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Regenerative Braking System, Function - GF42.22-P-0001GRH

MODEL 166.063 as of model year 2016 

G12079068

Block diagram 

Function requirements, general 

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).

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

The RBS assists the driver in dangerous situations which occur suddenly and thus ensures active safety.

To do so, the Electronic Stability Program control unit evaluates the data from the following components in order to detect the current driving situation:

The system is made up of the following subfunctions:

Function sequence RBS 

The driver's brake command is monitored by the pedal angle sensor and sent to the Electronic Stability Program control unit. This control unit continuously sends the value of the currently available regenerative braking torque to the ME-SFI [ME] control unit over chassis CAN 1. The value of the currently available regenerative braking torque is dependent on various factors including the charge level of the high-voltage battery. The charge level of the high-voltage battery is evaluated by the battery management system control unit, and sent over the hybrid CAN to the ME-SFI [ME] control unit. The Electronic Stability Program control unit requests a specific amount of regenerative braking torque from the ME-SFI [ME] control unit according to the driving condition. The ME-SFI [ME] control unit sends the amount of usable and/or generated regenerative braking torque over the chassis CAN 1 to the Electronic Stability Program control unit. The requested braking torque is generated by the electric machine.

The 3-phase AC voltage generated during regenerative braking by the electric machine is converted by the power electronics control unit into high-voltage DC voltage and supplied to the high-voltage battery.

The Electronic Stability Program control unit divides the overall braking torque moment requested by the driver according to the driving condition into a regenerative part (to be applied by the drive train) and a hydraulic part (to be applied over the brake).

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

Function sequence for radar-based regenerative braking system 

On vehicles with CODE 23P (Driving Assistance package Plus) 

The radar sensors control unit records the distance and the relative speed to the vehicle in front, and sends this over the chassis FlexRay, the chassis gateway control unit, chassis CAN 1 and the engine CAN using the powertrain control unit interface to the ME-SFI [ME] control unit.

A specified deceleration (negative acceleration) is calculated from the relative speed and the distance to the vehicle in front. This results in a desired force or desired torque which is provided as the engine drag torque. Deceleration by means of an increase in regenerative torque occurs when approaching a slower vehicle, when approaching a decelerating vehicle or when following a vehicle along a downhill stretch.

The ME-SFI [ME] control unit calculates the corresponding specified deceleration and sends the request for the regenerative braking torque over the engine CAN and the hybrid CAN using the powertrain control unit interface to the power electronics control unit. The power electronics control unit actuates the electrical machine accordingly by motor.

On vehicles except CODE 23P (Driving Assistance package Plus) 

The COLLISION PREVENTION ASSIST controller unit detects the distance and the relative speed to the vehicle in front and sends this information over the chassis CAN 1, the powertrain control unit and the engine CAN to the ME-SFI [ME] control unit.

A specified deceleration (negative acceleration) is calculated from the relative speed and the distance to the vehicle in front. This results in a desired force or desired torque which is provided as the engine drag torque. Deceleration by means of an increase in regenerative torque occurs when approaching a slower vehicle, when approaching a decelerating vehicle or when following a vehicle along a downhill stretch.

The ME-SFI [ME] control unit calculates the corresponding specified deceleration and sends the request for the regenerative braking torque over the engine CAN and the hybrid CAN using the powertrain control unit interface to the power electronics control unit. The power electronics control unit actuates the electrical machine accordingly by motor.

Function sequence for ESP® 

ESP® prevents the vehicle from breaking away when oversteering or understeering. It ensures that the vehicle does not deviate from the course specified by the driver (within physical limits). 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 Electronic Stability Program control unit processes the following measured quantities to determine the vehicle behavior:

Differentiation is made between the following intervention types:

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. In other words, the vehicle pushes via the front axle to the outer edge of the curve.

The resulting instability is counteracted by means of a reduction in drive torque and the following brake interventions at up to 3 wheels:

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 Electronic Stability Program control unit sends a signal requesting a reduction in drive torque over the chassis CAN 1, the engine CAN and the powertrain control unit to the ME-SFI [ME] control unit, which reduces engine power accordingly. A pending shift operation is suppressed for the duration of control intervention.

