COLLISION PREVENTION ASSIST function

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Function requirements, general

 No undervoltage or overvoltage

 COLLISION PREVENTION ASSIST coded in electronic ignition lock control unit (EIS)

 "Circuit 61 ON" (engine running)

 Dirt and function test of the radar sensor in the COLLISION PREVENTION ASSIST control unit completed successful

 Vehicle is not in backward movement

 The CDI control unit (engine 642 or 651) or the ME-SFI [ME] control unit (engine 271 or 272) transmits the "engine run" signal via the engine CAN to the COLLISION PREVENTION ASSIST control unit.

COLLISION PREVENTION ASSIST, general

The COLLISION PREVENTION ASSIST is a proximity warning system that warns the driver optically and acoustically about a possible collision with an obstacle or with the vehicle driving immediately ahead. The controlling takes place by the COLLISION PREVENTION ASSIST control unit, in that the warnings are generated in a vehicle speed range of v = 7 km/h up to the vehicle maximum speed.

The radar sensor and the controller unit is integrated in the COLLISION PREVENTION ASSIST control unit.

The following presentation shows the detection range of the integral radar sensor.

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COLLISION PREVENTION ASSIST function sequence

For "circuit 61 ON" (engine running) the COLLISION PREVENTION ASSIST is automatically active. The COLLISION PREVENTION ASSIST can, however, also be manually switched on and off using the COLLISION PREVENTION ASSIST button, which is located in the UCP (OBF) control unit.

The COLLISION PREVENTION ASSIST control unit leads to a soiling and function test after each engine start. The function test requires an object in the detection area of the radar sensor system which is used by the COLLISION PREVENTION ASSIST control unit as a reference value. If malfunctions are established or if no function test is possible then the COLLISION PREVENTION ASSIST cannot be activated.

The integral radar sensor operates in the 24 GHz frequency range and detects objects in front of the vehicle. The detection angle is  = 18° +/– 8° for s = 0 up to 80 m.

Apart from data from the radar sensor, the COLLISION PREVENTION ASSIST control unit also evaluates the following influencing factors:

 Direction of travel

 Vehicle speed

 Steering wheel angle

Direction of travel

The direction of travel is determined over the wheel rotation direction of the wheels. The wheel rotation direction for each individual wheel is detected by the rpm sensors. The ESP control unit® reads in the signals from the rpm sensors directly and transmits them via the engine CAN to the COLLISION PREVENTION ASSIST control unit.

Vehicle speed

The vehicle speed is calculated on the basis of the wheel speeds. The wheel speed for each individual wheel is detected by the respective rpm sensor. The ESP control unit® reads in the signals from the rpm sensors directly and transmits them to the instrument cluster via engine CAN. The instrument cluster uses the wheel speeds to calculate the vehicle speed and sends this information via the engine CAN to the COLLISION PREVENTION ASSIST control unit.

Steering wheel angle

The steering angle sensor records the steering wheel angle. The steering column tube module (MRM) control unit reads in the data from the steering angle sensor directly and sends them via the engine CAN to the COLLISION PREVENTION ASSIST control unit.

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COLLISION PREVENTION ASSIST warning output

The COLLISION PREVENTION ASSIST control unit calculates the distance to the obstacle and the time up to a possible collision. The AWS differentiates between the following modes:

 static distance warning

 collision critical distance warning

Static distance warning

Output of the static distance warning takes place purely optically. If the distance from the vehicle directly ahead goes below t = 0.8 s in flowing traffic and this critical distance is maintained for t > 3 s (e.g. at a speed of v = 100 km/h, distance about s = 22.2 m), the COLLISION PREVENTION ASSIST control unit sends the request to output the warning via the engine CAN to the instrument cluster. The instrument cluster reacts by actuating the COLLISION PREVENTION ASSIST warning lamp.

Collision critical distance warning:

Output of the collision-critical distance warning takes place optically and acoustically. If an obstacle within the detection range of the radar sensor system is classified as accident-critical (a collision will occur within t < 2.6 s), the COLLISION PREVENTION ASSIST control unit sends the request to output the warning via the engine CAN to the instrument cluster, which reacts by activating the COLLISION PREVENTION ASSIST warning lamp and the warning buzzer. The warning output cannot be acknowledged. Take-back of the warning only occurs if the situation has been defused.

If the driver reacts to the collision warning with emergency hazard braking, the braking support is activated by the BAS.

Timely warnings of obstacles ahead will only occur if a vehicle speed of v = 72 km/h is not exceeded.

 The hazard braking is defined over the actuation speed and the actuation strength with which the brake pedal is actuated.

The COLLISION PREVENTION ASSIST control unit also uses data from the radar sensor system and the brake servo assistance based on the situation and continuously calculates the necessary brake force while taking account of the clearance available and the closing speed to the obstacle.

If there is no actuation or the intensity of actuation of the brake pedal by the driver reduces, the brake servo assistance for the driver braking is reduced and the Brake Assist System is deactivated. If the driver presses the accelerator pedal, the BAS is also deactivated.