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BAS brake application, function - GF42.31-P-2000KJ

MODEL 453.0/3/4 with CODE 258 (Active Brake Assist System) 

Function requirements, general 

Brake Assist System (BAS)

In vehicles with combustion engine (except model 453.091/391/491), the ME-SFI control unit (N3/10) sends the signal of the engine speed via the interior CAN (CAN B) to the center SAM control unit (N10/10) and from there via the front-end CAN (CAN G) to the controller unit for the distance warning function with active brake application (A90). From this, the controller unit for the distance warning function with active brake application recognizes that the engine is running. In vehicles with electric drive (only model 453.091/391/491), the electric drive control unit (N127) sends the message "Drivetrain operational" via the interior CAN to the center SAM control unit and from there via the front-end CAN to the controller unit for the distance warning function with active brake application.

BAS brake application, function (distance warning function with active brake application), general 

Detailed information on the function sequence of collision warning can be found in the description "Active Brake Assist, function". If the driver fails to react to the warnings, Active Brake Assist (distance warning function with active brake application) with code 258 (Active Brake Assist) initiates autonomous partial braking or assists the driver in a full-stop braking action. 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 distance warning function system and routes the data between the controller unit for the distance warning function with active brake application, the ME-SFI control unit (except model 453.091/391/491) or the electric drive control unit (only model 453.091/391/491) and the instrument cluster (A1) to issue signals to the driver. Likewise it receives data from the Electronic Stability Program control unit (N30/4) to obtain information and transmits appropriate data to initiate autonomous braking. An autonomous brake application is only initiated when there is the possibility of a collision with dynamic (moving) objects. An object must have been detected as moving in front of the vehicle at least once. There is no autonomous braking in the case of purely static objects. Environmental influences or error sources outside the system can interfere with Active Brake Assist temporarily. The BAS brake application function is described in terms of two subfunctions:

Function sequence for braking assistance 

If the driver then reacts to the collision-critical distance warning with emergency braking, the Brake Assist System provides support for the best possible utilization of the braking distance to the obstacle. The braking assistance is available in a vehicle speed range of v ≥ 30 km/h to v approx. 140 km/h.

The emergency braking maneuver is defined over the actuation speed and the actuation strength with which the brake pedal is actuated. The BAS detects an emergency braking situation based on rapid actuation of the brake pedal and, if necessary, increases the brake pressure in order to achieve maximum deceleration. For this purpose, the Electronic Stability Program control unit evaluates the increase in pressure in the brake system and initiates an emergency stop if a certain activation threshold is exceeded. Additionally, in vehicles with code 258 (Active Brake Assist), in contrast to vehicles with a pure distance warning function (code 252 (Distance warning function)), the activation threshold of BAS is reduced when the controller unit for the distance warning function with active brake application detects a collision-critical situation. Thus, in a collision-critical situation, the driver is additionally aided by the system if he fails to brake strongly or quickly enough.

The controller unit for the distance warning function with active brake application evaluates the data from the internal radar sensor and sends the system status, whether the object ahead is stationary or moving, the relative speed in m/s and the distance to moving or stationary objects ahead 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 Electronic Stability Program control unit to initiate the braking assistance. The Electronic Stability Program control unit constantly calculates the necessary brake force. The initiation of the braking assistance depends on the relative speed to the vehicle ahead or to the obstacle. For example, at a relative speed of 30 km/h, the braking assistance is initiated approx. t ≤ 0.7 s before a potential collision. At a relative speed of 40 km/h, it is initiated approx. t ≤ 0.75 s before a potential collision. At a relative speed of 50 km/h, it is initiated approx. t ≤ 0.8 s before a potential collision. At a relative speed of 10 km/h, it occurs approx. t ≤ 0.5 s before a potential collision.

The speed and force of the brake application are detected by the brake vacuum sensor (B64/1). The Electronic Stability Program control unit reads in the signals from the brake vacuum sensor directly and calculates the braking torque requested by the driver. If an emergency braking action is necessary, the Electronic Stability Program control unit evaluates the braking torque currently being requested and initiates appropriate measures if necessary. If a collision is unavoidable, occupant protection measures are taken in addition to intervention by the brake system. Detailed information on this can be found in the separate function description "Supplemental Restraint System, function". If there is no actuation or the intensity of actuation of the brake pedal by the driver considerably decreases, the brake boost on the driver braking is reduced and the Brake Assist System is also deactivated. The Brake Assist System will also be deactivated if the driver presses the accelerator pedal.

Additional function requirements for autonomous braking 

Function sequence for autonomous brake application 

Even before an autonomous brake application in the event of a collision-critical warning, the Electronic Stability Program control unit pressurizes the brake system hydraulics slightly (precharging) so that the brakepads are brought into contact with the brake disks and, in wet weather, any existing water film can be scraped off without decelerating the vehicle. This means that, in the event of a subsequent braking action, the vehicle can be braked correspondingly earlier. If the driver fails to react to the collision-critical warning, Active Brake Assist initiates an autonomous brake application (gentle to moderate deceleration) and a reduction in the engine torque. Active Brake Assist calculates the necessary braking torque taking the following variables into account:

The controller unit for the distance warning function with active brake application sends the status of the FCW (system status), whether the object ahead is stationary or moving, the relative speed in m/s and the distance to moving or stationary objects ahead, and the request to initiate a braking torque in various CAN messages via the front-end CAN to the center SAM control unit. The center SAM control unit then sends this CAN message to initiate the braking torque via the interior CAN to the Electronic Stability Program control unit. The Electronic Stability Program control unit evaluates the incoming requests, actuates the traction system hydraulic unit directly and sends the request for the engine torque reduction via the interior CAN to the electric drive control unit (model 453.091/391/491) or to the ME-SFI control unit (except model 453.091/391/491).

The Brake Assist in model 453 can react to moving and preceding obstacles ahead at vehicle speeds up to approx. 140 km/h. The Brake Assist does not react to static obstacles.

The time of initiation of the autonomous brake application depends on the relative speed to the vehicle ahead. For example, at a relative speed of 30 km/h, the autonomous brake application is initiated approx. t ≤ 0.8 s before a potential collision. At a relative speed of 40 km/h, it is initiated approx. t ≤ 0.95 s before a potential collision. At a relative speed of 50 km/h, it is initiated approx. t ≤ 1.1 s before a potential collision. At a relative speed of 10 km/h, it occurs approx. t ≤ 0.5 s before a potential collision.

If a collision is unavoidable, occupant protection measures are taken in addition to intervention by the brake system. Detailed information on this can be found in the separate function description "Supplemental Restraint System, function". Detailed information on warning messages can be found in the separate function description "Active Brake Assist, function".

  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
  Overview of system components for driver assistance systems   GF54.00-P-9998KJ
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