GF42.45-P-0001T Electronic Stability Program (ESP) Function
GF42.45-P-0001T Electronic stability program (ESP) function
15.03.07 The document has been updated to this date and will no longer be subject to revision.
The "Electronic Stability Program" (ESP) is an active safety system for improving the stability of the vehicle in all driving situations.
It operates by means of individual brake control cycles at one or several wheels of 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 completes the familiar functions of the antilock brake system (ABS), the acceleration slip regulation (ASR) and 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 Electronic Stability Program (ESP) comprises the following system connections.
ABS prevents the wheels from locking up when braking and thus maintains the steerability and directional control of the vehicle during deceleration.
ASR prevents the drive wheels from spinning while driving. It also improves directional stability 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 physical limits). Brake forces are produced selectively at the individual wheels to correct this.
System correlations
Motor 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 stage from the electronic transmission control is also evaluated to calculate the ESP drive torque.
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 traction system hydraulic unit (A7/3).
The function for the electronic accelerator (EFP), for the speed control system (cruise control) is contained in the engine control unit.
A data is exchanged between the ESP control unit (N47-5), the ME-SFI [ME] control unit (N3/10) or CDI control unit (N3/9) and transmission control unit via a CAN data line.
The ESP control unit (N47-5) communicates with the traction system control unit (A7/3) 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.
^ Increases the level of active driving safety since only non-spinning wheels provide optimum traction without impairing side stability.
^ Reduces the risk of skidding under all road conditions by automatic stabilization when braking, accelerating or in the case of rolling freely.
^ Automatically adapts the engine torque to the respective transmission possibilities of the wheels to the road if the driver accelerates too hard.
^ 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 got close to the physical driving limits with his vehicle.
^ Via an ESP Off switch (N72/1s1) the ESP and ASR can be switched off which is signaled by the warning lamp in the instrument cluster (A1) lighting up permanently. This can provide better traction (grinding effect) in deep snow or when snow chains are fitted.
Function overview
All the forces acting on a vehicle from outside always want to turn the vehicle about the center of gravity, irrespective of whether these are one-sided brake forces or drive forces or lateral forces.
The Electronic Stability Program (ESP) measures the vehicle characteristics and introduces brake forces selectively at the individual wheels for correction.
Example A: under steering vehicle (in left-hand curve)
The vehicle pushes outwards over the front wheels.
A precisely calculated braking action is performed on the left rear wheel.
Example B: over steering vehicle (in left-hand curve)
The tail of the vehicle breaks away.
A precisely calculated braking action is performed on the right front wheel.
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 with ASR. A distinction is made between:
1.) Sensors which detect the driver's requirement
^ Steering angle sensor (N49)
^ Accelerator pedal position (throttle valve actuator)
2.) Sensors which measure the actual vehicle response
^ Yaw speed and lateral acceleration sensor (B24/15)
^ Left front pressure sensor (A7/3b3), right front pressure sensor (A7/3b4), left rear pressure sensor (A7/3b5), right rear pressure sensor (a7/3b6)
^ Left front rpm sensor (L6/1), right front rpm sensor (L6/2), left rear rpm sensor (L6/3), right front rpm sensor (L6/4) (N47-5) The wheel speeds, steering angle, vehicle yaw speed, the lateral acceleration and brake pressure of the individual wheels are recorded and processed in the ESP control unit.
The ESP control unit (N47-5) is connected to the control units of the engine/transmission management via a data bus. This digital line connection makes possible a rapid data exchange between the ESP control unit (N47-5), engine control unit and transmission control unit.
The ESP control unit (N47-5) is constantly supplied with the current data on engine torque, accelerator pedal position and transmission ratio.
Via the yaw rate and lateral acceleration sensor (B24/15) the forces are detected which want to turn the vehicle about the center of gravity.
The longitudinal and lateral forces acting on the wheels can be calculated based on the data listed above.
If these values exceed certain control thresholds, then the corresponding control valves are actuated by the ESP control unit (N47-5) via the SBC control unit (A7/3n1) in order to regulate a specific defined brake pressure at one or more wheels.
Simultaneously commands are passed on to the engine control unit and transmission control unit via the CAN data bus. For drive torque reduction a torque requirement is made to the ME-SFI [ME] 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:
ABS control (ESP braking torque control circuit)
If for example one wheel tends to lock, then the brake pressure in this wheel brake is regulated. The regulation in the SBC hydraulic unit (A7/3) takes place through the control valves by the control phases pressure buildup, pressure holding and depressurization.
ASR control (ESP braking torque control circuit)
To brake the spinning wheel the brake pressure is lead into the brake caliper of the rear axle via a pressure system in the hydraulic unit (pressure buildup).
As a result the other wheel can transmit the optimum drive force (locking differential effect).
The braking torque is regulated by pressure buildup, pressure holding and depressurization via control valves in the SBC hydraulic unit (A7/3).
ASR control (ESP drive moment control circuit)
In order to reduce an excessive drive torque and therefore to achieve optimum traction, drive torque is reduced via the CAN data bus between the ESP control unit (N47-5) and engine control unit via the CAN data bus.
In the ESP control unit (N47-5), a check is constantly made as to whether, for example, the control functions can be canceled as a result of a sudden improvement in road adhesion so that the drive torque specified by the driver via the accelerator pedal can be permitted again as quickly as possible.
MSR control (ESP drive torque control circuit)
If wheel slip occurs at the drive wheels when the throttle is released, this is likewise detected in ESP control unit (N47-5). The signal is reported to the engine control unit via the CAN data bus.
Using this information wheel slip is reduced by increasing the drive torque and as a result the side stability of the vehicle is increased. This process takes place without informing the driver.
ESP control (ESP brake torque control circuit and drive torque control circuit)
If over steering or under steering is detected, then a calculated intervention in the braking system at the front and rear axle is performed via the ESP control unit (N47-5) and traction system hydraulic unit (A7/3). This braking action deliberately counteracts the undesirable vehicle motion.
By a signal via the CAN data bus to the engine control unit the drive torque reduction to suit requirements is achieved by engine torque reduction.