To do so, the Electronic Stability Program control unit sends a signal to the fully integrated transmission control unit over the chassis CAN 1, powertrain control unit and the drive train CAN. This suppresses the shift operation.

EBD function sequence 

The EBD [EBV] function provides assistance when the driver applies medium force to the brake pedal. EBD [EBV] prevents overbraking of the rear axle and increases vehicle stability when braking in a 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. 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 rate (speed of vehicle rotation about vertical axis).

The yaw rate, 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 shift stage and brake pressure at each wheel.

If the measured yaw rate does not match the specified value or if the determined side-slip angle is too large, the Electronic Stability Program control unit generates a signal for brake force buildup or reduction for the relevant wheel. The resulting forces stabilize the vehicle.

Function sequence of exhaust test, dynamometer test mode 

For vehicle test purposes, the regenerative braking system (RBS) can be set to dynamometer test mode if the workshop menu is activated using the left multifunction steering wheel button group and the right multifunction steering wheel button group, and the engine is then started. ESP®, ABS and ASR are then switched to passive. The Electronic Stability Program warning lamp and the antilock brake system indicator lamp in the instrument cluster light up. Additionally, a corresponding message is shown in the instrument cluster. The roller dynamometer mode can also be activated via Xentry Diagnostics.

Function sequence for Torque Vectoring Brake 

Torque Vectoring Brake is the active influence of the yaw angle or the yaw acceleration on the rear axle through interventions by the brake system. When cornering, a short intervention by the brake system on the rear wheel on the inside of the curve causes the vehicle to rotate to a certain extent. The vehicle can be steered more precisely and easily and there is a noticeable improvement in the responsiveness and driving safety. Torque Vectoring Brake enables stable cornering without any noticeable loss in accelerating power while driving in the sporty driving mode.

ABS function sequence 

The ABS prevents the wheels from blocking when braking and as a result maintains the steerability and driving stability during vehicle deceleration. If a locking wheel is detected by the Electronic Stability Program control unit on the basis of the signals from the rpm sensors, the brake pressure is reduced at the appropriate wheel until the wheel begins to turn again.

Function sequence for ASR, ETS 

ASR and ETS prevent the drive wheels from spinning when driving.

ASR and ETS also serve to provide improved directional stability and road adhesion for increased traction potential over the entire vehicle speed range. Spinning of the drive wheels is detected by the Electronic Stability Program control unit using the signals from the rpm sensors. Wheel spinning is countered by reduction of the drive torque.

To do so, the Electronic Stability Program control unit sends a signal requesting a reduction in drive torque over the chassis CAN 1, the engine CAN and the powertrain control unit to the ME-SFI [ME] control unit, which reduces engine power accordingly. A check is continuously performed to establish whether the drive torque specified by the driver via the accelerator pedal sensor can be permitted again, e.g. due to an improvement in road surface adhesion. The drive torque is transmitted to the opposite, stable drive wheel by means of intervention by the brake system on the spinning wheel.

BAS function sequence 

The BAS detects emergency braking situations based on rapid actuation of the brake pedal and, if necessary, increases the brake pressure in order to achieve maximum deceleration. For this purpose, the Electronic Stability Program control unit evaluates the increase in pressure in the brake system and initiates an emergency braking if a certain triggering threshold is exceeded.

Function sequence for Brake Assist System PLUS (with code 23P (Driving Assistance package Plus)) 

BAS Plus increases the brake pressure depending on the speed of the brake pedal operation and the distance of the vehicle driving in front.

Additional function requirements for adaptive brake lights flashing (except with code 494 (USA version)) 

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

In the event of an emergency stop, the Electronic Stability Program control unit sends a corresponding signal via chassis CAN 1 and the interior CAN with the electronic ignition lock control unit interface to the SAM control unit.

The SAM control unit actuates the left rear lamp unit, the right rear lamp unit and the center brake lamp with a flashing frequency of f = 5 Hz for the duration of the signal (min. t = 1 s). If the vehicle comes to a standstill from a speed > 70 km/h (no urban traffic) the brake lights and the additional 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.

