Electronic stability program (ESP), function - GF42.45-P-0001VC
MODEL 203.08 /265 /28 with CODE (472a) Electronic Stability Program (ESP)
The "E lectronic S tability P rogram" (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, while braking or when coasting without power and holds it safely on course.
ESP complements the familiar functions of the antilock brake system (ABS), acceleration slip regulation (ASR) and engine braking regulation (EBR).
The stability control with its function takes priority over the ABS and ASR control systems. In addition to the active brake control of the ESP, it also influences the engine-transmission management.
The electronic stability program (ESP) comprises the following system:
ABS prevents the wheels from locking up during braking and thus maintains the steerability and directional control of the vehicle during deceleration.
ASR/ETS prevents all 4 drive wheels from spinning while driving. It also improves directional stability and road adhesion and increases traction potential across the entire vehicle speed range. The effect of up to 3 differential locks can be simulated.
EBR reduces brake slip at the drive wheels during deceleration and ensures directional control.
ESP prevents the vehicle from breaking away when oversteering or understeering. 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 correlations
The engine electronics (ME) are 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 included in the ESP electronics of the brake control circuit.
The basic ABS and ASR components are also combined in the ESP hydraulic unit.
The engine control module includes the functions for the electronic accelerator (EA) and the cruise control system.
A data exchange takes place 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.
Benefits 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.
- a Shortens the stopping distance when cornering or on slippery roads.
- A flashing warning lamp in the speedometer notifies the driver of ESP control and informs him that his 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. Deactivation can help to improve traction ("cutting" 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 forces, drive forces or lateral forces.
The electronic stability program (ESP) detects the behavior of the vehicle and specifically controls the braking forces of the individual wheels as a corrective measure.
Example A: understeered vehicle (cornering left)
The vehicle pushes outwards over the front wheels.
A precisely calculated braking action is performed on the left rear wheel.
Example B: oversteered vehicle (cornering left)
The tail of the vehicle breaks away.
A precisely calculated braking action is performed on the right front wheel.
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:
- Sensors which register driver requirements
- Steering angle sensor
- Accelerator pedal position (throttle valve actuator)
- Sensors which measure the actual vehicle behavior
- ESP yaw rate sensor
- Lateral acceleration sensor
- ESP brake pressure sensor
- Wheel speed sensor
The wheel speeds, the steering angle, the yaw rate of the vehicle, the lateral acceleration and the brake pressure are detected and processed in the ESP control module (N47-5).
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 module (N47-5) is continuously supplied with current data on engine torque, accelerator pedal position and transmission ratio.
The forces attempting to rotate the vehicle about its center of gravity are detected via the yaw rate and lateral acceleration sensors.
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 appropriate solenoid valves and the high-pressure / return pump in the hydraulic unit are actuated via the ESP control module (N47-5) in order to produce a defined brake pressure selectively at one or more 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 control functions are performed during this process:
ESP braking torque 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 build-up, pressure hold and pressure reduction control phases.
ESP braking torque control circuit
ASR/ETS control
To brake the wheel which is slipping, the brake pressure is directed into the appropriate brake caliper by means of a pressure system in the hydraulic unit (increasing pressure).
The other wheel can then transmit optimum driving power (locking differential effect).
Solenoid valves in the ESP hydraulic unit (A7/3) regulate the braking torque with pressure build-up, pressure hold and pressure reduction phases.
ESP drive torque 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 throttle valve position is reduced by the actuator in contrast to the accelerator pedal position input by the driver, and the ignition timing point is reduced.
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
If wheel slip at the drive wheels occurs when the gas pedal is released, this is also detected in the ESP control module. The signal is sent to the ME control module over the CAN data bus. This information is processed to reduce the braking slip by defined opening of the throttle valve and to thus increase the cornering stability of the vehicle. This process occurs without notifying the driver (ESP warning lamp).
ESP braking and drive torque control circuit
ESP control
If oversteer or understeer is detected, a calculated braking action is performed at 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.
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