PRE-SAFE function - GF91.40-P-0001RF

Model 205, 253 with code 299 (PRE-SAFE® system)

Fig 1: PRE-SAFE - Block Diagram

Courtesy of MERCEDES-BENZ USA

Function requirements, general

No overvoltage or undervoltage
Circuit 15 ON (with seat belt adjustment function)
"Engine running" or "Drive train operational" signal set (except with seat belt adjustment function)

The electronic ignition lock control unit sends the status of circuit 15 via chassis FlexRay to the Electronic Stability Program control unit. The CDI control unit (diesel engine) or the ME-SFI [ME] control unit (gasoline engine) sends the "Engine running" or "Drive train operational" signal via drive train CAN, powertrain control unit and the chassis FlexRay to the Electronic Stability Program control unit.

PRE-SAFE®, general

The PRE-SAFE® preventive occupant protection system can recognize critical driving situations in advance, and activate functions as a precaution when necessary, that reduce the risk of injury to the vehicle occupants. To do so, PRE-SAFE® permanently evaluates the sensor data from systems such as the Electronic Stability Program (ESP®) and the Brake Assist System PLUS (BAS PLUS), which then enables it, e.g. to recognize excessive understeer or oversteer and emergency braking. If a critical driving situation is recognized, corresponding measures can be started within fractions of a second. These measures allow the seat belts and airbags to provide their full protective effect in the event of a collision.

The PRE-SAFE® function is controlled by the Electronic Stability Program control unit.

The PRE-SAFE function® includes the following subfunctions:

Function sequence for sensing the vehicle status
Function sequence for reversible tensioning retraction
Function sequence for seat belt adaptation
Function sequence for positioning the front passenger seat (with CODE 242 (Electrically adjustable front passenger seat with memory function))
Function sequence for closing side windows
Function sequence for closing sliding roof (with CODE 414 (Sliding roof)) or Panoramic sliding sunroof (with CODE 413 (Panoramic sliding sunroof))
Function sequence for display of fault messages
Function sequence for terminating PRE-SAFE® functions

Function sequence for sensing the vehicle status

The current vehicle status is defined by the Electronic Stability Program control unit by evaluating the following variables:

Wheel speed
Natural dynamics of the vehicle (yaw speed, lateral acceleration and longitudinal acceleration)
Steering angle

In vehicles with CODE 23P (Driving assistance package Plus), the Electronic Stability Program control unit also evaluates the following signals:

"Autonomous brake application" signal
"Collision-critical warning" status
Relative speed to opposing object

The radar sensors control unit sends the signals via chassis FlexRay to the Electronic Stability Program control unit.

The Electronic Stability Program control unit differentiates between the following critical driving states:

Emergency braking: Fast brake-pedal operation, which indicates a startled response by the driver.
Panic braking: The driver's deceleration request is significantly higher than the physically possible vehicle deceleration on a slippery road (e.g. aquaplaning, ice or snow). Strong interventions of the anti-lock braking system (ABS).
Strong oversteer: Swerving of rear end combined with strong ESP® stabilization interventions.
Strong understeer: Vehicle pushes for a certain time over the front wheels. Strong ESP® stabilization interventions.
Critical steering movements: Rapid steering movements at speeds of > 140 km/h that indicate a startled reaction from the driver, making it highly likely that the driver is unable to cope with this situation.

Also in vehicles with CODE 23P (Driving assistance package Plus):

Strong braking support by BAS PLUS: Strong support of the brake application initiated by the driver with detected risk of collision in linear traffic on the basis of radar-assisted evaluation of the surroundings.
Autonomous brake application through PRE-SAFE® brake: Vehicle deceleration with detected risk of collision in linear traffic by the PRE-SAFE® brake based on radar-assisted environmental evaluation of the surroundings.
Front collision object detection: Detection of an imminent frontal impact (under certain conditions) on the basis of radar-assisted evaluation of the surroundings.
Rear collision object detection: Detection of an imminent rear-end collision (under certain conditions) on the basis of radar-assisted evaluation of the surroundings.
Autonomous course correction by Active Blind Spot Assist: Significant course-correcting intervention by the brake system in the event of a registered risk of a side collision with vehicles in the blind spot on the basis of radar-assisted evaluation of the surroundings.

