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Regenerative braking system, function - GF42.22-P-0001FLH

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MODEL 212.095/098/298 up to model year 2014 

Fig 1: Regenerative Braking System Block Diagram
G10242305Courtesy of MERCEDES-BENZ USA

Function requirements for regenerative braking system, general 

Regenerative braking system, general 

The regenerative braking system (RBS) is a modular brake and vehicle dynamics control system (in the event of a fault, only the affected functions are switched off).

The regenerative braking system is a further development of the Electronic Stability Program (ESP) and also contains the ESP functions.

The regenerative braking system control unit evaluates the data from the following directly connected components to monitor the driving situation at any given time:

The regenerative braking system control unit communicates through the CAN network with the following control units and sensors to monitor the driving situation at any given time:

DISTRONIC PLUS or code (237) Active Blind Spot Assist)

IMPORTANT CAN network

The regenerative braking system control unit exchanges data with other control units integrated in the CAN network, via 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 subfunctions:

Function sequence for regenerative braking system (RBS) 

The driver's braking request is recorded by the pedal angle sensor and sent to the regenerative braking system control unit. This control unit continuously sends the value of the currently available regenerative braking torque to the CDI or ME-SFI control unit via the chassis CAN. The regenerative braking system control unit requests a specific amount of regenerative braking torque from the CDI control unit or ME-SFI [ME] control unit to match the driving condition. The CDI control unit or the ME-SFI [ME] control unit sends the amount of generated regenerative braking torque to the regenerative braking system control unit.

The required braking torque is generated by the electrical machine (A79/1).

The regenerative braking system control unit divides the overall braking torque moment requested by the driver according to the driving condition into a regenerative (to be implemented by the drivetrain) part and a hydraulic (to be implemented over the wheel brake) braking torque moment.

In the event of an emergency stop or a fault message (drivetrain fault or faults that initiate a deactivation of the antilock brake system (ABS)) the braking torque is then generated hydraulically only.

Function sequence for electronic Stability Program (ESP) 

ESP prevents break-away 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 driving stability.

The regenerative braking system control unit processes the following performance indicators to determine the vehicle characteristics:

The sideslip 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 transverse forces acting on the wheels can be determined from the yaw angle velocity, lateral acceleration and steer angle of the front wheels, which is calculated from the steering wheel angle. 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 sideslip 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 sends a signal requesting a reduction in drive torque, via the chassis CAN to the CDI or ME-SFI control unit, which then reduces the engine power accordingly. A pending shift operation is suppressed for the duration of control intervention.

For this purpose, the regenerative braking system control unit sends a signal via the chassis CAN to the CDI control unit, which sends the information via the drive train CAN (CAN C) to the fully integrated transmission control unit. The fully integrated transmission control unit suppresses the shift operation.

Exhaust emission test/rolling mode 

For vehicle test purposes, the regenerative braking system can be set to roller dynamometer mode if the workshop menu is activated through the instrument cluster (A1) and the engine is then started. ESP, ABS and ASR are then switched passive.

The Electronic Stability Program warning lamp (A1e41) and the antilock brake system indicator lamp (A1e17) in the instrument cluster light up.

A message is also displayed in the multifunction display (A1p13) of the instrument cluster.

The roller dynamometer mode can also be activated via Xentry Diagnostics.

Electronic brake force distribution (EBD) function sequence 

EBD 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 at all wheels, the brake pressure is reduced at the appropriate wheel until the wheel begins 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.

For this purpose, the regenerative braking system control unit sends a signal requesting a reduction in drive torque, via the chassis CAN to the CDI or ME-SFI control unit, which then reduces the engine power accordingly (drive torque reduction).

A check is continuously performed to establish whether the drive torque specified by the driver via the accelerator pedal sensor can be implemented again, e.g. due to an improvement in road surface adhesion. As a result of targeted brake intervention at the spinning wheel (without traction), the drive torque is transferred to the opposite solidly rotating wheel (with traction) (braking torque control).

Brake Assist System (BAS) function sequence 

The BAS detects emergency braking situations from fast actuation of the brake pedal and, if necessary, then increases the brake pressure through the BAS solenoid valve (A77/y1) in order to achieve greater deceleration. In this way, the regenerative braking system control unit supports the driver in the event of an emergency braking operation.

Additional function requirements for tire pressure loss warning 

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 (A1p13).

The following information is included in the calculation:

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.

IMPORTANT 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 for HOLD function 

Function sequence for HOLD function 

HOLD makes life easier for the driver optionally:

The HOLD function holds the vehicle in position without the driver having to keep the brake pedal depressed.

The HOLD function can be activated by briefly and rapidly pressing the depressed brake pedal further when the vehicle is stationary. Successful activation of the HOLD function is indicated in a status bar in the instrument cluster. When the driver releases the brake pedal, an incline-dependent hold pressure is set and maintained. The braking effect is canceled when the driver depresses the acceleration pedal. Pressure reduction when starting off is regulated by the standstill coordinator to match the respective situation.

