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Home >> Mercedes Benz >> 2019 >> GLC300 Base >> Repair and Diagnosis >> Brakes >> Brakes Control Systems >> Brakes - Hydraulic & Mechanical Systems - 253 Chassis - 1 Of 2 >> Basic Knowledge >> Regenerative Braking System, Function - GF42.22-P-0001RFH

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

Model 205.012/013/047/053/054/147/212/213/247/253, 253.354/954/99 

G12819782Courtesy of MERCEDES-BENZ USA

Function requirements, general 

Regenerative braking system (RBS), general 

The regenerative braking system (RBS) assists the driver in suddenly occurring dangerous situations, and thus ensures active safety. During regenerative braking, a portion of the braking energy is generated solely through alternator operation of the electric machine. The vehicle's kinetic energy is converted to electrical energy and then used to charge the high-voltage battery. Once the high-voltage battery is fully charged, braking is then through hydraulic operation only. Regenerative braking is then not permitted.

The Electronic Stability Program control unit evaluates the following variable to implement the function:

The Electronic Stability Program control unit directly reads in the status of the electric parking brake switch.

The Electronic Stability Program control unit as master control unit for the brake system and the CDI control unit or the ME-SFI [ME] control unit as master control unit for the high voltage on-board electrical system management, communicate continuously with each other over the chassis FlexRay, powertrain control unit and the engine CAN. Depending on the charge level of the high-voltage battery, the CDI control unit or the ME-SFI [ME] control unit, sends information about the maximum possible regenerative braking torque over the engine CAN, powertrain control unit and the chassis FlexRay to the Electronic Stability Program control unit. The Electronic Stability Program control unit calculates the calculated regenerative portion of the total braking torque and then requests it over the chassis FlexRay, powertrain control unit and the engine CAN from the CDI control unit or the ME-SFI [ME] control unit. The CDI control unit or the ME-SFI [ME] control unit calculates the specified torque for the electric machine, and requests it over the engine CAN, powertrain control unit and the hybrid-CAN from the power electronics control unit. The power electronics control unit actuates the electric machine accordingly. The energy generated by the electric machine is used to charge the high-voltage battery. Information on the charge level of the high-voltage battery is sent by the battery management system control unit over the hybrid CAN, powertrain control unit and the engine CAN to the CDI control unit or the ME-SFI [ME] control unit.

In the event of an emergency braking or a fault message (drive train fault or faults that initiate a deactivation of the anti-lock braking system (ABS)), the braking torque is then generated hydraulically only. The RBS encompasses the following functions:

Additional function requirements for radar-based recuperation (model 205.047/147/247, 253.354/954 with code 258 (COLLISION PREVENTION ASSIST) or code 23P (Driving Assistance Package)) 

Radar-based recuperation (model 205.047/147/247, 253.354/954 with code 258 (COLLISION PREVENTION ASSIST) or code 23P (Driving Assistance Package)) 

When driving in the ECO transmission mode, the distance to a vehicle driving in front is determined by the radar sensor system. If the vehicle in front reduces its speed within certain limits, the driver's vehicle engages in regenerative braking. I.e. the electric machine is used as an alternator, charges the high-voltage battery and slows the vehicle down through alternator mode.

On vehicles with code 258 (COLLISION PREVENTION ASSIST) the COLLISION PREVENTION ASSIST controller unit sends information on the distance and the speed differential to the vehicle driving in front over the peripherals CAN, powertrain control unit and the engine CAN to the ME-SFI [ME] control unit.

On vehicles with code 23P (Driving Assistance Package) the radar sensors control unit Sends information on the distance and speed differential to the vehicle driving in front over the suspension FlexRay, powertrain control unit and the engine CAN to the ME-SFI [ME] control unit.

This information is used by the ME-SFI [ME] control unit to calculate the deceleration torque (negative acceleration).

Under the following conditions, deceleration is introduced through regenerative braking:

The ME-SFI [ME] control unit calculates the specified torque for the electric machine, and requests it over the engine CAN, powertrain control unit and the hybrid CAN from the power electronics control unit. The power electronics control unit actuates the electric machine accordingly.

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 for this. Furthermore, the engine torque is reduced to increase the driving stability.

The Electronic Stability Program control unit evaluates the following variables 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 arises that leads to the vehicle turning in with a simultaneous speed reduction, leading to a considerable stabilizing effect. The Electronic Stability Program control unit sends a signal to reduce engine torque over the chassis FlexRay, powertrain control unit and the engine CAN to the CDI control unit or the ME-SFI [ME] control unit, which then 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 via chassis FlexRay, powertrain control unit and drive train CAN. The latter control unit suppresses the shift operation.