Additional function requirements - dry braking 

The status of the windshield wiper system is sent by the SAM control unit over the interior CAN, electronic ignition lock control unit and the chassis CAN 1 to the Electronic Stability Program control unit.

Dry braking function sequence 

The dry braking function enhances the performance of the brakes on the front axle in wet driving conditions. The brake pads are applied cyclically for a short time (approx. 0.5 s) with a brake pressure p = 1 bar to remove the film of water from the brake disk. This improves the response time of the brake.

Function sequence for precharging dependent on accelerator pedal operation 

Upon detection of a possible emergency braking situation, 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 is achieved.

Emergency braking is detected on the basis of the release gradient of the accelerator pedal. If a rapid release of the accelerator pedal is detected, the system assumes that the driver intends to brake. The function is activated and a brake pressure of p = 2 to 3 bar is requested.

IMPORTANT In order to adapt the calculated triggering threshold of the release gradient to the driver behavior, a learning algorithm is implemented which places the driver in the category "normal driver" or "hectic driver" on the basis of the average rate at which the accelerator pedal is released.

Function sequence for precharging depending on lateral acceleration 

During dynamic cornering, the brake pads are brought into contact with 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 HOLD function 

The HOLD function assists the driver during waiting times in traffic or when starting off on a hill. The HOLD function is activated by quickly pressing down hard on the actuated brake pedal after coming to a standstill.

Successful activation of the HOLD function is indicated in a status line in the instrument cluster.

The HOLD function can be activated if:

If the driver releases the brake pedal, an incline-dependent hold pressure is set and maintained until the driver quickly depresses the brake pedal again or drives off. Pressure reduction when starting off is performed by the standstill control depending on the respective situation.

The HOLD function is deactivated automatically if:

Hill Start Assist function sequence 

When starting off, the Hill-Start Assist prevents the vehicle from rolling back opposite the travel direction of the engaged gear range during the time it takes for the driver to change from the brake pedal to the accelerator pedal. The function is released automatically if, when the vehicle is stationary, an uphill or downhill gradient which would cause the vehicle to roll opposite the travel direction of the engaged gear range is detected by the Electronic Stability Program control unit. The Electronic Stability Program control unit also reads in the pedal angle sensor directly and thereby identifies the brake pedal status.

The brake pressure applied by the driver is maintained in the brake system by the traction system hydraulic unit.

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. It 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 Hill-Start Assist is deactivated and the vehicle starts off without rolling in the undesired direction of travel.

If the driver does not depress the brake pedal, the pressure in the brake system is reduced after t = 1 s.

The Hill-Start Assist is terminated immediately if:

The Hill-Start Assist does not function if:

Additional function requirements for speed control during downhill driving 

v < 30 km/h

The downhill speed regulation serves to provide greater safety when driving on steep terrain. The corresponding menu in the instrument cluster can be used to set the relevant setting speed for the downhill speed regulation function, in a range of v = 4 km/h up to v = 18 km/h in increments of 2 km/h.

The system is activated via the Downhill Speed Regulation button in the lower control panel control unit. Activation is indicated by an LED in the Downhill Speed Regulation button. The defined vehicle speed that matches the corresponding set speed (shown on the instrument cluster) is now maintained. The speed setting can be varied while actually driving downhill.

The request to activate speed control during downhill driving is read in by the lower control panel control unit and sent via the center console switch LIN to the SAM control unit.

The SAM control unit sends the signal to the interior CAN. The electronic ignition lock control unit receives the signal and sends this information to the chassis CAN 1.

The Electronic Stability Program control unit receives the signal and regulates the required speed by means of brake interventions and corresponding requests to the engine management and the transmission control. If the driver accelerates the vehicle by pressing the accelerator pedal while the function is active, the downhill speed regulation switches to passive mode. If the vehicle speed is v > 35 km/h when the accelerator pedal is not pressed, it is actively regulated to the configured setting speed.

If the driver then accelerates the vehicle using the accelerator pedal during an active function to a speed of v > 35 km/h, the downhill speed regulation switches off automatically and the LED in the Downhill Speed Regulation switch goes out. A message also appears on the instrument cluster and a warning tone is emitted. If the speed regulation for downhill driving is switched off manually via the Downhill Speed Regulation button, similarly, a message appears on the instrument cluster. A warning tone does not sound in this case.