The triggering threshold for the PRE-SAFE® functions is increased if the ESP® has been manually switched to passive via the menu on the instrument cluster. The instrument cluster transmits the status of the ESP® via the user interface CAN, the electronic ignition lock control unit and the chassis FlexRay to the Electronic Stability Program control unit.

Additional function requirements, reversible emergency tensioning retraction

Front seat belt applied

Function sequence for reversible tensioning retraction

The reversible emergency tensioning retractors reduce any seat belt slack in hazardous situations by tensioning the front seat belts using an electric motor. The driver and the front passenger are held in position better in order to optimize the protective effect of the conventional restraint equipment. Depending on the situation, the belt strap of the seat belt is tightened with varying force or speed. After the PRE-SAFE® measure, the belt strap of the seat belt is slackened again (unless a roll-over has occurred).

The Electronic Stability Program control unit transmits information about the PRE-SAFE® status and the PRE-SAFE® stage via the chassis FlexRay, the electronic ignition lock control unit and the user interface CAN to the reversible emergency tensioning retractors. The supplemental restraint system control unit sends the crash signal (in the event of a rollover) via user interface CAN to the reversible emergency tensioning retractors. In order to tension the seat belt, the respective reversible emergency tensioning retractor actuates an integrated actuator motor.

Additional function requirements for seat belt adjustment

Function activated in head unit
PRE-SAFE® has not been activated

The head unit transmits the status of the seat belt adaptation function via the user interface CAN to the reversible emergency tensioning retractor.

Function sequence for seat belt adaptation

The seat belt adaptation function reduces the amount of seat belt slack by tensioning the seat belts of the driver and front passenger seats using an electric motor. The function is triggered when the belt lock tongue is inserted into the respective belt buckle. The insertion of the belt tab of the seat belt into the belt buckle is recorded via the status of the respective belt buckle switch.

The supplemental restraint system control unit reads in the status of the respective seat belt buckle switch directly and transmits this via the user interface CAN to the reversible emergency tensioning retractors. In order to tension the seat belt, the respective reversible emergency tensioning retractor actuates an integrated actuator motor. This tightens the belt strap at a constant speed until the seat belt slack has been reduced.

Additional function requirements for positioning the front passenger seat (with CODE 242 (Electrically adjustable front passenger seat with memory function))

Front passenger seat occupied
Seat adjustment normalized
Front passenger seat outside of PRE-SAFE® position

The supplemental restraint system control unit sends the status of the front passenger seat occupied recognition over the user interface CAN, electronic ignition lock control unit and interior CAN to the front passenger seat control unit.

Function sequence for positioning the front passenger seat (with CODE 242 (Electrically adjustable front passenger seat with memory function))

In order to optimize the protective effect of the conventional restraint equipment, the front passenger seat is moved by the electrical seat adjustment into a position that is favorable from a safety point of view before a potential accident. The Electronic Stability Program control unit sends information about the PRE-SAFE® status and the request for PRE-SAFE® adjustment via the chassis FlexRay, electronic ignition lock control unit and interior CAN to the front passenger seat control unit. An active setting, if given, is interrupted.

Depending on the current seat position, the following adjustments are made:

Backrest inclination
Seat height
Seat fore/aft position
Seat inclination

The PRE-SAFE® setting continues until:

The PRE-SAFE® position is reached
The impact occurs
End of PRE-SAFE® situation

The PRE-SAFE® adjustment of the front passenger seat can be canceled at any given time through manual adjustment requests.

Following completion of the PRE-SAFE® situation, the front passenger seat must be manually set to the desired position again.

Seat adjustment if the front passenger is not wearing his/her seat belt:

The system moves the seat into a position that is adapted to the unsafe conditions. The supplemental restraint system control system reads in the status of the front passenger seat belt buckle switch directly and transmits this via the user interface CAN, the electronic ignition lock control unit and the interior CAN to the front passenger seat control unit.