EBD 

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 determined using the yaw rate (speed of vehicle rotation about vertical axis).

The yaw rate, the lateral acceleration and the steer 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, the transmission stage and the 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 build up or a reduction in brake force for the relevant wheel. The resulting forces stabilize the vehicle.

Exhaust-gas test, dynamometer test mode 

For vehicle test purposes, the RBS can be set to the exhaust-gas test, roller dynamometer mode if the workshop menu is activated through the instrument cluster whereupon the engine is then started. ESP®, ABS and ASR are switched to passive. The Electronic Stability Program warning lamp and the anti-lock brake system indicator lamp in the instrument cluster light up. Additionally, a corresponding message is shown in the instrument cluster.

The instrument cluster sends the status of the exhaust test and roller dynamometer mode via the user interface CAN, electronic ignition lock control unit and chassis FlexRay to the Electronic Stability Program control unit.

IMPORTANT The exhaust-gas test, roller dynamometer mode can also be activated using the diagnostic device.

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 

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

ASR, electronic traction system 

The ASR and electronic traction system functions prevent the drive wheels from spinning when in driving mode. In addition, they result in improved driving stability with an increased traction potential over the entire vehicle speed range. The Electronic Stability Program control unit detects the spinning of the drive wheels using the signals of the rpm sensors. If a drive wheel spins, its drive torque is reduced. Targeted interventions by the brake system at the spinning wheel transfer the drive torque to the opposite wheel. If necessary, the engine torque is reduced. To do so, the Electronic Stability Program control unit sends the "Reduce drive torque" request via chassis FlexRay, powertrain control unit and engine CAN to the CDI control unit or the ME-SFI [ME] control unit. The CDI control unit or the ME-SFI [ME] control unit reduces the engine power accordingly.

A check is continuously performed of whether the drive torque specified by the driver can be permitted again, e.g. due to an improvement in road surface adhesion.

BAS 

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

BAS PLUS (with CODE 23P (Driving assistance package)) 

The BAS PLUS function increases the brake pressure depending on the speed of the brake pedal actuation and the distance of the vehicle driving in front.

Hill Start Assist 

The Hill-Start Assist prevents the vehicle from rolling back when driving off when the driver switches from the brake pedal to the accelerator pedal.

The function is triggered automatically when the Electronic Stability Program control unit detects an uphill or downhill gradient when the vehicle is at a standstill which would cause the vehicle to roll back. The Electronic Stability Program control unit reads in the signal from the pedal angle sensor directly and by doing so it detects the status of the brake pedal.

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 the 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 operate the brake pedal, the pressure in the brake system is reduced after 1 s.

The CDI control unit or the ME-SFI [ME] control unit sends information about the accelerator pedal position via drive train CAN, powertrain control unit and chassis FlexRay to the Electronic Stability Program control unit.

In the following situations, the Hill-Start Assist is immediately interrupted:

In the following situations, the Hill-Start Assist is non-functional:

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

Adaptive brake lights, flashing (except with CODE 494 (USA version)) 

In the event of emergency braking, the Electronic Stability Program control unit sends a corresponding signal via chassis FlexRay, electronic ignition lock control unit and interior CAN to the rear SAM control unit. The rear SAM control unit then actuates for the duration of the signal (for at least 1 s) all the brake lights on the vehicle at a flasher frequency of 5 Hz. If the vehicle comes to a standstill from a speed in excess of 70 km/h (not urban traffic), the brake lights 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, the hazard warning system is automatically switched off again.

Additional function requirements - dry braking 

IMPORTANT The front SAM control unit sends the status of the windshield wiper system via interior CAN, electronic ignition lock control unit and chassis FlexRay to the Electronic Stability Program control unit.

Dry braking 

The dry braking function enhances the performance of the brakes on the front axle in wet driving conditions. Cyclical application of the brake pads for a brief period (approx. 0.5 s) at a brake pressure of 1 bar, ejects the water film from the brake disk. This improves the response time of the brake.

Precharging, depending on accelerator pedal actuation 

Upon detection of a possible emergency braking situation, the brake pads are brought into contact with the brake disks. The clearance between the brake lining 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 2 to 3 bar is requested.

IMPORTANT In order to adapt the calculated activation 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.

Precharging, depending on lateral acceleration 

During dynamic cornering, the brake linings are brought into contact with the front axle brake disks. The air gap between the brake lining and brake disk is therefore compensated to achieve the required braking power at the corresponding wheel as quickly as possible during braking.

