GF42.45-P-0001SX Electronic Stability Program (ESP) Function

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





Function on example of an understeering vehicle (A)
If the vehicle understeers the drive torque is reduced as a first measure. If as a result the developing instability cannot be prevented, a brake pressure is built up at the left rear wheel. Due to the reduction in lateral stability and the build up of brake force at the left rear wheel a yawing moment occurs which causes the desired turning in of the vehicle. The speed decreases as a result of the brake force at the rear wheel, which also enhances stability.

Function on the example of an oversteering vehicle (B)
If the vehicle starts to oversteer, the drive torque is first reduced. If this cannot prevent the developing instability, brake pressure is built up at the right front wheel. This causes a yawing moment which counteracts the turning in motion of the vehicle. The speed decreases as a result of the brake force at the front wheel, which also enhances stability.

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
^ Reduction in the risk of skidding under all road surface conditions by automatic stabilization when braking, 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:
^ ABS: Avoidance of locking of individual wheels by reducing the brake pressure at the wheels in question
^ ASR: Prevents a too high slip of the driven wheels when starting off and accelerating due to torque reduction
^ MSR: Reduction in wheel slip of the drive wheels in overrun mode and as a result better directional stability and road adhesion

Component Identification:

Function overview
Brake forces, driving forces and side forces which act on the vehicle endeavor to turn the vehicle about the vertical axis. The ESP records the vehicle characteristics and for correction specifically brakes individual wheels or several wheels. 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 requirement (e.g. the steering angle sensor) and sensors which record the actual vehicle handling. These include:
^ Yaw rate, lateral and longitudinal acceleration sensor
^ Wheel speed sensor

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

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

For drive torque reduction a torque requirement is specified to the ME-SFI [ME] control unit or CDI control unit. 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 brake torque control circuit

- ABS control
- ASR control

^ ESP drive torque 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. The brake pressure is controlled via the ESP control unit.

ESP braking torque control circuit
ASR control

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

ESP drive torque control circuit
ASR control

To reduce a too large drive torque and as a result to achieve optimum traction, a drive torque reduction takes place via the chassis CAN between the ESP control unit and the ME-SFI [ME] control unit or CDI control unit. The ESP control unit checks constantly whether, the control function can be canceled for example, as a result of a sudden improvement in road adhesion. So the drive torque specified by the driver via the accelerator pedal sensor (B37) can be permitted again as soon as possible.

ESP drive torque control circuit
EBR control

If slip occurs at the drive wheels in overrun mode, this is detected by the ESP control unit. The signal is transmitted via the chassis CAN to the ME-SFI [ME] control unit or the CDI control unit. 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 take place without the ESP warning lamp being switched on.

ESP brake torque and drive torque control circuit
ESP control

If oversteer or understeer is detected, a calculated brake intervention is arranged at the appropriate wheel. This braking action deliberately counteracts the undesirable vehicle motion.

A signal to the ME-SFI [ME] control unit or CDI control unit via chassis CAN achieves the reduction in drive torque to suit the requirements by reducing the engine torque.

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 warning lamp (A1e41) in the instrument cluster which lights up permanently. 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 only controls through individual wheel brake interventions at the drive wheels, but not through influencing the engine management. ABS cannot be deactivated by the driver.

Optical status indication
The current condition of the ESP can be reported to the driver via the ESP warning lamp.

There are the following displays:
^ ESP warning lamp flashes

- ESP currently controlling
- ESP is switched off and there is slip at at least one wheel

^ ESP warning lamp lights up

- ESP is switched off there no slip at any wheel
- ESP is inoperative (fault), a corresponding text message is also output in the multifunction display (A1p13)