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

GF42.45-P-0001M Electronic stability program (ESP) function
- with CODE (472a) Electronic Stability Program (ESP)

Electronic Stability Program (ESP), function:

The "Electronic Stability Program" (ESP) is an active safety system which improves vehicle stability in all driving situations.

It operates by actuating the brakes individually on one or more wheels on the front or rear axle. ESP stabilizes the vehicle when cornering, when braking or when coasting without power and holds it reliably in track.

ESP complements the familiar functions of the anti-lock brake system (ABS), acceleration slip regulation (ASR) and engine braking regulation (EBR).

The function of the stability control overrides the ABS and ASR control systems. In addition to the active braking control of the ESP, it also influences the engine-transmission management.

The Electronic Stability System (ESP) includes the following subsystems:

ABS prevents the wheels from locking up during braking and thus maintains the steerability and directional control of the vehicle during deceleration. --

ASR/ETS inhibits wheelspin at all four drive wheels during vehicle operation. It also improves directional control with better traction across the entire speed range. It can simulate the functionality of up to three limited-slip differentials.

EBR reduces brake slip at the drive wheels during deceleration and ensures directional control.

ESP prevents the vehicle from breaking away when it is oversteered or understeered. In all situations it ensures that the vehicle does not deviate from the course specified by the driver (within the bounds of physical limits). Brake forces are produced selectively at the individual wheels to correct this.

System interplay
The engine electronics (ME) is used to adjust the drive torque. This system controls the engine by regulating the throttle valve and the firing point.

The gear setting is also evaluated by the electronic transmission control (EATC) to calculate the ESP drive torque control.

The ABS, ASR and BAS functions are integral components of ESP, which administers these subsystems through electronic closed-loop control intervention using the brake system.

The ESP hydraulic unit also houses the main components of the ABS and ASR.

The engine control module includes the functions for the electronic accelerator (EA) and the cruise control (Tempomat).

Data are exchanged over the CAN databus between the ESP control module, the BAS control module (integrated), the engine control module and, if fitted, the transmission control module.

Advantages of ESP
^ Improves moving-off and acceleration capabilities by increasing traction; especially useful on road surfaces with different levels of grip and when cornering.
^ Improves active dynamic safety, since only a wheel which is not spinning can provide optimum traction with no loss of lateral stability.
^ Automatically adapts the engine torque to suit the ability of the wheels to transmit this to the road when the driver applies too much throttle.
^ Reduces the risk of skidding under all road conditions by automatically stabilizing the vehicle when braking, accelerating or coasting smoothly.
^ Significantly improves the directional stability of the vehicle when cornering up to the limit range.
^ Shortens the stopping distance in corners or on slippery roads.
^ A flashing warning lamp in the speedometer notifies the driver of ESP control and that the vehicle is approaching the physical limits.
^ ESP or ASR can be shut down using the ESP OFF switch, which is indicated by the warning lamp in the speedometer shining constantly. This can provide better traction (grinding effect) in deep snow or when snow chains are fitted.

Function overview
Essentially, all the forces acting on a vehicle from outside attempt to rotate the vehicle about its center of gravity, regardless of whether these are one sided braking or drive forces or lateral forces.

The electronic stability program (ESP) detects vehicle behavior and specifically controls the braking forces of the individual wheels to correct this.

Component Identification:

ESP operates:
^ when cornering (vehicle understeering or oversteering)
^ when driving straight ahead (vehicle deviates off course due to uneven road conditions)

To be able to perform these extremely precise control interventions, an expanded system of sensors is required compared with ASR.

A distinction is made between:
1) ^ Sensors which recognize the driver requirement
^ Steering angle sensor
^ Accelerator pedal position (throttle valve actuator)
2) ^ Sensors which measure the actual vehicle behavior
^ ESP yaw rate sensor
^ Lateral acceleration sensor
^ ESP brake pressure sensor
^ Wheel speed sensor

The ESP control unit (N47-5) monitors and processes data on wheel speeds, steering angle, the vehicle's yaw rate, its lateral acceleration rate and the braking pressure.

The ESP control module (N47-5) is linked to the control modules of the engine/transmission management system over a CAN data bus. This digital link enables fast data exchange between the ESP, ME and ETC control modules.

The ESP control unit (N47-5) receives a continuous stream of information on current engine torque, accelerator pedal travel and gear selection.

The forces attempting to rotate the vehicle about its center of gravity are detected via the yaw rate and lateral acceleration sensors. By detecting all the data mentioned, it is possible to calculate the longitudinal and transverse forces acting at the wheels.

Once these data exceed specific, predefined levels, the ESP control unit (N47-5) enters its closed-loop control mode, in which it triggers selected solenoid valves as well as the hydraulic unit's high-pressure/ return pump to dial in a precisely programmed braking pressure at one or several wheels.

At the same time, commands are sent to the ME and EATC control modules via the CAN data bus. Computed values for throttle valve position and ignition timing point are set for reducing the drive torque, as well as to prevent any possible downshift.

The precise and accurately proportioned intervention is completed within 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 moment control circuit
ABS control
If, for example, a wheel has a tendency to lock, the brake pressure in this wheel brake is regulated. The pressure in the ESP hydraulic unit (A7/3) is regulated via the solenoid valves by the pressure buildup, pressure hold and pressure reduction control phases.

ESP braking torque control circuit
ASR/ETS closed-loop control
A spinning wheel is braked by a pressure system in the hydraulic unit which directs brake pressure into the appropriate brake caliper (pressure build-up).

This allows the other wheel to transmit the optimum drive force (differential lock effect).

Solenoid valves within the ESP hydraulic unit (A7/3) furnish closed loop control of braking force with selective activation of three operating modes: pressurization, pressure maintenance and pressure discharge.

ESP drive moment control circuit
ASR control
In order to reduce an excessive drive torque and thereby obtain optimum traction, the drive torque is reduced over the CAN data bus between the ESP and ME control modules.

The position of the throttle valve is reduced by the actuator contrarily to the accelerator pedal position input by the driver, and the ignition timing point is retarded.

The ESP control module 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.

ESP drive torque control circuit
EBR control
The ESP control module also detects if brake slip occurs at the drive wheels when the accelerator pedal is released. The signal is sent to the ME control module over the CAN data bus. On the basis of this information, wheel slip is reduced by defined opening of the throttle valve, and, as a result of this, the cornering stability of the vehicle is enhanced. This process occurs without notifying the driver (ESP warning lamp).

ESP brake and drive moment control circuit
ESP control
If over- or understeering is detected, calculated braking force is applied to the front or rear axle via the ESP control module and the hydraulic unit. This brake intervention counteracts the undesired driving behavior.

A signal sent to the ME control unit via the CAN data bus triggers the demand-based reduction in drive torque by decreasing engine torque.