Brakes and Traction Control: Description and Operation
GF42.47-P-0001CEA Adaptive Brake (ABR), Function
MODEL 204.077/ 277/ 377
Function requirements, general
^ Engine running
^ Drivetrain ready
Adaptive Brake (ABR), general
ABR assists the driver in dangerous situations which occur suddenly and thus serves active safety.
To do so, the Electronic Stability Program control unit (N30/4) evaluates the data from the following components in order to detect the current driving situation:
- Yaw rate, lateral and longitudinal acceleration sensor (B24/15)
- Left front axle rpm sensor (L6/1)
- Right front axle rpm sensor (L6/2)
- Left rear axle rpm sensor (L6/3)
- Right rear axle rpm sensor (L6/4)
- Steering angle sensor (N49)
- Stop lamp switch (S9/1)
ABR on AMG vehicles is only available as the ABR Premium system (with or without code (233) DISTRONIC PLUS).
The status of the brake light switch is read in by the Electronic Stability Program control unit and directly looped through to the rear SAM control unit with fuse and relay module (N10/2) to allow brake light actuation.
The ABR system is comprised of the following partial functions:
^ Electronic Stability Program (ESP) function sequence
^ Antilock brake system (ABS) function sequence
^ Acceleration Slip Regulation (ASR) and Electronic Traction System (ETS) function sequence
^ Function sequence for ABS, ASR and ESP sport or OFF
^ Brake Assist (BAS) function sequence
^ BAS PLUS (with code (233) DISTRONIC PLUS)
^ Hill Start Assist function sequence
^ Function sequence for precharging depending on accelerator pedal actuation
^ Function sequence for precharging depending on lateral acceleration (air gap)
^ Dry braking function sequence
^ Fault handling function sequence
^ System fault display function sequence
^ Failed Boost function sequence (with code (B03) ECO start/ stop function)
^ Over Boost function sequence (with code (B03) ECO start/ stop function)
Additional function requirements for ESP
^ ESP active
Function sequence, ESP
ESP prevents the vehicle from breaking away when oversteering or understeering. 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 Electronic Stability Program control unit processes the following measured quantities to determine the vehicle behavior:
- Yaw velocity
- Steering angle
- Brake pressure
- Engine torque
- Transmission stage
- Lateral acceleration
The sideslip 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 steer angle of the front wheels (calculated from the steering wheel angle) are used to determine the lateral forces on the wheels. The rotation angle is recorded by the steering angle sensor, this is then read in by the steering column tube module control unit (N80) and sent over the chassis CAN (CAN E) to the traction system hydraulic unit.
The longitudinal forces on the wheels are calculated using the engine torque, transmission stage and brake pressure at each wheel. If the measured yaw angle velocity does not match the specified value or if the determined sideslip 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.
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 at this point the driver depresses the accelerator pedal, the drive torque is first of all reduced.
If this is not enough or the accelerator pedal is not depressed, the incipient instability is prevented by brake application at up to three wheels:
Stage 1: Brake pressure reduction at inner rear wheel
Stage 2: Stage 1 plus brake pressure buildup at outer rear wheel
Stage 3: Stage 2 plus brake pressure buildup at inner front wheel
Depending on the brake force a torque occurs which leads to the turning in of the vehicle for a simultaneous reduction in speed. This produces a considerable stabilizing effect.
The Electronic Stability Program control unit sends a signal requesting a reduction in drive torque via the chassis CAN to the ME-SFI [ME] control unit (N3/10) which reduces the engine power accordingly.
A pending shift operation is suppressed for the duration of control intervention.
For this purpose, the Electronic Stability Program control unit sends a signal via the chassis CAN to the ME-SFI [ME] control unit, which sends the information to the fully integrated transmission control unit (Y3/8n4) via the drive train CAN (CAN C). The fully integrated transmission control unit then suppresses the shift operation. The status of the electronic selector lever module control unit (N15/5) is read in by the ME-SFI [ME] control unit via the drive train CAN.
Exhaust test/dynamometer mode:
For vehicle test purposes, the ABR can be set to dynamometer test 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 Electronic Stability Program warning lamp (A1e41) and the antilock brake system indicator lamp (A1e17) in the instrument cluster light up. In addition, a message is displayed in the multifunction display (A1p13) of the instrument cluster.
The roller test mode can also be activated through the ISE - tester".
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 Electronic Stability Program control unit on the basis of the signals from the rpm sensors, the brake pressure is reduced at the appropriate brake cylinder until the wheel begins to turn again.
Function sequence for ASR and ETS
ASR and ETS prevent the drive wheels from spinning when driving. In addition it causes improved driving stability for increased traction potential over the entire 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. The Electronic Stability Program control unit sends a signal requesting a reduction in drive torque via the chassis CAN to the ME-SFI [ME] control unit which then reduces the engine power accordingly.
A check is continuously performed to establish whether the drive torque specified by the driver via the accelerator pedal sensor (B37) 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.
Function sequence for ABS, ASR and ESP Sport or OFF
ESP can be operated in three different operating modes. The operating states are selected by pressing the ESP button (S6/1s5) in the instrument panel switch group (S6/1). The ESP button only has to be pressed briefly to switch between ESP ON and ESP Sport mode. The ESP button has to be pressed for a longer time of t > 2.5 s to switch off the ESP. The operating states are:
- ESP ON:
ABS, ASR and ESP operate as on series production vehicles with adjusted control thresholds.
