GF42.47-P-0001CE Adaptive Brake (ABR), Function
GF42.47-P-0001CE Adaptive brake (ABR), function
General function requirements
^ Circuit 61 ON
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 sensor for lateral and longitudinal acceleration (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)
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
^ Anti lock brake system (ABS) function sequence
^ Acceleration Slip Regulation (ASR) and Electronic Traction System (ETS) function sequence
^ Brake Assist (BAS) function sequence
^ Function sequence for Trailer Stability Assist (TSA) (with code (550) Trailer hitch)
^ Hill Start Assist function sequence
^ Function sequence for downhill speed regulation (on model 204.9 with code (430) Off-road package)
^ Function sequence for off-road mode (on model 204.9 with code (430) Off-road package) Function sequence for recharging depending on accelerator pedal actuation
^ Function sequence for recharging depending on lateral acceleration (air gap)
^ Dry braking function sequence
^ Passive ASR and ESP switching function sequence
^ Fault handling function sequence
^ System fault display function sequence
Additional function requirements for ESP
^ ESP active
Function sequence, ESP
ESP prevents the vehicle from breaking away when over steering or under steering. 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 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 steer angle of the front wheels (calculated from the steering wheel angle) are used to determine the lateral forces on the wheels. The steering wheel angle is acquired by the steering angle sensor. This angle is read in by the steering column tube module control unit (N80) and sent to the traction system hydraulic unit via the chassis CAN (CAN E).
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 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.
A distinction is made between under steering and over steering 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.
Via the chassis CAN, the Electronic Stability Program control unit sends a signal to the CDI control unit (N3/9) (with diesel engine) or ME-SFI [ME] control unit (N3/10) (with gasoline engine) requesting a reduction in drive moment. The engine power is then reduced 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 CDI control unit or ME-SFI [ME] control unit. The CDI control unit or ME-SFI [ME] control unit then sends the information via the drive train CAN to the electronic transmission control control unit (N15/3) (with transmission 722.6) or to the fully integrated transmission control control unit (Y3/8n4) (with transmission 722.9).
The electronic transmission control control unit or fully integrated transmission control control unit then suppresses the shift operation. The status of the electronic selector lever module control unit (N15/5) is read in by the CDI control unit or 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. Dynamometer test mode can also be activated via STAR DIAGNOSIS.
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. For this purpose, the Electronic Stability Program control unit sends a signal requesting a reduction in drive torque via the chassis CAN to the CDI control unit or ME-SFI [ME] control unit, which 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.
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.
TSA function sequence
TSA detects pendular swinging motion in a tractor/trailer combination based on the yawing vibrations caused in the tractor vehicle by the trailer.
The trailer recognition control unit (N28/1) detects a trailer and sends the status to the front SAM control unit with fuse and relay module (N10/1) via the interior CAN (CAN B).
The front SAM control unit places the information on the chassis CAN. This information is received by the Electronic Stability Program control unit.
If vehicle/trailer instability occurs, this is detected by the yaw rate, lateral and longitudinal acceleration sensor.
The Electronic Stability Program control unit receives this information via the vehicle dynamics CAN (CAN H).
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.
If several TSA interventions occur in quick succession, the simple stabilization intervention is no longer allowed by the system, regardless of the damping detected.
This prevents the tractor/trailer combination from constantly driving at a critical speed and prevents the brake system from being subjected to too much stress due to interventions in quick succession.
Active vehicle/trailer stabilization does not change the critical speed.
Additional function requirements for the Hill Start Assist
^ Transmission not in position "N" (with transmission 722)
^ 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.
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 directly 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.
Additional function requirements for speed control during downhill driving
^ v < 30 km/h
Function sequence for speed control during downhill driving
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 from v = 4 km/h to v = 18 km/h in increments of 2 km/h.
The system is activated using the Downhill Speed Regulation switch (N72/1 s24) in the upper control panel control unit (N72/1).
The activation is indicated by a light-emitting diode in the Downhill Speed Regulation switch.
The vehicle speed is then maintained in accordance with the speed setting (this is shown on the multifunction display of the instrument cluster).
While driving downhill, the speed setting can be varied via the cruise control lever (S40/4).
The request to activate the downhill speed regulation function is read in directly by the upper control panel control unit.
The upper control panel control unit transmits the signal on the interior CAN. The instrument cluster receives the signal and places this information along with the current speed setting on the engine compartment CAN.
The ESP control unit receives the signals and adjusts the vehicle speed to comply with the setting by means of brake interventions and appropriate requests to the engine control and transmission control systems. 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 accelerates the vehicle to a speed of v > 35 km/h using the accelerator pedal while the function is active, the downhill speed regulation function switches off automatically and the LED in the Downhill Speed Regulation switch goes out. A message also appears on the multifunction display of the instrument cluster and a warning tone sounds. If the downhill speed regulation function is switched off manually using the Downhill Speed Regulation switch, a message also appears in the multifunction display of the instrument cluster. A warning tone does not sound in this case.
Additional function requirements for off-road mode
^ Circuit 61 ON
^ Gear range "N"
^ active v < 40 km/h
Function sequence for off-road mode
The off-road mode partial function is comprised of the following sub functions:
^ Function sequence for activating the off-road mode
^ Function sequence for adaptation of the threshold values
^ Function sequence for engine torque support
Function sequence for activating off-road mode
The off-road mode is activated using the off-road program switch (N72/1s25) in the upper control panel control unit.
Activation is indicated by an illuminated light emitting diode in the off-road program switch. The off-road symbol also appears on the multifunction display of the instrument cluster.
Off-road mode is deactivated by pressing the off-road program switch again. The LED in the off-road program switch goes out and the indicator in the multifunction display disappears.
Adjustment of threshold values function sequence
By activating off-road mode:
- The thresholds for potential ESP intervention are widened i.e. ESP intervention only occurs under high vehicle dynamics.
- The ABS lockup phases are extended. For example, during a poor stretch of road the wheels are allowed to lock to reduce the braking distance (a loose surface supports a braking effect through a corresponding buildup of material in front of the front wheels).
- Ground with a low friction value is detected during ASR control. The ASR activation thresholds are varied depending on the tire characteristics to improve traction.
The above-specified changes with respect to ESP, ABS and ASR are dependent on speed.
This means that the threshold values when off-road mode is active 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 engine torque support
If braking torque can no longer be applied for at least one wheel (wheel is stationary because there is no load on it) with the downhill speed regulation activated, off-road mode activated and differential locks disengaged, brake pressure is built up actively at the wheels with traction via the Electronic Stability Program control unit.
Engine torque support is active up to a speed of v =18 km/h, coming from a lower speed.
Function sequence for recharging depending on accelerator pedal actuation
The recharging 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. recharging 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 recharging 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.
Passive ASR and ESP switching function sequence
The ASR and ESP functions can be activated passively via the ESP button (S6/1s5) in the dashboard switch group (S6/1). The status of the ESP button is read in by the front SAM control unit via the instrument panel LIN. The status is then sent to the Electronic Stability Program control unit via the chassis CAN.
If the system is activated passively, the Electronic Stability Program warning lamp lights up in the instrument cluster. The control thresholds are raised in the case of passive switching. ABS cannot be deactivated. ESP is always active during a brake application.
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:
- Anti lock 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.
