Electronic stability program (ESP) function - GF42.45-P-0001TM

MODEL 211.0/2, 219.3 as of 1.6.06 except CODE (494) USA version except CODE (498) Japan version 

MODEL 211.0/2, 219.3 as of 1.7.06 with CODE (494) USA version with CODE (498) Japan version 

Schematic representation 

Fig 1: Identifying Understeering And Oversteering Vehicle

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Example of understeering vehicle (A) 

A vehicle understeers on a left curve. The vehicle pushes outwards over the front wheels. Precisely calculated braking force is applied to the left rear wheel.

Example of oversteering vehicle (B) 

A vehicle oversteers in a left curve. In the process the rear of the vehicle breaks away. A precisely calculated braking action is performed on the right front wheel.

General 

The Electronic Stability Program (ESP) is an active safety system for improving the vehicle stability in all driving situations. It operates by individual brake intervention at one or more wheels on the front or rear axle. The ESP stabilizes the vehicle when cornering, when braking or when rolling without drive and keeps it safely on course. It supplements the familiar functions of the antilock brake system (ABS), the acceleration slip regulation (ASR) and the engine braking regulation (MSR).

The function of the stability control overrides the ABS and ASR control systems.

In addition to active ESP brake intervention there is also an influence on the engine/transmission management

The ESP controls under the following driving conditions:

• when cornering (vehicle under- or oversteered)
• when driving straight ahead (vehicle deviates off course due to uneven road conditions)

Advantages of ESP:

• Improvement in the starting-off and acceleration capability due to increased traction, particularly on a road with different adhesion and on curves
• Automatic adaptation of the engine torque depending on the traction condition of the wheels
• Reduces the risk of skidding under all road surface conditions through automatic stabilization when braking, when accelerating or rolling
• Improvement in the lane stability of the vehicle when cornering, also up to the critical limits
• Increase in active driving dynamics safety, as only one wheel which is not spinning permits the optimum traction without loss of lateral stability

The ESP contains the following systems:

• Anti-lock system (ABS)

  Avoids the individual wheels locking by reducing the brake pressure at the wheels in question

• Acceleration slip regulation (ASR)

  Prevents a too high slip of the drive wheels when starting off and accelerating by torque reduction

• Engine braking regulation (MSR)

  Reduces wheel slip of the drive wheels in overrun mode and as a result better directional stability and road adhesion

  Fig 2: View Of Electronic Stability Program Function Diagram

  Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Function overview 

Brake forces, drive forces and side forces acting on one side, which act on the vehicle from outside endeavor to turn the vehicle about its vertical axis. The ESP detects the vehicle handling and selectively brakes individual wheels for correction. It regulates when cornering (vehicle understeers or oversteers) and when driving straight ahead (vehicle deviates from course due to dissimilar road conditions).

Sensor system 

A distinction is made between sensors which detect the driver's wish

(e.g. the steering angle sensor) and sensors which record the actual vehicle handling. These include:

• Yaw rate and lateral acceleration sensor
• Wheel speed sensor

The ESP control unit is constantly supplied with the current data on engine torque and transmission ratio.

Via the yaw rate and lateral acceleration sensor the brake forces, drive forces and side forces which want to turn the vehicle about the perpendicular axis are recorded (yaw rate). The torque acting on the vehicle can be calculated by the information listed. If this value exceeds certain thresholds then the corresponding wheels are braked by the ESP control unit to neutralize the torque which is acting.

At the same time commands are passed on to the ME-SFI [ME] control unit or CDI control unit and VGS control unit or ETC [EGS] control unit via CAN C. For drive torque reduction a torque requirement is given to the ME-SFI [ME] control unit or CDI control unit. If necessary a downshift is prevented by the VGS control unit or ETC [EGS] control unit. The selective and accurately modulated intervention takes place in a few fractions of a second.

Active brake intervention and drive torque reduction by the ESP ensure optimum vehicle stability. The following processes are performed:

• ESP braking torque control circuit
  • ABS control
  • ASR control
• ESP drive moment control circuit
  • ASR control
  • MSR control

ESP braking torque control circuit 

ABS control

If one wheel tends to lock, the brake pressure at this wheel is reduced. Brake pressure is controlled via the ESP control unit.

ESP braking torque control circuit 

ASR control

To brake the spinning wheel the brake pressure at the spinning wheel is increased via the ESP control unit. A locking differential effect can be achieved as a result.

ESP drive torque control circuit 

ASR control

In order to reduce an excessive drive torque and as a result to achieve optimum traction, a drive torque is reduced between the ESP control unit and the ME-SFI [ME] control unit or the CDI control unit via CAN C. The ESP control unit constantly checks whether the control functions can be canceled, e.g. as a result of a sudden improvement in road surface adhesion. So the drive torque specified by the driver via the accelerator pedal can be permitted again in good time.

ESP drive torque control circuit 

EBR control

If wheel slip occurs at the drive wheels in overrun mode, this is detected by the ESP control unit. The signal is reported to the ME-SFI [ME] control unit or to the CDI control unit via CAN C. With this information wheel slip is reduced by increasing drive torque and as a result the lateral control of the vehicle is increased. This process takes place without switching on the ESP warning lamp.

ESP brake torque and drive torque control circuit 

ESP control

If oversteer or understeer is detected, a calculated brake intervention is arranged at the corresponding wheel via the ESP control unit. This braking action deliberately counteracts the undesirable vehicle motion. By a signal via CAN C to the ME-SFI [ME] control unit or to the CDI control unit the drive moment reduction to suit the requirements is achieved by engine torque reduction.

ESP OFF mode 

The response threshold for ESP can be set up via the ESP OFF switch. The ESP "shutoff" is displayed by the ESP and ABS warning lamp lighting up constantly in the (A1e41)instrument cluster (A1). This can provide better traction (grinding effect) in deep snow or when snow chains are fitted. ESP is always active during a brake application. ASR still only regulates through individual wheel brake interventions at the drive wheels, but not by influencing the engine management. ABS cannot be deactivated by the driver.

Visual status indication 

The current condition of the ESP is reported back to the driver via the ESP warning lamp.

There are the following displays:

• ESP warning lamp flashes
  • ESP just regulates
  • ESP is switched off and at least one wheel has slip
• ESP warning lamp lights up
  • ESP is switched off and no wheel has slip
  • ESP is inoperative (fault), a corresponding text message is output in the multifunction display (A1p13)