Electrical systems: Chassis

From an electrical point of view, the chassis has been modified with respect to the following systems:

• Dynamic Stability Control 

  The vehicle is equipped as standard with Dynamic Stability Control (DSC).

  DSC is a development of the DSC 8Plus.

  For ease of identification this chassis control system is known as "Premium".

  The control functions of DSC have been further optimized. The improvements have been achieved through the use of a return pump with 6 pistons as well as more efficient electronics. The extended functions of the DSC are:

  • Brake standby by preemptive application of brake shoes in case of need (creation of low anticipatory brake pressure)
  • "Dry braking" of the brake discs on wet roads when wiper switched on
  • Detection of fading and increase in brake pressure during fading
  • Start-off assistant
  • Cruise control with brake function (option 544)
  • Parking brake (electromechanical and hydraulic parking brake)
    • from 09/2007: additionally with automatic-hold function

  New from 10/2007: 

  The DSC is equipped with the function "Drive dynamic brake intervention". To this end, the software in the DSC has been changed. No additional hardware is required.

  Drive dynamic brake intervention increases agility and reduces understeering. This improves the handling characteristics of the vehicle.

  As soon as the front wheels push unduly outward, the rear wheel on the inside of the curve will be specifically braked by the DSC.

  The resulting loss in forward drive is compensated for by an increase in engine power. Thus even on slippery roads, curves can be taken more precisely.

  Whether and to what extent intervention of this kind is required, is determined with the data on yaw rate, steering angle and engine power.

• Active front steering 

  For the 3rd generation of Active Steering the following features are new:

  • Yawing moment compensation has been fully integrated as a function in the Active Steering control unit
  • Cumulative steering angle sensor withdrawn
  • Digital motor position sensor
  • Commissioning exclusively via service function in BMW diagnosis system

  On the X5, Active Steering is available for the first time on an all-wheel drive vehicle.

  The variable steering transmission ratio offer the optimum gear ratio for every speed. With speeds of up to approx. 100 km/h, a very direct and agile vehicle performance is realized in conjunction with a small steering angle.

  It is hardly necessary for the driver to span the steering wheel with his hands. This means that the controls on the steering wheel are easier and safer to control. Thus for parking you can steer from the left stop to the right stop with only two turns of the steering wheel.

  At high speeds, steering becomes increasingly indirect, thus improving the straight-running stability and the general driving comfort.

  The road speed dependent Servotronic adjusts the force required to turn the steering wheel to a reasonable degree, while the Active Steering influences the steering angle requirement. Thus Active Steering and Servotronic have different effects and perfectly complement each other. For this reason Active Steering is only offered in combination with the Servotronic.

  Drive stability functions:

  The possibilities of Active Steering are also exploited to stabiles the vehicle in oversteering situations.

  The Active Steering can countersteer more rapidly and precisely that the average driver and stabilizes the car largely automatically. Another function increases vehicle stability during braking maneuvers on non-uniform road surfaces (μsplit braking). The differences in frictional coefficients result in vehicle reactions and the driver has to compensate for these with conventional systems by counter steering.

  Active Steering takes over this function and stabilizes the vehicle automatically. On one hand, it is easier to stay in control in this kind of situation and, on the other, the braking distance is reduced. This produces a tangible increase in safety compared with an identical vehicle without Active Steering.

• Adaptive Drive 

  With the optional extra Adaptive Drive the following are functionally combined:

  Active Roll Stabilization Dynamic Drive (ARS = Dynamic Drive) and variable Impact Damping Adjustment (VDM = Vertical Dynamic Management).

  The integration of both systems maximizes safety, comfort and responsiveness and is not available with any other SAV.

  • Vertical Dynamic Management 

  The Vertical Dynamic Management offers the following advantages:

  • Increased driving comfort
  • Increased vehicle agility
  • Improved independent steering performance and response to changing loads

  The VDM increases driving comfort. Increased driving comfort is attained when the vertical movement of the vehicle body is reduced to the minimum: whether this is due to agitation of the vehicle by the road surface (unevenness, joints) or on curves.

  The VDM controls the damper satellites in the actuation of the shock absorbers (impact force for individual wheel). To this end, the VDM control unit is connected with the damper satellites via the very high speed FlexRay databus. With a data transfer rate of 10 Mbit/s, FlexRay is significantly faster than the databuses employed on comparable vehicles (in the areas of body and powertrain/suspension).

  In addition to the higher bandwidth, FlexRay supports a realtime datatime transmission. FlexRay can be configured to be error-tolerant.

  The damper satellites (= control units) electronically regulate the impact forces.

  The damper satellites and the EDC valves are directly connected to the shock absorbers. Each damper satellite contains its own vertical acceleration sensor.

