GF42.45-P-0001SL Electronic Stability Program (ESP), Function
GF42.45-P-0001SL 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, while braking or when coasting without power and holds it safely on course.
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. As well as actively intervening with the brakes, ESP also has an influence on engine/transmission management.
The Electronic Stability Program (ESP) comprises the following system correlations.
ABS prevents the wheels from locking when the brakes are applied so that the vehicle remains steerable and stable as it decelerates.
ASR prevents the drive wheels from spinning while driving. It also improves directional control with better traction across the entire speed range.
EBR reduces brake slip at the drive wheels during deceleration and ensures directional control.
ESP prevents the vehicle from breaking away when over steering or under steering. 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.
To calculate the ESP drive torque control the gear stage is also evaluated by the electronic transmission control (ETS).
The ABS, ASR and MSR functions are included in the ESP electronics of the brake control circuit.
The controls for ABS, ASR and ESP are carried out in the SBC hydraulic unit.
The function for the electronic accelerator pedal (EFP), the speed control system (cruise control) are included in the engine control unit.
Data is exchanged between the ESP control unit, the engine control unit and transmission control unit via a CAN data line.
The ESP control unit communicates with the SBC control unit via a second CAN data line.
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 the respective wheel-to-road-surface traction possibilities if the driver accelerates too heavily.
^ Reduces the risk of skidding under all road conditions by means of automatic stabilizing when braking, when accelerating or in the case of freewheeling.
^ 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 signals ABS or ESP closed-loop operation to the driver and informs him that he has approached the physical limit of driving with his vehicle.
^ Via an ESP OFF switch an ESP and ASR shutoff can occur which is recognized by the warning lamp permanently lighting up in the instrument cluster. 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) analyzes the vehicle behavior and applies specific braking force to individual wheels to correct any instability.
ESP operates:
^ when cornering (vehicle under steering or over steering)
^ when driving straight ahead (vehicle deviates off course due to uneven road conditions)
In order to be able to perform these control intervention functions, an expanded system of sensors is required compared to ABR.
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
^ Yaw rate sensor
^ Lateral acceleration sensor
^ Brake pressure sensors
^ Wheel speed sensor
In the ESP control unit (N47-5) the wheel speeds, steering angle, vehicle yaw rate, lateral acceleration and brake pressure of the individual wheels is recorded and processed.
The ESP control unit (N47-5) is linked to the control units of the engine/transmission management system over a CAN data bus. This digital link permits fast data exchange between the ESP control unit, engine control unit, and transmission control unit.
The ESP control unit (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 particular control thresholds, the corresponding control valves are actuated by the ESP control unit (N47-5) via the SBC control unit (A7/3n1) in order to selectively regulate a defined brake pressure at one or more wheels.
At the same time commands are passed on to the engine and transmission control units over the CAN data bus. For drive moment reduction, a torque requirement is given to the motor electronics control unit as well as, if necessary, a downshift prevented by the ETC [EGS] control unit.
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 braking torque control circuit
ABS control
If a wheel tends to lock, then the brake pressure in this wheel brake is regulated.
The control in the traction system hydraulic unit (A7/3) takes place via the control valves by the control phase pressure buildup, pressure holding and pressure reduction.
ESP braking torque control circuit
ASR control
A spinning wheel is braked by a pressure system in the hydraulic unit which directs brake pressure into the brake caliper on the rear axle (pressure build-up).
This allows the other wheel to transmit the optimum drive force (differential lock effect).
The braking torque is regulated by pressure buildup, pressure holding and depressurization (A7/3) via control valves in the traction system hydraulic unit.
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 control unit and the engine control unit.
The ESP control unit 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 unit also detects if brake slip occurs at the drive wheels when the accelerator pedal is released. The signal is sent to the engine control unit over the CAN data bus.
With this information the brake slip is reduced by increasing the drive torque, thus improving the lateral stability of the vehicle. This process occurs without notifying the driver (ESP warning lamp).
ESP brake moment and drive torque control circuit
ESP control
If over steering or under steering is detected, a calculated brake intervention at the front or rear axle is carried out via the ESP control unit and the SBC hydraulic unit. This braking action deliberately counteracts the undesirable vehicle motion.
A signal via the CAN data bus to the engine control unit reduces the drive moment as required by reducing the engine torque.
