GF42.22-P-0001LEH Regenerative Braking System, Function
GF42.22-P-0001LEH Regenerative braking system, function
Function requirements for regenerative braking system, general
^ Circuit 15 ON
^ Adaptive brake (ABR) is functional
Regenerative braking system, general
The regenerative braking system is a modular designed brake and vehicle dynamics control system (in case of errors only affected functions are switched off).
Sensor data are read in directly and through the CAN network by the regenerative braking system control unit (N30/6) and then the corresponding actuators are actuated.
The regenerative braking system control unit evaluates the data from the following directly connected components to monitor the driving situation at any given time:
- BAS diaphragm travel sensor (A7/7b1)
- Vacuum sensor (A7/7b3)
- Yaw rate, lateral and longitudinal acceleration sensor (B24/15)
- Pedal angle sensor (B37/1)
- Left front rpm sensor (L6/1)
- Right front rpm sensor (L6/2)
- Left rear rpm sensor (L6/3)
- Right rear rpm sensor (L6/4)
The regenerative braking system control unit communicates through the CAN network with the following control units to monitor the driving situation at any given time:
- DT control unit DTR (A89) (with code (219) DISTRONIC)
- ME-SFI [ME] control unit (N3/10)
- Power electronics control unit (N129/1)
- Electric controller unit (VGS) (Y3/8)
- Steering column tube module (N80)
CAN interconnection
The regenerative braking system control unit exchanges data with other control units, which are integrated into the CAN network, over the connected chassis CAN (CAN E) and vehicle dynamics CAN (CAN H).
Function sequence for regenerative braking system
The system is made up of the following component functions:
^ Function sequence for electronic Stability Program (ESP)
^ Electronic brake force distribution (EBD) function sequence
^ Antilock brake system (ABS) function sequence
^ Acceleration skid control (ASR) function sequence
^ Brake Assist System (BAS) function sequence
^ Function sequence for tire pressure loss warning
^ Function sequence HOLD (starting-off aid)
^ Function sequence for precharging, depending on accelerator pedal actuation (Ready Alert Breaking) (RAB)
^ Dry braking function sequence
^ Passive ASR and ESP switching function sequence
^ Function sequence for adaptive brake lights flashing (except with code (494) USA version)
^ Function sequence - PRE-SAFE
^ System fault display function sequence
^ Function sequence for Hill Start Assist (HSA)
^ Function sequence - Failed Boost
^ Function sequence - Fading Brake Support
^ Function sequence for clearance reduction (air gap)
^ Function sequence - Standstill coordinator
Function sequence for electronic Stability Program (ESP)
ESP prevents breakaway when the vehicle oversteers or understeers. Within physical limits it ensures that the vehicle does not deviate from the course specified by the driver. Brake forces are produced selectively at the individual wheels to correct any deviations. Furthermore, reduction of the drive torque takes place in order to increase directional stability and road adhesion.
The regenerative braking system control unit processes the following performance indicators to determine the vehicle characteristics:
- Yaw angle velocity
- Steering wheel angle
- Brake pressure
- Engine torque
- Transmission stage
- Lateral acceleration
The side-slip angle (angle between vehicle longitudinal axis and direction of movement of the vehicle's center of gravity) is calculated using the yaw angle velocity (speed of vehicle rotation about vertical axis). The yaw angle velocity, the lateral acceleration and the turning angle of the front wheels (calculated from the steering wheel angle) can be used to determine the lateral forces on the wheels. The longitudinal forces on the wheels are calculated using the engine torque, transmission stage and brake pressure at each wheel.
If the yaw angle velocity measured does not match the specified value or if the determined side-slip angle is too large, the regenerative braking system control unit generates a signal for brake force build-up or reduction for the relevant wheel. The resulting forces stabilize the vehicle.
A distinction is made between understeering and oversteering vehicle behavior.
Intervention in the case of oversteer:
If the vehicle begins to oversteer, brake pressure is built up at the outer front wheel. The resulting reduction in lateral force at the outer front wheel generates a yawing moment which counteracts the tendency of the vehicle to rotate inward. The vehicle speed decreases as a result of the brake force at the front wheel, which also enhances stability.
Intervention in the case of understeer:
If the vehicle understeers, the maximum possible lateral force at the front axle has been exceeded. This means that the vehicle pushes itself over the front axle and toward the outer edge of the corner. If vehicle understeer is detected, the drive torque is reduced first.
If this is not sufficient, brake pressure is built up at the rear wheel on the inside of the curve. Depending on the brake force, a torque is generated which causes the vehicle to rotate inward with a simultaneous reduction in speed. This has a considerable stabilizing effect.
The regenerative braking system control unit transmits a signal for drive torque reduction over the chassis CAN to the ME-SFI [ME] control unit which reduces the engine output accordingly.
A pending shift operation is suppressed for the duration of control intervention.
To this end, the regenerative braking system control unit sends a signal over the chassis CAN to the ME-SFI [ME] control unit, which sends the information over the drive train CAN (CAN C) to the electric controller unit (VGS). The electric controller unit (VGS) suppresses the shift operation.
