GF42.31-P-0001TX Braking Assistant (BAS) - Operation
GF42.31-P-0001TX Braking Assistant (BAS) - Operation
MODEL 219.3 up to 31.5.06
except CODE (494) USA version
MODEL 219.3 up to 30.6.06
with CODE (494) USA version
Braking behavior
Studies when driving normally on the driving simulator and on the test track have shown that most drivers in emergency braking situations, above all in the initial phase of brake application, in fact press the brake pedal quickly but not hard enough.
A distinction is made between:
^ Hesitant braking, e.g. in the initial phase the driver applies too little pedal force and increases it too slowly.
^ Inadequate braking, e.g. the driver applies too little pedal force throughout the entire braking application.
Brake Assist (BAS) has been developed to improve this situation.
Task
BAS is part of the active safety system. In interaction with the SBC multiple value function "Precharging", the BAS is already active when the accelerator pedal is released quickly. If the driver brakes very quickly after that, the system detects whether an emergency braking situation exists by means of the speed with which the brake pedal is operated.
If this is the case, the high-pressure charging pump (A7/3m1) in the traction system hydraulic unit (A7/3) is activated. The full brake force is reached as a result. If the driver releases the brake, this is detected and the high-pressure charging pump is switched off again.
Advantages:
^ By optimal braking deceleration BAS reduces the accident hazard in dangerous situations and thus assists the driver considerably.
^ BAS shortens the brake distance significantly in the event of a hesitant or inadequate brake application.
^ Due to the combination of the digital BAS with the SBC multiple value function "Precharging", the brake basically responds more quickly compared with the old conventional BAS which functions via venting of the vacuum brake booster.
^ The effect of the digital BAS is increased continuously during the brake application. Thus a virtually constant, maximum deceleration of the vehicle is achieved.
^ Variable brake pedal characteristic:
In order to achieve a more spontaneous brake response at high speed, as the speed increases a higher deceleration is already adjusted for a small pedal travel.
Function overview
With the introduction of the Sensotronic Brake Control (SBC) BAS has been completely integrated in the SBC system. Compared with the old conventional BAS which functioned via the venting of the vacuum brake booster, the new digital BAS is based purely on an electronic basis. In an emergency braking situation the response of the brake was able to be improved again by the digital BAS. Thus the stopping distance is also shortened again particularly at high speeds. The electronics are located in the ESP control unit (N47-5). Using the speed of the pressure rise in the front axle pre-pressure sensor (A7/3b1) and the signal from the SBC pedal value sensor (B37/1) the ESP control unit calculates the operating speed of the brake pedal.
The speed of actuation of the brake pedal and the vehicle speed are recorded in the ESP control unit. A BAS brake application is introduced if the pedal speed is greater than the threshold value specified for the respective vehicle speed.
At the start of a BAS brake application the high-pressure charging pump in the traction system hydraulic unit is activated and the separation valves in the hydraulic unit separate the closed loop from the hydraulic circuit for emergency operation. The pressure reservoir of the traction system hydraulic unit makes possible an immediate provision of brake pressure from approx. 140 bar to 160 bar. Within the shortest time, the maximum brake force is applied to the brake pistons of the brake calipers, thus achieving an immediate maximum deceleration of the vehicle
Once the engine has started, a vehicle identification and self-monitoring function of the complete SBC system takes place including BAS.
BAS is ready to operate if no fault exists and after starting off the vehicle exceeds a speed of 10 km/h.
BAS is effective up to a speed of more than 3 km/h.
In the event of a defect BAS is not used, this is displayed by an SBC error message via the multifunction display (A1p13) in the instrument cluster (A1). The normal function of the brake is not affected by this.
