehicle traveling in front or the obstacle. The time before a warning of a possible collision also depends on whether the driver is already active or not and, for example, if the brakes have been actuated. For example, a warning will be issued at a relative speed of 30 km/h approx. t ≤ 2.0 s (with an inactive driver) and approx. t ≤ 1.7 s (with an active driver) before a possible collision. With a relative speed of 40 km/h, a warning will be issued approx. t ≤ 2.2 s (with an inactive driver) and approx. t ≤ 1.8 s (with an active driver) before a possible collision. With a relative speed of 10 km/h, a warning will be issued approx. t ≤ 1.5 s (with an inactive driver) and approx. t ≤ 1.3 s (with an active driver) before a possible collision. For this, the AWF controller unit with active brake intervention sends the request to output a warning via the front end CAN to the center SAM control unit, which sends this request via the interior CAN to the instrument cluster. The instrument cluster reacts by actuating the AWF warning lamp and the warning horn (intermittent tone). The warning output cannot be acknowledged. Take-back of the warning only occurs if the situation has been defused. Furthermore, with a collision-critical warning from the Electronic Stability Program control unit, slight pressure is applied to the hydraulic system of the brake system (priming) so that the brake linings are placed on the brake disks without decelerating the vehicle in the process. The autonomous braking that follows facilitates an earlier braking of the vehicle. A warning will only be issued for a stationary obstacle if this obstacle has already been recognized as being dynamic (moving). After a dynamic warning, no further dynamic warning for the same obstacle occurs for a time threshold of t = 3 s.

IMPORTANT Detailed information about the information flow can be taken from the chapter "Collision warning" in the sub-chapter "Reading in of input signals". Relative speed is the speed with which the actual vehicle is approaching the vehicle driving in front.

In addition to the vehicle speed and the distance to an obstacle, the AWF also records the driver activity. The driver activity is detected, for example, through modifications of the accelerator pedal position, through actuation of the turn signal or through pressing of the brake pedal. The route ahead is defined over the yaw rate, the steering wheel angle and the direction of travel.

System limits 

The collisions critical warning is not issued for a collision with:

Function sequence for torque adjustment (radar-supported regenerative braking) (only model 453.091/391/491) 

Additional function requirements:

IMPORTANT The radar-supported regenerative braking is significantly reduced for a completely charged high-voltage battery (95 %). In ECO transmission mode (high-voltage battery not fully charged) there is constantly the highest recuperation stage set.

For vehicles with an electric drive (model 453.091/391/491), the AWF controller unit with active brake intervention also sends the status of the FCW (system status), stationary objects or those moving immediately ahead, relative speed in m/s, as well as the distance to the objects moving immediately ahead or stationary objects in various CAN messages via the front end CAN to the center SAM control unit. The center SAM control unit then sends these CAN messages via the interior CAN to the EDCU. The EDCU uses this information to adapt regulation of the electric drive via the power electronics control unit, e.g. by means of torque reduction (reduction of the regenerative braking and the drag delay) or increasing the regenerative braking. The regenerative braking stage is set optimally to the current situation. For a free road coasting is activated.

  Electrical function schematic for Active Brake Assist Model 453.0 (except 453.091), 453.3 (except 453.391), 453.4 (except 453.491) with code 258 (Active Brake Assist System) PE30.30-P-2070-97BBA 
Model 453.091/391/491 with code 258 (Active Brake Assist System) PE30.30-P-2070-97BEA 
  BAS brake application, function   GF42.31-P-2000KJ
  Overview of system components for driver assistance systems   GF54.00-P-9998KJ
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