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 Electric Stability Program (ESP) analyzes the vehicle behavior and applies specific braking force to individual wheels to correct any instability.
See Fig 1 for function overview.
Example A: Vehicle understeering (on left-hand bend). The vehicle pushes outwards over the front wheels. Precisely calculated braking force is applied to the left rear wheel.
Example B: Vehicle oversteering (on left-hand bend). The tail of the vehicle breaks away. Precisely calculated braking force is applied to the right front wheel.
ESP operates:
- When cornering (vehicle under- or oversteered)
- 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:
- Yaw rate sensor.
- Lateral acceleration sensor.
- Brake pressure sensor.
- Wheel speed sensor.
The ESP control module (N47-5) records and processes the wheel speeds, the steering angle, the yaw rate of the vehicle, the lateral acceleration and the brake pressure at the front axle. 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 permits fast data exchange between the ESP control module, engine control unit, and transmission control module.
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 this data acquisition. If these values exceed certain control thresholds, the appropriate solenoids as well as the high-pressure/return-flow pump in the hydraulic unit are actuated via the ESP control module (N47-5) in order to apply precisely defined brake pressure to one or more wheels. At the same time, commands are sent to the engine and transmission control module via the CAN data bus. Calculated values for throttle valve position and ignition timing are predefined and downshifting is suppressed in order to reduce driving torque.
The precise and accurately proportioned intervention is completed within a few fractions of a second. Active brake intervention and driving torque reduction by the ESP ensure optimum vehicle stability.
The following control functions are performed during this process:
The normal brake servo amplifies the force of the driver's foot proportionally. If braking is not taking place, a vacuum created by the engine or by a vacuum pump exists in both chambers. A mechanical control valve in the brake servo operated by the force of the driver's foot allows air at atmospheric pressure to flow into the rear chamber when braking. The pressure difference existing between the two chambers and the amplification force created is therefore proportional to the force applied by the driver's foot. Amplification of the force of the foot takes place up to the point at which full atmospheric pressure exists in the rear chamber.
With the BAS brake servo, the basic version is expanded with the following parts:
- BAS diaphragm travel sensor for measuring the brake pedal travel.
- BAS solenoid valve for active implementation of braking force support.
- BAS release switch to detect the end of the brake application.
- BAS control module to measure the values and to control the BAS braking intervention.
After the engine is started, a vehicle identification and self-check of the system takes place. The BAS is ready to function if no fault is present and the vehicle has exceeded a speed of 8 km/h after driving off. Following this, BAS can be activated down to a speed of 3 km/h. The actuating speed of the brake pedal and the vehicle speed are measured in the BAS control module (N48) / ESP (N47-5). If the pedal speed is greater than the threshold value specified for the particular vehicle speed, a solenoid valve in the brake servo is activated and full servo force is achieved. In the case of a defect, the BAS is not brought into use (indication by malfunction indicator lamp or multi-function display in the instrument cluster). The normal function of the brake servo is not affected by this.