Additional function requirements for off-road program 

Function sequence for off-road program 

The function sequence for the off-road program is made up of the following subfunctions:

Function sequence for activation of off-road program 

The off-road program is activated by the On/Off-road menu button in the lower control panel control unit.

Activation is displayed by an LED lighting up in the Off-road program button. The off-road symbol also appears in the instrument cluster. The off-road program is deactivated by renewed actuation of the off-road program button. The LED in the On/Off-road menu button and indicator in the instrument cluster goes out.

Function sequence for adaptation of threshold values 

Through activation of the off-road program:

The changes mentioned with regard to ESP®, ABS and ASR are dependent on the speed.

This means that the threshold values for an active off-road program match those of on-road mode as of a speed of approx. v = 50 km/h.

If the vehicle decelerates to below a speed of approx. v = 50 km/h, the thresholds return to the off-road values.

Function sequence for switching ESP® to passive mode 

The ESP® function can be switched to passive mode using the ESP OFF button. The ESP Off button status is read in over a direct line by the upper control panel control unit and the signals are forwarded over the battery sensor LIN to the SAM control unit, over the interior CAN to the electronic ignition lock control unit and over the chassis CAN 1 to the Electronic Stability Program control unit. If the system is switched passive, the Electronic Stability Program warning lamp lights up in the instrument cluster. The control thresholds are raised in the case of passive switching.

IMPORTANT ABS cannot be deactivated. ESP® is always active during a brake application.

Function sequence for ESP® trailer stabilization (with CODE 550 (Trailer hitch)) 

The ESP® trailer stabilization detects rocking motion in the tractor/trailer combination based on the yawing vibrations caused in the tractor vehicle by the trailer. If vehicle/trailer instability occurs, this is detected in the regenerative braking system (RBS) by means of the signals from the yaw rate sensor for lateral and longitudinal acceleration. The Electronic Stability Program control unit receives this information via the vehicle dynamics CAN.

The ESP® trailer stabilization engages more sensitively if a trailer hitch is installed at the factory. Stabilization is ensured through standard intervention for retrofitted trailer hitches.

The ESP® trailer stabilization 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 engine torque and by building up pressure at all wheels.

Active vehicle/trailer stabilization does not change the critical speed.

Function sequence for PRE-SAFE® system (with code 23P (Driving Assistance package Plus)) 

The PRE-SAFE® system is activated in critical driving conditions, i.e. in a potential accident situation. The goal is to create the most favorable conditions possible for the occupants during the expected accident by activating the functions before the actual impact.

The PRE-SAFE® system is integrated in the Electronic Stability Program control unit. If the Electronic Stability Program control unit detects one of the following situations, the PRE-SAFE® system is activated:

IMPORTANT If emergency braking or panic after-braking is detected in the Electronic Stability Program control unit, there is no automatic engine stop by the ECO start/stop function.

IMPORTANT The PRE-SAFE® system is described in a separate function description.

Function sequence for standstill control (SSC) 

The standstill control monitors the braking driver assistance system DISTRONIC PLUS (with CODE 23P (Driving Assistance package Plus)) and the HOLD function below a speed of 3 km/h until the vehicle comes to a safe standstill. Within this vehicle speed range, the SSC is responsible for the transfer to a safe state when a securing event occurs.

When the vehicle is at a safe standstill, the rolling monitoring function of the SSC is requested by the driver assistance systems. If vehicle rolling is detected, the brake pressure is increased until the vehicle is stationary again. To ensure that a safe standstill is achieved, leakage compensation is also active. If the brake pressure measured by means of the circuit pressure sensors drops relative to the specified pressure request of the SSC, the specified pressure of the SSC is restored by means of an active pressure buildup.

Securing the vehicle by means of the SSC is necessary if the following events occur:

Door contact plausibility check 

The plausibility check detects any faults along the signal path from the left front door rotary tumbler microswitch or the right front door rotary tumbler microswitch (depending on global national coding for right-hand or left-hand drive vehicle), read in by the left front door control unit or the right front door control unit and sent over the interior CAN and chassis CAN 1, with the electronic ignition lock control unit interface to the Electronic Stability Program control unit. If, as the result of an electrical or mechanical fault, "Driver door closed" is detected by mistake although the driver door is open, this is detected by the door contact plausibility check in the SSC and a corresponding fault is stored in the Electronic Stability Program control unit.

