PRE-SAFE rear, function - GF30.30-P-0015RF
Model 205, 253 with code 253 (Rear impact warning and protection system)
Function requirements, general
- No undervoltage or overvoltage
- Engine running or drivetrain operational
- The rear end PRE-SAFE® function switched on over the "Driver Assistant" menu of the IC
- No trailer recognized (with CODE 550 (Trailer hitch))
The CDI control unit or the ME-SFI [ME] control unit sends the "Engine running" or "Drivetrain operational" signal to the radar sensors control unit via the drive train CAN, powertrain control unit and chassis FlexRay.
The status of the rear end PRE-SAFE® function is sent by the instrument cluster via the user interface CAN, electronic ignition lock control unit and the chassis FlexRay to the radar sensors control unit. The trailer recognition control unit (with CODE 550 (Trailer hitch)) sends the "Trailer recognized" status via the interior CAN, electronic ignition lock control unit and chassis FlexRay to the radar sensors control unit.
Rear end PRE-SAFE®, general
The rear end PRE-SAFE® function extends the preventative occupant protection in dangerous situations associated with following traffic. The rear center bumper radar sensor detects the traffic occurring behind one's own vehicle. The radar sensors control unit uses the information from the center rear bumper radar sensor to determine the closing speed and the distance from the traffic to the rear. Various measures are introduced on this basis that may be able to reduce the consequences of an accident for the passengers.
The possible measures are shown in chronological order in the following graphic:
Shown: Model 205.0
The rear end PRE-SAFE® function consists of the following subfunctions:
- Function sequence for vehicle detection and warning about the traffic to the rear (Escalation level t1)
- Function sequence for preparing the stationary vehicle for a collision (escalation level t 2 )
- Function sequence for activating the occupant protection measures (escalation level t 3 )
- Function sequence for securing the vehicle after the collision (escalation level t 4 )
Function sequence for vehicle detection and warning about the traffic to the rear (Escalation level t 1 )
Impact in approx. t = 1.4 ms:
If an approaching vehicle is detected by the center rear bumper radar sensor and is deemed accident-critical by the radar sensors control unit, the driver of this following vehicle is warned by activating the rear hazard warning flasher function (except CODE 460 (Canada version) and except CODE 494 (USA version)).
The flashing frequency of the rear turn signal indicator is f = 5 Hz.
Model 205, 253
with code 253 (Rear impact warning and protection system)
with code 460 (Canada version)
with code 494 (US version)
The following traffic is not warned by the rear hazard warning flasher function.
Technical implementation
The radar sensors control unit reads in the signals from the center rear bumper radar sensor via radar CAN 2, evaluates these and transmits the request to output a warning via the suspension FlexRay, electronic ignition lock control unit and interior CAN to the front SAM control unit. This evaluates the incoming request and activates the function rear end hazard warning flasher.
Additional function requirements for preparing the standing vehicle for a collision (escalation level t2)
- Vehicle at rest
- A braking request is recognized by through actuation of the brake pedal or actuation of the parking brake or engaging the gear range "P"
- Brake request sent by the hold function or by DISTRONIC PLUS or by DISTRONIC PLUS with Steer Assist and Stop & Go Pilot
The Electronic Stability Program control unit sends the status of the braking system via the chassis FlexRay to the radar sensors control unit.
The Electronic Stability Program control unit activates the parking brake and transmits the status of the electric parking brake switch via the suspension FlexRay to the radar sensors control unit. The fully integrated transmission control unit sends information about the selected gear range via drive train CAN, powertrain control unit and suspension FlexRay to the radar sensors control unit.
The IC transmits the vehicle speed via the user interface CAN, electronic ignition lock control unit and suspension FlexRay to the radar sensors control unit.
Function sequence for preparing the stationary vehicle for a collision (escalation level t2)
Impact in approx. t = 600 ms:
If the radar sensors control unit registers no or only inadequate deceleration of the approaching vehicle via the closing speed, the vehicle is "braked hard" as a preventive measure by increasing the brake pressure. The hard braking on the one hand significantly reduces the occupant load (whiplash), while on the other hand serves the protection of other road users. Uncontrolled movements of the vehicle after a rear-end collision that could lead to subsequent collisions are minimized (collision with a vehicle in front, collisions with pedestrians or other road users in situations involving an intersection).
Technical implementation
The radar sensors control unit sends the braking request via the suspension FlexRay to the Electronic Stability Program control unit which then directly actuates the traction system hydraulic unit appropriately.
Function sequence for activating the occupant protection measures (escalation level t 3 )
Impact in approx. t = 100 - 50 ms:
If the radar sensors control unit calculates that it is likely that there will be an imminent rear end collision based on the closing speed of the vehicle closing in from the rear, preventative measures are instigated for occupant protection. The driver and the front passenger are held in position in their seat in order to optimize the protective effect of the conventional restraint equipment.
Technical implementation
The radar sensors control unit transmits the following information via the suspension FlexRay to the Electronic Stability Program control unit:
- Closing speed of the vehicle to the rear
- Time until the possible impact of the vehicle to the rear
- Deployment recommendation for the front reversible emergency tensioning retractors
The Electronic Stability Program control unit initiates PRE-SAFE® measures to provide protection to occupants.
Function sequence for securing the vehicle after the collision (escalation level t 4 )
About. t = 2 s after the impact:
After a rear end collision the brake pressure is held for about t = 2 s.
After a rear-end collision, the rear-end PRE-SAFE® function is no longer available until it is activated using a diagnostic tester.
Technical implementation
The technical implementation of "Maintain brake pressure" corresponds to the technical implementation of the escalation level t2
.
| Electrical function schematic for PRE-SAFE® rear end | PE30.30-P-2061-97FBA |
| Overview of system components for driver assistance systems | GF54.00-P-9998RF |