Additional function requirements for closing the side windows

Power windows normalized
Side windows opened further than PRE-SAFE® position

Function sequence for closing side windows

The side windows are closed up to a residual gap by the power window lifters before a potentially impending accident. This ensures that no objects can penetrate into the vehicle interior and no body parts (e.g. arms) of the occupants can protrude from the vehicle. This residual gap allows the smoke particles generated by the pyrotechnical restraint equipment to disperse quickly.

Left front door control unit
Right front door control unit
Left rear door control unit (model 205.0/1/2.253)
Right rear door control unit (model 205.0/1/2, 253)

Any automatic opening or manual setting that is active is aborted. The automatic reversing function is active when closing the side windows using PRE-SAFE®.

The PRE-SAFE® setting continues until:

The side windows reach the PRE-SAFE® position
The impact occurs

The closing of the respective side window by PRE-SAFE® can be interrupted at any time by manual adjustment requests. Following the PRE-SAFE® situation, the side windows must be manually moved back into the desired position.

Additional function requirement for closing sliding roof (with CODE 414 (Sliding roof)) or Panoramic sliding sunroof (with CODE 413 (Panoramic sliding sunroof))

Sliding roof NORMALIZED
Sliding roof opened further than PRE-SAFE® position

Function sequence for closing sliding roof (with CODE 414 (Sliding roof)) or Panoramic sliding sunroof (with CODE 413 (Panoramic sliding sunroof))

The sliding roof is closed completely by PRE-SAFE® if it was already lifted and open before the PRE-SAFE® measure. If the sliding roof was open in the sliding direction before the PRE-SAFE® measure occurred, it is closed apart from a residual gap. This ensures that no objects can penetrate into the vehicle interior and no body parts (e.g. arms) of the occupants can protrude from the vehicle in the event of a potential accident. This residual gap allows the smoke particles generated by the pyrotechnical restraint equipment to disperse quickly.

The Electronic Stability Program control unit sends information about the PRE-SAFE® status and the request for PRE-SAFE® adjustment via the chassis FlexRay, electronic ignition lock control unit and interior CAN to the panoramic sliding sunroof control unit or the sliding roof control module. Any automatic movement or manual setting that is active is aborted. The automatic reversing function is active when the sliding roof is being closed.

The PRE-SAFE® setting continues until:

The sliding roof reaches the PRE-SAFE® position
The impact occurs

Closing of the sliding roof can be aborted at any time by means of manual adjustment requests. Following completion of the PRE-SAFE® situation, the sliding roof must be manually moved back into the desired position.

Function sequence for display of fault messages

If the reversible emergency tensioning retractors detect faults, appropriate messages are displayed in the multifunction display of the instrument cluster. Faults in other components that are involved in PRE-SAFE® are not displayed as PRE-SAFE® faults.

The reversible emergency tensioning retractors transmit the request to display the fault message in the multifunction display via the user interface CAN to the instrument cluster.

Function sequence for terminating PRE-SAFE® functions

The PRE-SAFE® function is deactivated as soon as the prevailing situation is evaluated as no longer critical and the vehicle deceleration is a < 0.3 g. For this purpose, the Electronic Stability Program control unit obtains the information about the PRE-SAFE® status from the chassis FlexRay.

The adjustment of the front passenger seat is canceled insofar as the PRE-SAFE® position has not been reached. Closing of the side windows or the sliding roof is not stopped. The automatic closing function continues until the PRE-SAFE® position has been reached. The reversible emergency tensioning retraction is released as soon as the vehicle acceleration has dropped below a value of a = 0.3 g.

The reversible emergency tensioning retractors are equipped with a mechanical acceleration sensor, which locks the belt extension in the event of any vehicle acceleration of a > 0.3 g. For this reason, the belt may remain mechanically locked even when the reversible emergency tensioning retraction has been completed. In the event of an accident that is detected by the supplemental restraint system control unit, all PRE-SAFE® settings are immediately stopped.

	Electrical function schematic, pre-safe		PE91.40-P-2051-97FBA
	Overview of PRE-SAFE system components		GF91.40-P-9997RF