Exhaust test/roller dynamometer mode
For vehicle test purposes, the ABR can be set to roller dynamometer mode if the workshop menu is activated via the instrument cluster (A1) and the engine is then started. ESP, ABS and ASR are then switched passive.
The ESP warning lamp (A1e41) and the ABS indicator lamp (A1e17) light up in the instrument cluster.
In addition, a message is displayed in the multifunction display (A1p13) of the instrument cluster.
The dynamometer mode can also be activated using the Diagnosis Assistance System (DAS).
Electronic brake force distribution (EBD) function sequence
EBV assists the driver in the partial braking area. It prevents overbraking of the rear axle and increases vehicle stability when braking in the curve, by reducing the pressure at the rear wheel on the inside of the curve or increasing it at the front wheel on the outside of the curve as required.
Antilock brake system (ABS) function sequence
ABS prevents the wheels from locking up when braking and as a result maintains the steerability and directional stability and road adhesion during vehicle deceleration.
If a locking wheel is detected by the regenerative braking system control unit on the basis of the signals from the rpm sensors on all the wheel, the brake pressure is reduced at the appropriate wheel until it starts to turn again.
Acceleration skid control (ASR) function sequence
ASR prevents the drive wheels from spinning while driving. In addition it causes an improved directional stability and road adhesion with an increased traction potential over the entire vehicle speed range. The ASR comprises the brake torque control and the drive torque control. The regenerative braking system control unit records the spinning of the drive wheels via the signals from the rpm sensors. Wheel spinning is countered by reduction of the drive torque.
To this end, the regenerative braking system control unit sends a signal to reduce drive torque over the chassis CAN to the ME-SFI [ME] control unit, which reduces the engine output accordingly (drive torque control).
The system constantly checks whether the drive torque specified by the driver via the accelerator pedal sensor can be realized again e.g. due to improved road adhesion. Targeted brake interventions on the slipping wheel enable the drive torque to be transferred to the opposite drive wheel which is running in a stable manner (brake torque control).
Brake Assist System (BAS) function sequence
BAS detects emergency braking situations by means of a rapid brake pedal operation and if necessary increases the brake pressure via the BAS solenoid valve (A7/7y1) in order to achieve maximum possible deceleration. The regenerative braking system control unit evaluates the extent of the pressure rise in the brake system and by doing so assist the driver in the event of any emergency stop.
Additional function requirements for tire pressure loss warning
^ Vehicle speed 15 km/h
^ No active ABS/ASR or ESP control intervention function
^ No snow chain mode
^ No active brake application
Function sequence for tire pressure loss warning
The tire pressure loss warning detects a significant loss of pressure at a tire, based on the dependence between the rolling circumference (rotational speed) of a wheel and the tire's internal pressure, and issues a corresponding warning message in the instrument cluster's multifunction display.
The following information is included in the calculation:
- Lateral acceleration
- Turn rate
- Drive torque
The tire pressure loss warning is active after a few minutes driving. In the subsequent ignition sequences, a reminder message is displayed in the instrument cluster, provided that the tire pressure loss warning (RDW) system has not been reactivated.
A uniform loss of pressure at all 4 wheels cannot be detected. After changing one or more tires, the tire pressure loss warning has to be restarted in the menu guidance in the instrument cluster.
At a lateral acceleration of more than 1.5 m/s2 or a longitudinal acceleration of more than 1.0 m/s2, the function is inactive.
Additional function requirements - HOLD
^ Transmission not in position "P"
^ Vehicle not secured with parking brake
^ Incline detected with vehicle at standstill
Function sequence HOLD (starting-off aid)
HOLD assists the driver when starting off on a hill or when waiting in traffic. HOLD is activated by briskly continuing to depress the brake pedal at a standstill.
The activation of the HOLD function, is notified to the driver by the message "HOLD" in the instrument cluster's multifunction display. The set brake pressure is held, until the driver applies a specific pressure to the brake pedal again (until "HOLD" in the instrument cluster goes out) or he drives off.
Furthermore, the HOLD function will be deactivated automatically under the following conditions:
- Electric parking brake is actuated
- The DIRECT SELECT gear selector switch (S16/13) shifts the transmission to the "P" position
A deactivation of the HOLD function can be forced as follows before the driver leaves the vehicle:
- With circuit 15 ON and the driver door open (notification in multifunction display)
- With circuit 15 OFF and the driver door open (discreet honking of horn)
Should the function nevertheless not be deactivated, the honking gets louder.
The circuit 15 status is sent by the EIS [EZS] control unit (N73) and the status of the driver door by the left front door control unit (N69/1).
Additional function requirements for RAB
^ No active control intervention function by ABS, ASR or ESP
^ ESP system not switched passive
Function sequence for precharging, depending on accelerator pedal actuation (Ready Alert Breaking) (RAB)
The RAB function applies the brake pads to the brake disks in the event of a supposed emergency braking situation being detected The clearance between the brake pad and brake disk is thus compensated before the driver operates the brake.
The response time of the brake is improved and a shorter braking distance can be achieved.