Furthermore, the HOLD function will be deactivated automatically under the following conditions:

Function sequence - standstill coordinator (SSK) 

The standstill coordinator monitors braking support systems below 3 km/h up to the point where the vehicle is securely held stationary and, within this speed range, the standstill coordinator is responsible for ensuring that the vehicle is in a safe condition when a securing event occurs (vehicle securing).

The SSK monitors the following support systems:

When the vehicle is in a safe stationary condition, the support systems request the following monitoring functions of the SSK:

If vehicle rolling is detected, the brake pressure is increased until the vehicle is stationary again.

In vehicles with an ESP system with circuit pressure sensors, leakage compensation is also active throughout a secure standstill. If the brake pressure measured by means of the circuit pressure sensors drops relative to the specified pressure request of the SSK, the specified pressure of the SSK is restored by means of an active pressure buildup.

The standstill coordinator must secure the vehicle if the following events occur:

"Engage P" request 

The vehicle is secured using the "DIRECT SELECT module" (DSM) securing system, which is prompted to engage gear range "P" through a signal sent to the CAN network. The vehicle is secured after this "P request" sent by the SSK has been implemented by the DSM and a feedback signal indicating that gear range P has been engaged has been sent to he CAN network. The existing brake pressure of the SSK is reduced and the SSK is inactive.

Emergency-P path 

If the DSM does not signal engagement of gear range P on the CAN network within a defined time after the request, the emergency-P functionality of the EIS forces gear range P to be engaged. For this purpose, the electronic ignition lock control unit (N73) receives via the chassis CAN (CAN E) the SSK request to the DSM to engage gear range P. When this SSK signal is received, the electronic ignition lock control unit also sends a request to engage gear range P to the DSM. When this prompt is sent, the electronic ignition lock control unit internally activates its feedback monitoring function to detect when gear range P has been engaged by the DSM (emergency-P path). If no feedback from the DSM is received within a defined time, the emergency-P path is energized by the electronic ignition lock control unit after enabling by the DSM, and engagement of gear range P in the DSM is forced by a spring mechanism.

SSK deactivation 

Deactivation of the SSK is signaled internally in the ESP and externally via the chassis CAN. The following applies: Activation of the support systems is not permitted. If the Assist System is active the secure vehicle function is also requested. Sliding and skidding When the vehicle is stationary, the SSK monitors whether the vehicle is sliding or skidding. These conditions are signaled internally in the ESP and externally over the CAN network. The following applies: Activation of the support systems is not permitted.

Active Assist Systems must be terminated when the brake pedal is depressed and the vehicle is sliding.

Driver status 

Assuming that the vehicle is not being misused, "driver seat belt buckle fastened" or shifting out of "P" with the driver door closed is regarded as a qualified signal indicating that the driver is present in the vehicle. If the driver door is open and the seat belt buckle is not fastened, the driver status is unclear and the driver is therefore recognized as absent. The driver status is signaled on the CAN network. The following applies: Activation of the DISTRONIC PLUS (with code (233) DISTRONIC PLUS) is not permitted.

Starting off 

The SSK executes the start-off algorithm for the support functions. Uphill starting-off takes place with torque balancing. The incline output torque, the total braking torque and the available drive torque are taken into consideration here. When the start-off torque is high enough, the start-off algorithm is terminated and the vehicle starts off without rolling in the undesired direction of travel.

The prerequisite for starting off is the start-off request detected from actuation of the acceleration pedal and a gear range engagement for vehicles with automatic transmission.

Pressure reduction in the case of starting off in a downhill direction or on the flat takes place with a hyperbolic characteristic.

Additional function sequence for precharging depending on accelerator pedal actuation 

Function sequence for precharging depending on accelerator pedal actuation 

If a possible emergency braking situation is detected, the brake pads are brought into contact with the brake disks.

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, and a brake pressure of 3 to 4 bar is requested.

Additional function requirements - dry braking 

IMPORTANT The status of the wiper system is transmitted by the front SAM control unit with fuse and relay module (N10/1) via 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 to passive mode via a menu in the instrument cluster. The instrument cluster sends the request via the chassis CAN to the regenerative braking system control unit.