The Electronic Stability Program warning lamp (A1e41) and the ESP OFF indicator lamp (A1e30) in the instrument cluster are unlit; the Electronic Stability Program warning lamp flashes during ESP intervention.
- ESP Sport:
ABS operates as on series production vehicles with adapted control thresholds.
When engine torque control is active, ASR operates with significantly reduced intervention strength and adjusted single-sided brake system interventions to improve traction. ESP operates as with reduced intervention strength and adapted control thresholds.
The Electronic Stability Program warning lamp in the instrument cluster remains lit; it flashes during ESP intervention. The "ESP SPORT" warning message appears in the multifunction display.
- ESP OFF:
ABS is configured for maximum deceleration. Ride comfort is of secondary importance.
ASR operates without engine torque control. Only the single- sided brake system interventions to improve traction are active. ESP is completely deactivated, no engine torque or brake system interventions are made.
The Electronic Stability Program warning lamp and the ESP OFF indicator lamp in the instrument cluster remain lit. The Electronic Stability Program warning lamp flashes during ESP intervention. The "ESP OFF" message appears in the multifunction display.
All Pre-Safe functions are deactivated in ESP OFF mode.
Brake Assist (BAS) function sequence
BAS detects emergency braking situations from rapid actuation of the brake pedal and, if necessary, increases the brake pressure in order to achieve maximum possible deceleration. For this purpose, the Electronic Stability Program control unit evaluates the increase in pressure in the brake system and initiates an emergency stop if a certain triggering threshold is exceeded.
BAS PLUS (with code (233) DISTRONIC 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 the Hill Start Assist
^ Transmission not in position "N"
^ Vehicle not secured with parking brake
^ Incline detected with vehicle at standstill
Hill Start Assist function sequence
Hill Start Assist prevents the vehicle from rolling back opposite the direction of the engaged gear range when starting off during the time it takes for the driver to move his/her 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.
This information is sent to the Electronic Stability Program control unit via the vehicle dynamics CAN (CAN H).
The Electronic Stability Program control unit reads in the status of the brake light switch directly and in this way detects the status of the brake pedal.
The status of the parking brake is detected via the parking brake indicator switch (S12). This status is read in by the front SAM control unit and sent to the Electronic Stability Program control unit via the chassis CAN.
The brake pressure applied by the driver is maintained in the brake calipers by the traction system hydraulic unit (A7/3).
Once the brake pedal is released, the brake pressure is modulated based on the balance of torques (downward torque due to slope, braking torque and drive torque).
When the torque is sufficient to move off, the Hill Start Assist function is deactivated and the vehicle starts off.
However, if the driver does not actuate the accelerator pedal, the pressure in the brake calipers is reduced to zero after t = 0.8 to 1.5 s.
Hill Start Assist is not a HOLD function.
Function sequence for precharging depending on accelerator pedal actuation
The precharging function depending on accelerator pedal actuation applies the brake pads to the brake disks when a supposed emergency braking situation is 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 from the release gradient of the accelerator pedal which is formed from the "accelerator pedal position" signal.
If a rapid release of the accelerator pedal is detected, the system assumes that the driver intends to brake. Precharging depending on accelerator pedal actuation is activated and a brake pressure of p = 2 to 3 bar is requested.
In order to adapt the calculated triggering threshold of the accelerator pedal 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 (air gap)
After dynamic cornering, the air gap function applies the brake pads to the front axle brake disks.
Consequently, the air gap between the brake pad and brake disk is compensated in order to receive the required braking power at the corresponding wheel as quickly as possible in the event of ESP intervention by the brake system.
Additional function requirements - dry braking
^ No brake intervention by a control system
^ Vehicle speed v > 30 km/h
^ Number of wiper cycles > 600
The status of the wiper system (WSA) is sent to the Electronic Stability Program control unit by the front SAM control unit via the chassis CAN.
Dry braking function sequence
Dry braking assists the performance capability of the front axle brake under wet conditions. The brake pads are applied cyclically for t = 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.
Fault handling function sequence
The range of functions can vary depending on the received signals.
The following table shows function deactivation owing to a component failure or defect.
System fault display function sequence
The driver is informed of the system status and any faults by means of the following:
- Antilock brake system indicator lamp
- Electronic Stability Program warning lamp
- Message in instrument cluster
If the Electronic Stability Program control unit fails, basic braking without ABS is always available.
The status of the brake fluid level switch (S11) and parking brake indicator switch is read in by the front SAM control unit and sent via the interior CAN to the instrument cluster.
Failed Boost function sequence (with code (B03) ECO start/ stop function)
The Failed Boost function serves to safeguard a sufficiently high brake pressure level in the system in the event of a shortage of brake boosting by the BAS brake booster.
If during a brake application by the driver, the brake vacuum pressure sensor (B64/1) (with code (B03) ECO start/stop function) recognizes that the BAS brake booster is not providing any driver assistance, then Failed Boost provides driver assistance through actuating the high pressure and return flow pump (A7/3m1). Hydraulic boosting occurs with a suitable boost factor.
Over Boost function sequence (with code (B03) ECO start/stop function)
If the brake booster reaches the limits of its pneumatic boost, the brake pressure is increased through actuation of the high pressure and return flow pump, thereby ensuring that the usual level of brake boost is then provided.