Emergency braking is detected via the release gradient of the accelerator pedal which is formed from the CAN signal "accelerator pedal position".
If a rapid release of the accelerator pedal is detected, the system assumes that the driver intends to brake. RAB is activated and a brake pressure of 3 to 4 bar requested.
Additional function requirements - dry braking
^ No active control intervention function by ABS, ASR or ESP
^ Windshield wiper active
The status of the wiper system (WSA) is sent by the front SAM control unit with fuse and relay module (N10/1) over the interior CAN (CAN B) to the central gateway control unit (N93) and forwarded from here over the chassis CAN to the regenerative braking system control unit.
Dry braking function sequence
Dry braking enhances the performance of the brakes on the front axle in wet driving conditions. The water film is wiped off the brake disks by cyclically applying the brake pads for 0.5 s with a brake pressure of 1 bar. This improves the response time of the brake.
Passive ASR and ESP switching function sequence
The ASR and ESP functions can be switched passive by operating the ESP OFF switch (S6/2s11) in the cockpit switch group (S6/2). The cockpit switch group is read in over the OBF-LIN (LIN B3) by the upper control panel control unit (N72/1) and sent over the interior CAN to the central gateway control unit. From here the signal is forwarded over the chassis CAN to the regenerative braking system control unit.
If the system is switched to passive, the ESP warning lamp lights up in the instrument cluster. The control thresholds are raised in the case of passive switching. ABS cannot be deactivated. ESP is always active during a brake application.
Additional function requirements for adaptive brake lights flashing (except with code (494) USA version)
^ Vehicle speed 50 km/h
^ Deceleration 7.5 m/s2
Function sequence for flashing adaptive brake lights (except with code (494) USA version
In the event of an emergency stop, the regenerative braking system control unit generates a corresponding signal on the chassis CAN. The central gateway control unit relays this to the rear SAM control unit with fuse and relay module (N10/2) via the interior CAN.. Then for the duration of the signal (min. 1 s), the rear SAM control unit actuates the brake light, taillight and left parking light (E3e9), brake light, taillight and right parking light (E4e9) and the center high-mounted brake lamp (E21) with a flashing frequency of 5 Hz. If the vehicle goes from a speed of > 70 km/h (no urban traffic) to standstill, then the brake lights and the center high-mounted brake lamp are permanently actuated again. In addition, the automatic hazard warning flasher function is activated.
If the vehicle then exceeds a speed of 10 km/h again, the hazard warning system are automatically switched off again.
Function sequence - PRE-SAFE
The PRE-SAFE function is integrated into the regenerative braking system control unit. If the regenerative braking system control unit detects one of the following situations, the PRE-SAFE function is activated:
- Panic post-braking (the driver's wish for deceleration is greater than physically possible)
- Severe oversteer (vehicle rear breaks away in combination with powerful ESP control intervention functions)
- Severe understeer (the vehicle pushes powerfully for a longer time over the front wheels)
- Rapid steering movements at higher speeds which suggest a shock reaction of the driver and can lead to vehicle instabilities
System fault display function sequence
The driver is informed of the system status and any faults by means of the following:
- ABS indicator lamp
- ESP warning lamp
- Message in instrument cluster
In the event of a failure of the regenerative braking system control unit, a basic brake function without ABS is always available.
Additional function requirements for Hill Start Assist (HSA)
^ Transmission not in position "N"
^ Vehicle not secured with parking brake
^ Incline detected with vehicle at standstill
Function sequence for Hill Start Assist (HSA)
HSA prevents the vehicle from rolling back contrary to the direction of the gear range engaged when starting off during the time it takes for the driver to move his foot from the brake pedal to the accelerator pedal.
The function is triggered automatically, if when the vehicle is at a standstill an incline is detected via the yaw rate sensor for lateral and longitudinal acceleration which would cause rolling in the opposite direction to the gear range engaged. For this the brake pressure applied by the driver is maintained for a period of 0.8 to 1.5 s by the traction system hydraulic unit (A7/3) and switched off by a suitably large starting off torque.
Function sequence - Failed Boost
The failed boost function serves to ensure a sufficiently high brake pressure level in the system with a shortage of brake boost through the BAS brake booster (A7/7).
Function sequence - Fading Brake Support
Fading brake support assists the driver when the brake is overheated and therefore the brake force is reduced. With extreme overheating, a message may be shown in the multifunction display of the instrument cluster.
Function sequence for clearance reduction (air gap)
After dynamic cornering, the air gap function applies the brake pads to the front axle brake disks.
This closes the air gap between the brake pad and brake disk in order to achieve the required braking power at the corresponding wheel as quickly as possible in the event of ESP intervention by the brake system.
Function sequence - standstill coordinator
When support systems such as HOLD are actuated and the driver status is unclear, the standstill coordinator requests that the vehicle be transferred to a safe state.
The vehicle-related component functions in the regenerative braking system are described in the following documents:
^ Regenerative brake mode, function
^ Regenerative and hydraulic brake mode, function
^ Hydraulic brake mode, function
^ Emergency brake mode, function
^ Vacuum supply, function