If the system is switched to passive, the ESP®/ASR OFF warning lamp (A1e32) 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) 

Function sequence for flashing adaptive brake lights (except with code (494) USA version 

In the event of emergency braking, the regenerative braking system control unit sends a corresponding signal via the chassis CAN to the front SAM control unit with fuse and relay module. The front SAM control unit with fuse and relay module forwards this over the interior CAN to the rear SAM control unit with fuse and relay module (N10/2). The rear SAM control unit with fuse and relay module actuates the left brake light, taillight and parking light (E3e9), the right brake light, taillight and parking light (E4e9) and the center brake lamp (E21) for the duration of the signal (min. 1 s) at a flasher frequency of 5 Hz. If the vehicle comes to a standstill from a speed >70 km/h (not urban traffic), the brake lights and the center high-mounted brake lamp is 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:

System fault display function sequence 

The driver is informed of the system status and any faults by means of the following:

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) 

Function sequence for Hill Start Assist (HSA) 

The HSA supports the driver when moving off (forward and backward) in an uphill direction. The brake pressure fed by the driver and required to hold the vehicle is maintained for a short time even after the brake pedal is released. The holding time is long enough to allow the driver to move his foot from the brake pedal to the accelerator comfortably without the vehicle immediately starting to roll downhill. The HSA is activated automatically when the vehicle is stationary and if an incline which will cause the vehicle to roll opposite the desired direction of travel (forward travel with reverse gear engaged and reverse travel with forward gear engaged) is detected.

The incline is calculated using the signal from the yaw rate sensor for lateral and longitudinal acceleration when the vehicle is stationary. At the very least, the brake pressure applied by the driver and, at the very most, the incline-dependent holding pressure in the case of active HSA is maintained by the traction system hydraulic unit (A7/3) in the brake system. Following release of the brake pedal and detection of a start-off request, reduction of the brake pressure is controlled by means of torque balancing system. This system takes into consideration the incline output torque, the total braking torque and the available drive torque. When the start-off torque is high enough, the HSA is terminated and the vehicle starts off without rolling in the undesired direction of travel. The prerequisite for torque balancing is the start-off request detected from actuation of the acceleration pedal and engagement of a gear range in the case of vehicles with automatic transmission. If the driver does not depress the accelerator pedal, the supplied pressure is rapidly reduced to 0 bar 1 second after the brake pedal is released.

Hill-Start Assist is terminated immediately if:

Function sequence - Fail boost 

The fail boost function ensures brake boosting. It is activated if the pneumatic brake booster cannot adequately support driver braking because of a system error in the vacuum supply.

The system error in the vacuum supply is displayed in the instrument cluster by the Electronic Stability Program warning lamp.

Failure of the pneumatic brake booster is detected via the vacuum sensor which is integrated in the pneumatic brake booster.

The fail boost function implements the hydraulic brake boost by means of a suitable boost factor and actuates the solenoid valves and hydraulic pump in the traction system hydraulic unit accordingly. If this fault profile exists, the corresponding fault entry must be checked by means of diagnosis in the regenerative braking system control unit and the fault must be rectified in accordance with the repair instructions.

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 for Electronic Stability Program trailer stabilization (TSA) 

TSA detects pendulum swinging motion in a tractor/trailer combination based on the yawing vibrations caused in the tractor vehicle by the trailer. The front SAM control unit with fuse and relay module places the information on the chassis CAN.

This is received by the regenerative braking system control unit. If vehicle/trailer instability occurs, this is detected by the yaw rate, lateral and longitudinal acceleration sensor.

The regenerative braking system control unit receives this information via the vehicle dynamics CAN.

IMPORTANT No additional sensors are used on the trailer or trailer hitch. TSA stabilizes the tractor/trailer combination by means of brake pressure requests on alternating sides of the front axle and, if necessary, slows down the tractor/trailer combination by reducing torque and by building up pressure at all wheels.

The vehicle is stabilized only via the front axle. If heavy negative damping is detected, the system intervenes to decelerate the vehicle and stabilize the trailer.

Function sequence for cruise control (CC) 

The cruise control maintains the constant speed specified by the driver, irrespective of engine load condition and without the accelerator pedal being actuated.

The cruise control also includes the following functions:

Further information on the cruise control (CC) can be found in the following document:

Function sequence - Over boost 

Like the fail boost function, the over boost function ensures brake boosting. The overboost function is activated if the pneumatic brake booster can no longer adequately support driver braking because of a sub-optimal vacuum level. Unlike fail boost, over boost represents a normal operating state. There is no system error in the vacuum supply.

This reduced vacuum level operating state can occur, for example, in the following instances:

The over boost function implements hydraulic brake boosting using a suitable boost factor and actuates the solenoid valves and hydraulic pump in the ESP system for it accordingly.

IMPORTANT The trailer stabilization engages more sensitively if a trailer hitch is installed at the factory. If the trailer hitches are retrofitted, the standard intervention ensures stabilization.

The vehicle-related component functions in the regenerative braking system are described in the following documents:

  Electrical function schematic for regenerative braking system   PE42.22-P-2050-97DAH
  Regenerative brake mode, function   GF42.22-P-1000FLH 
  Regenerative and hydraulic brake mode, function   GF42.22-P-1001FLH 
  Hydraulic brake mode, function   GF42.22-P-1002FLH 
  Emergency brake mode, function   GF42.22-P-1003FLH 
  Vacuum supply, function   GF42.22-P-1004FLH 
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