ESP braking moment control circuit, function

P42.45-2537-09

1 Stop lamp "on"

2 Special CAN bus for the sensor B24/15 (dynamics CAN)

A1 Instrument cluster

A1e17 ABS indicator lamp

A1e41 ESP warning lamp

A1p13 Multifunction display

A7/7 BAS brake booster

A7/7b1 BAS diaphragm travel sensor

A7/7s1 BAS release switch

A7/7y1 BAS solenoid valve

B24/15 Micromechanical yaw rate sensor AY pickup

N72/1s1 ESP OFF switch

N80 Steering column module

N93 Central gateway control unit

S11 Brake fluid indicator switch

S12 Parking brake indicator switch

Y3/8 Electric control unit (VGS)

CAN B Controller Area Network bus Class B (interior compartment) (CAN-B)

CAN-C Controller Area Network bus Class C (engine compartment) (CAN-C)

B34 ESP brake pressure sensor

B37 Accelerator pedal sensor

L6/1 Left front rpm sensor

L6/2 Right front rpm sensor

L6/3 Left rear rpm sensor

L6/4 right rear rpm sensor

N3/10 ME-SFI [ME] control unit

N10 Front SAM control unit

N10/8 Rear SAM control unit

N47-5 ESP control unit

N49 Steering angle sensor

N51 AIRmatic with ADS control unit

N72/1 Upper control panel control unit

Electronic Stability Program
(ESP) regulates

 when cornering (vehicle understeers or oversteers),

 When driving straightahead (vehicle deviates from course due to uneven road conditions).

In order to be able to perform these control intervention functions, an expanded sensor system is required compared to acceleration slip regulation (ASR).

A distinction is generally made between:

 Signals which convey the driver's requirement .

 Steering angle sensor signal

 Accelerator pedal sensor signal

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 Signals which convey the effective vehicle characteristics.

 Signals (yaw rate and lateral acceleration) from micromechanical yaw rate sensor AY pickup

 Signal (brake pressure) from brake pressure sensor

 Signals (wheel speeds) from left front rpm sensor, right front rpm sensor, left rear rpm sensor, right rear rpm sensor

 Signals (transmission shift stages) from electric control unit (VGS)

The wheel speeds, rotation angle, vehicle yaw rate, lateral acceleration and brake pressure of the individual wheels are recorded and processed in the ESP control unit.

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The ESP control unit is connected to the control units of the engine/transmission management via the Controller Area Network bus Class C (engine compartment). This digital line connection makes possible a rapid data exchange between the ESP control unit, ME-SFI [ME] control unit and the electric control unit (VGS).

The ESP control unit is continuously informed with the current data on engine torque, accelerator pedal position and transmission ratio. The forces which want to turn the vehicle about the center of gravity are detected via a micromechanical yaw rate sensor AY pickup.

The longitudinal and lateral forces acting on the wheels can be calculated by means of the data acquisition described. If these values exceed certain control thresholds, the corresponding control valves are actuated by the ESP control unit in order to specifically regulate defined brake pressure at one or more wheels.

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Commands for torque requirement transmitted via CAN-C to the ME-SFI [ME] control unit and the electric control unit (VGS) cause a drive torque reduction, in addition a downshift is prevented by the electric control unit (VGS). The precise and accurately proportioned intervention is completed within a few fractions of a second. An active intervention by the brake system and drive torque reduction by the ESP take care of optimum vehicle stability.

The following processes are performed:

Start-off assist

If the driver changes from the brake pedal to the accelerator pedal, the vehicle may roll away in the opposite direction to the gear stage engaged as of a certain ascent or as of a certain descent. A built-in inclination sensor detects the exceeding of a set ascent or a set descent. With the aid of a booster the brake pressure is held until the accelerator pedal is pressed.

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Antilock brake system (ABS)

If, for example, a wheel tends to lock during braking, the brake pressure in this wheel brake is regulated. The regulation in the traction system hydraulic unit (A7/3) takes place via control valves by the pressure buildup, pressure holding and depressurization control phases.

Electronic traction system (ETS)

To brake the spinning wheel (slippery surface), brake pressure is led into the brake caliper of the spinning wheel (pressure buildup) via the pressure system of the traction system hydraulic unit. As a result the other wheel can transmit driving power again (locking differential effect).

The brake torque is regulated by pressure buildup, pressure holding and depressurization via control valves in the traction system hydraulic unit.

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Acceleration slip regulation (ASR)

In order to prevent the drive wheels spinning due to too large drive torque for the surface, the drive torque is reduced via the ME-SFI [ME] control unit. The ESP control unit detects this condition by evaluating the wheel sensor signals and conveys this to the engine control unit via CAN C.

The ESP control unit continuously checks whether the control functions can be canceled owing to, for example, a sudden improvement in road grip so that the drive torque requested by the driver via the gas pedal can then be made available again.

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Engine braking regulation control (MSR)

If wheel slip occurs at the driving wheels due to engine brake torque when the driver releases the accelerator pedal, this is likewise detected by the ESP control unit. The signal is reported to the ME-SFI [ME] control unit via CAN C. On the basis of this information, wheel slip is reduced by increasing the drive torque, and the side stability of the vehicle is thus increased. This process takes place without informing the driver (ESP warning lamp (A1e41)).

Electronic Stability Program (Electronic-Stability-Program) (ESP)

If oversteer or understeer is detected, a calculated intervention by the brake system is performed at the front or rear axle via the ESP control unit. This intervention by the brake system counteracts the undesired driving behavior.

The drive torque reduction to suit the requirements effected by reducing the engine torque is achieved by a signal to the ME-SFI control unit via CAN C.

Electronic brake force distribution (EBV)

A dynamic center of gravity shift occurs due to braking, the vehicle center of gravity moves forwards. The wheels on the rear axle are relieved of load and tend to overbrake as a result. The lack of lateral support at the rear wheels means that an unstable driving condition would occur. This condition is recorded by the ESP control units by means of wheel sensors. The ESP control unit now reduces the brake pressure at the rear wheels and as a result prevents overbraking of the rear wheels.