Dynamic Stability Control (DSC III)
DYNAMIC STABILITY CONTROL (DSC III)
DSC FOR THE X5 INTRODUCTION
The X5 is equipped with DSC III for its stability control system. The components are the same as the DSC III system (version 5.7) currently installed in the E38/E39 vehicles. However the software and programming are changed to allow for new functions associated with all wheel drive and off-road use.
New/changed functions of the DSC III include:
- New Hill Decent Control (HDC) function
- New Differential Brake Control (ABD) function
- Changed traction control function
- Changed ABS function
DSC - X5 OVERVIEW
The DSC system for the X5 provides all of the functions and features of the DSC III system currently in the E38/E39 vehicles. these functions/features include:
- Maintaining traction under all driving conditions
- Maintaining the directional Control (Longitudinal Stability)
- Providing Lateral Stability (Correction for oversteer/understeer)
- Providing Corner Braking Control (CBC)
- Providing Dynamic Braking System features of - Dynamic Braking Control and Maximum Braking Control (DBC and MBC)
- Providing Electronic Brake Proportioning (EBV)
TRACTION CONTROL SYSTEM OVERVIEW (ASC)
THEORY OF TRACTION CONTROL
Traction Control Systems are referred to as Automatic Stability Control (ASC), Automatic Stability + Traction Control (ASC+T), Dynamic Stability Control (DSC) or All Season Traction (marketing term). The traction control system has gone through several changes since its introduction in Model Year 1988. While each new version offered new innovations, the principle of traction control remains the same for any of these versions.
The link between the vehicle and its driving environment is established by the frictional contact between the tires and the road surface. The ability to control the vehicle while driving, is lost as a result of excessive wheel spin or slip if either rear tire exceeds its ability to:
- Maintain traction
- Accept side forces to maintain directional control (lateral locating forces)
The primary function of the traction control system is to maintain traction and stability of the vehicle regardless of the road surface condition. This Is achieved by reducing the drive torque applied to the rear wheels or pulsing the rear wheel brakes to eliminate wheel slip depending on the version of traction control installed.
The electronics for traction control operation are incorporated in the ABS control module and share many of the same components and sensors. The module determines the speed of the vehicle through a calculated average of the four wheel speed sensor inputs. Wheel slip/spin is then recognized by comparing the speed of the driven wheels to the front wheels.
A critical slip ratio of greater than 5% between the wheels will cause the traction control regulation to begin. This slip ratio is established when the control module detects a wheel speed difference of 2 MPH or higher.
THEORY OF ELECTRONIC CONTROL
An electronic control system is used because of its ability to react quicker and more precisely than the driver. It backs up the driver's decisions and contributes to active safety, even under extreme driving conditions.
Advantages:
- optimum traction at all speeds
- increased stability by retaining lateral (side to side) locating forces.
NOTE: The system is limited to the extent that no system can overcome the laws of physics.
DYNAMIC STABILITY CONTROL (DSC II)
Dynamic Stability Control II (DSC II) is a further development to the ABS/ASC+T5 traction control system. DSC I was introduced in other markets on the E32 and E31 vehicles with the M70 engine.
The DSC system is designed to improve the lateral stability of the vehicle in all driving situations. Whereby ASC+T is primarily designed for longitudinal stability and providing the optimum traction for driving off, DSC adds lateral stability control to the already proven system.
For any given turning angle and speed of the vehicle, there is a set difference between the rotational speeds of the front wheels. If the rotational speeds of the front wheels vary from this set difference, it means the vehicle is understeering or oversteering through the turn. This could lead to an unstable condition and possible loss of control.
The DSC system is designed to monitor this rotational difference and react to any changes or deviations that might possibly occur. The DSC provides ASC control for the vehicle while driving through corners or any time the vehicle is not moving straight ahead.
In essence, the ASC+T5 becomes a Dynamic Stability Control System with this added feature.
All of the familiar braking and straight line traction control features and system communication carry over from DSC II. Based on select high/select low logic, the DSC III control module selects a vehicle stabilizing strategy based on the specific input signal values it is monitoring at the moment. For all DSC strategies this begins with engine intervention to reduce torque:
- For the 750iL this is handled via CAN communication, DSC III to EML to minimize the throttle angle of the DK motors (750iL). For the 740i/iL and 54OiA this is handled by direct DSC III activation of the ADS II throttle housing.
- If additional torque reduction is necessary, DSC III informs DME over CAN to:
- Retard ignition timing
- Shut down the fuel injection to individual cylinders
DSC III monitors under/oversteer conditions through the following components:
- The driver's desired steering angle - steering angle signal over CAN bus.
- Vehicle speed and speed differential at front wheels - wheel speed sensors
- Dynamic forces of lateral acceleration and yaw placed on the vehicle. This is possible with two new components which are discussed further on. The results are as follows:
The expanded hydraulic control of individual wheel circuits is apparent when stabilizing a vehicle exhibiting an oversteer condition as follows:
Transmission system intervention also occurs during any ASC/DSC regulating phase. Through CAN bus communication the AGS control module is informed to delay any gear changes during regulation. This prevents any unwanted driveline dynamic changes during DSC regulation.
Though DSC III provides state of the art, electronic correction of undesirable vehicle handling characteristics, it is important to remember vehicle stability is always subject to the physical laws of centrifugal force and extreme road conditions. Good judgement and common sense on the part of the driver are still required.