SSK deactivation 

Deactivation of the SSK is signaled internally in the Electronic Stability Program control unit and externally over the chassis CAN 1. Activation of the driver assistance systems is no longer possible.

Securing of the vehicle is additionally requested immediately if the SSK is deactivated and a driver assistance system is active.

Securing the vehicle 

The vehicle is secured by the comfort securing system of the electric parking brake, which is prompted to lock by the SSC. After the locking request has been implemented by the Electronic Stability Program control unit, the vehicle is secured. The existing brake pressure at the wheels is reduced and the SSC becomes inactive.

If the close request is not implemented by the electric parking brake within a defined time or the electric parking brake signals a system fault to the SSC, the electric parking brake is detected as being defective and the safety strategy becomes active.

Safety strategy, request engage "P" or Emergency-P path 

The securing strategy is performed via the fully integrated transmission control unit. The Electronic Stability Program control unit transmits a request to engage gear range "P" to the fully integrated transmission control unit over the chassis CAN 1, powertrain control unit and the drive train CAN. The vehicle is secured once the fully integrated transmission control unit has engaged gear range "P" and following feedback to the Electronic Stability Program control unit. The existing brake pressure at the wheels is reduced and the SSC becomes inactive.

If the fully integrated transmission control unit does not signal engagement of gear range "P" within a defined time, the Emergency-P function of the electronic ignition lock control unit forces gear range "P" to be engaged.

For this purpose, the electronic ignition lock control unit also receives the request for engaging gear range "P" from the Electronic Stability Program control unit via chassis CAN 1 and itself transmits a request to the fully integrated transmission control unit. If feedback from the fully integrated transmission control unit is not received by the electronic ignition lock control unit within a defined time, the emergency-P path is energized by the electronic ignition lock control unit following an enable command from the fully integrated transmission control unit and the engagement of the "P" gear range is forced in the fully integrated transmission control unit.

Steer Assist function sequence 

Steer Assist is intended to help the driver achieve the optimal steering characteristics in critical situations. Steering assistance is provided with the aid of the electric power steering control unit and Electronic Stability Program control unit.

The following functions are performed here:

The current vehicle status is sensed by the Electronic Stability Program control unit. When required, the "steering assistance request" message is sent over chassis CAN 1 and chassis CAN 2 using the electronic ignition lock control unit interface to the electrical power steering control unit, which then actuates the actuator motor for the electric power steering accordingly.

Additional function requirements for Crosswind Assist 

Function sequence for Crosswind Assist 

The Crosswind Assist function assists the driver in the event of strong crosswinds and minimizes the vehicle's yaw response and the track offset. Through course-correcting brake interventions. The yaw rate sensor for lateral and longitudinal acceleration records the movement of the vehicle in the event of quickly changing winds and sends the signals to the Electronic Stability Program control unit over the vehicle dynamics CAN. The Electronic Stability Program control unit also processes the steering wheel angle from the steering wheel angle sensor over chassis CAN 1, which is read in by the steering column tube module control unit, the wheel speeds, brake pressures and the toothed rack position from the electrical power steering control unit.

System fault display function sequence 

The driver is informed about the system status and about faults by the following displays:

If the Electronic Stability Program control unit fails, basic braking without ABS is always available.

IMPORTANT The status of the brake fluid level switch is read in by the SAM control unit and sent via the interior CAN to the instrument cluster.

IMPORTANT In the semi-automatic parking space entry/exit function, the Electronic Stability Program control unit receives the information on the remaining distance to a defined obstacle from the parking system control unit (with CODE 235 (Active Parking Assist)) over the chassis CAN 1 and chassis CAN 2 with the electronic ignition lock control unit interface. The Electronic Stability Program control unit calculates the required braking torque and actuates the traction system hydraulic unit accordingly upon reaching the maneuvering points or the target parking position. The vehicle is braked.

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