  • Active Roll Stabilization  (Marketing designation Dynamic Drive)

  Dynamic Drive provides active stabilizers on the front and rear axles. These active stabilizers support continuous adaptation to the current driving situation.

  There is thus no necessity to make compromises between the basic attunement of the chassis to sporty or comfortable driving. When you are driving straight ahead Dynamic Drive is "sensitively" activated, thus offering the greatest possible degree of suspension comfort. Depending on the lateral acceleration, the lateral inclination when taking cures or avoiding obstacles will be minimized by a "sporty" setting of the stabilizers. The active stabilizers are vertically distributed in the center. The active stabilizer consists of the oscillating motor and the stabilizer halves mounted on the oscillating motor.

  As one stabilizer half is connected with the shaft and the other with the housing, the stabilizer halves counter-rotate. The torque thus produced exerts a stabilizing effect on the rolling motion. The maximum system pressure is 180 bar.

  The most important control variable is the lateral acceleration. In addition the following signals are evaluated on the PT-CAN:

  • Lateral acceleration
  • Yaw rate
  • Vehicle road speed
  • Steering wheel angle
• Electronic height control (simple level control) 

  Ride height is controlled by air being fed into or withdrawn from the air springs. The EHC control unit receives information about the ride height on the left and right-hand sides of the vehicle from the level sensors.

  If the ride height is outside defined tolerances, the system will automatically activate the air supply unit to regulate to the specified level.

  The system is activated and starts to regulate whenever a door or the tailgate is opened. The system is deactivated after a certain time (approx. 20 minutes).

  The signals from the level sensors are filtered to distinguish between changes in load and other disturbances such as an uneven road surface. For correct use, two signals filtered at different frequencies are computed for each signal (low-pass filter with fast and slow filtering).

  Fast filtering is always employed when regulation is in progress. This prevents the regulated height from exceeding the specified level. Slow filtering is employed when the vehicle is in motion. This mode of filtering eliminates oscillations induced by the road surface.

  Both sides of the vehicle are regulated independently.

  In other words, the comparison of specification to actual value is performed separately for each side (Exception: testing the minimum height and the control function "Intermeshing".)

  With these control functions, the mean values of the left and right-hand sides of the vehicle are taken into account. The EHC control unit recognizes the different operating conditions from the incoming signals and messages. Depending on operating conditions, appropriately adapted control functions are activated.

• Parking brake (electromechanical parking brake = EMF) 

  The vehicle is equipped as standard with an electromechanical parking brake (EMF). The parking brake acts as a hand brake. The parking brake is actuated via the parking brake button in the central console.

  • Exclusion of all safety-critical conditions
  • Optimum functionality
  • Highest availability
  • Highest degree of comfort

  The parking brake is designed to secure the parked vehicle against rolling away. When the engine is turned off, the EMF takes over braking: electromechanically via the servo unit.

  When the engine is running, braking is control by the Dynamic Stability Control (DSC): hydraulically via the 4 disc brakes.

  New from 10/2007: 

  The vehicle has an automatic-hold function. The automatic-hold function is activated by a button behind the parking brake switch. The switch is linked to the EMF.

  When the function is selected, the vehicle is hydraulically braked from the first application of the brake by the DSC until it comes to a complete halt. Stepping on the gas pedal releases the disc brakes and the vehicle continues in motion. The automated braking and release process supports driving in city traffic or in jams, for example.

  The start assistant for starting on uphill slopes is integrated in the DSC (via signal longitudinal acceleration sensor).

• Tyre pressure control (US version only) 

  The tire pressure control (RDC) is a system for monitoring the tire pressure when driving.

  To this end, the tire pressure and the tire air temperature are measured at certain intervals on request by the RDC control device and telemetrically transmitted to the RDC aerial via a high-frequency transmission path.

  The RDC aerial relays the signal back to the RDC control device via a sub-bus. The control device evaluates the received data.

  Then the RDC control device forwards the data if necessary to the instrument cluster (KOMBI).

  The driver can thus be informed about any correction required to the tire pressure or about a flat tire (check-control message).

  The RDC monitors the tire pressure throughout the journey.

  The driver sets the tire pressure to be monitored. He instructs the system via the control function in iDrive or via the RDC button to save the current tire pressure as the target pressure value (Reset).

  The RDC control unit tests the target value for plausibility before saving it (axle by axle comparison of target pressures, minimum pressures).

  A reset is only possible when the air pressure in all wheels is at least 1.6 bar. If the tire pressure of a wheel is under this limit, a check-control message will be issued immediately.

  If the pressure difference between the wheels on an axle is > 0.4 bar, the reset will be rejected after the plausibility test. A check-control message will be issued. Remedy: Set the tire pressures to the correct values and repeat the reset.