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 Electronic Stability Program control unit sends the status of the HOLD function via the chassis FlexRay, electronic ignition lock control unit and user interface CAN to the instrument cluster.

The HOLD function can be activated under the following conditions:

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 (SSC) to match the respective situation.

The HOLD function is automatically switched off under the following conditions:

Switch ESP® passive 

The ESP® function can be switched to passive mode using the "Assistance" menu and the "ESP®" submenu in the instrument cluster when in driving mode.

The instrument cluster sends the corresponding request over the user interface CAN, electronic ignition lock control unit and the chassis FlexRay to the Electronic Stability Program control unit. If the ESP® function is switched to passive, the Electronic Stability Program warning lamp comes on in the instrument cluster. The control thresholds are raised when the ESP® is in passive mode. ABS cannot be deactivated. ESP® is always active during a brake application.

Additional function requirements for ESP® trailer stabilization (with code 550 (Trailer hitch)) 

IMPORTANT The trailer recognition control unit sends the Trailer detected status via interior CAN, electronic ignition lock control unit and chassis FlexRay to the Electronic Stability Program control unit.

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 by the Electronic Stability Program control unit based on the signals from the supplemental restraint system control unit. The Electronic Stability Program control unit receives this information over the vehicle dynamics CAN.

IMPORTANT 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.

PRE-SAFE® system (with CODE 23P (Driving assistance package)) 

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

SSC 

The standstill control (SSC) monitors the braking driver assistance systems (with code 23P (Driving Assistance Package)) 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. On vehicles with Premium ADAPTIVE BRAKE (with code 23P (Driving Assistance Package)) a leakage compensation function is also active throughout the safe standstill condition. 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:

The driver is considered absent if the driver door is open and the driver seat belt is not fastened. The supplemental restraint system control unit sends the status of the seat belt via vehicle dynamics CAN to the Electronic Stability Program control unit. The left front door control unit sends the status of the left front door (open or closed) via interior CAN, electronic ignition lock control unit and chassis FlexRay to the Electronic Stability Program control unit.

Door contact plausibility check 

The plausibility check is used to detect a fault in the signal path. If, as the result of an electrical or mechanical fault, "Driver door closed" is wrongly detected although the driver door is open, this is detected by the door contact plausibility check and a corresponding fault is stored in the Electronic Stability Program control unit.

SSC deactivation 

Deactivation of SSC is communicated internally in the Electronic Stability Program control unit and externally via the suspension FlexRay. Activation of the driver assistance systems is no longer possible. Securing of the vehicle is additionally requested immediately if the SSC is deactivated and a driver assistance system is active.

Securing the vehicle 

The vehicle is secured by the comfort securing system of the electronic 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 electronic parking brake within a defined time or the electronic parking brake signals a system fault to the SSC, the electronic 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 through 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 FlexRay, powertrain control unit and the drive train CAN. The vehicle is secured after gear range "P" is engaged 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 the suspension FlexRay and itself transmits a request to the fully integrated transmission control unit. If the electronic ignition lock control unit does not receive any feedback from the fully integrated transmission control unit within a specific period of time, the Emergency-P path is energized by the electronic ignition lock control unit after power disconnect, and gear range "P" is then forcibly engaged by means of a spring mechanism.

Steer Assist 

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. If necessary, the message "Steer Assist request" is sent over the chassis FlexRay to the electrical power steering control unit that actuates the electric power steering actuator motor accordingly.

Additional function requirements for Crosswind Assist 

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 Electronic Stability Program control unit uses the following signals to determine a wind interference (e.g. wind gusts) which is reacted to with a one-sided brake intervention:

System fault display 

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 Electronic Stability Program control unit reads in the signals from the brake vacuum sensor directly.

IMPORTANT The status of the brake fluid level switch is read in by the front SAM control unit and sent over the interior CAN, electronic ignition lock control unit and the user interface CAN to the instrument cluster.

IMPORTANT During semi-automatic parking and driving out of a parking space, the Electronic Stability Program control unit receives information about the remaining distance to the defined obstacle from the parking system control unit via the suspension FlexRay. 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.

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 pneumatic brake booster failure is detected by the brake vacuum sensor, which is integrated into the pneumatic brake booster. The Electronic Stability Program control unit reads in the signals from the brake vacuum sensor directly.

The fail boost function performs hydraulic brake boosting using a suitable boost factor and for this purpose actuates the corresponding components in the traction system hydraulic unit.

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