3022488 - 3350E vehicle direction control system vdc

CONSTRUCTION FEATURES

GENERAL VIEW
1 - ABS/VDC control unit 2 - Steering angle sensor 3 - Engine management control unit 4 - Yaw/lateral acceleration sensor 5 - Front right wheel sensor 6 - ASR off button 7 - Rear right wheel sensor 8 - Rear right wheel sensor 9 - VDC warning light 10 - EBD warning light 11 - ABS warning light 12 - Front left wheel sensor

SPECIFCATIONS

Function

Driving a car under all conditions that may lead to the physical limit of grip and thus of stability may be a difficult task for a normal driver even the car tends to be stable. To improve driving safety, it is therefore necessary to fit an electronic system able to help the driver in this difficult task.Systems currently used to make driving safer under certain conditions are mainly ABS for braking control and TC and ASR/MSR systems. These systems control traction during acceleration by acting on the brakes (T.C.) and traction/drive torque during acceleration and over-run by acting on the brakes and engine control system motorised throttle body (ASR/MSR).

The VDC system (Vehicle Dynamic Control) incorporates all the functions listed above and optimises vehicle control with the addition of specific sensors:

  • steering angle sensor on the steering wheel
  • yaw/side acceleration sensor located near the centre of gravity beneath the central tunnel
μ - wheel grip S - slip A - EBD intervention area B - ABS intervention area C - VDC intervention area 1 - Side force curve 2 - Longitudinal force curveAs may be seen from the grip/slip diagram, the VDC system covers a bigger area than a conventional ABS/EBD. The VDC cannot be cut out because it is a safety system; the ASR/MSR function can, however, be cut out by means of a button on the central faciaVDC system intervention is indicated by a special warning light on the instrument panel node (Italian acronym = NQS)

COMPOSITION

Structure

The VDC system consists of:

  • a special A.B.S.5.7 hydraulic/electronic control unit (with V.D.C. function)
  • interface with the C-CAN line resident in the ABS control unit for communication with the engine management control unit, robotised gearbox and body computer
  • magnetoresistive wheel speed sensors
  • steering angle sensor with CAN interface (steering angle node)
  • yaw/side acceleration sensor integrated within a single component
  • brake fluid pressure sensor fitted to the ABS control unit
  • VDC warning light controlled via CAN line
  • ASR deactivation switch with warning light on button
ASR deactivation is indicated at the NQS by means of a message on the DOT matrix.

OPERATION

Introduction

The VDC system continuously detects loss of longitudinal and transverse wheel grip under all driving conditions, from braking to acceleration, to ensure vehicle directionality and stability. The VDC system is managed by a Bosch 5.7 ABS ECU integral with a special hydraulic control unit that makes it possible for the brake system to be operated independently of the driver.

The control unit processes the following signals:

  • steering angle/steering wheel rotation speed
  • side acceleration and yaw
  • motorised throttle position
  • wheel rpm
  • hydraulic braking system pressure

and uses special algorithms in the software to obtain the values for dynamic control of the vehicle:

  • longitudinal and transverse slip between wheels and road surface
  • axle drift.
The system calculate actual vehicle dynamics from these values; and identifies all critical conditions due to environmental factors (e.g. surface with poor grip) or any mistakes made by the driver (e.g. panic situations) to restore the car to efficient handling conditions by adjusting the brakes and drive torque.

The system interfaces with:

  • NCM for drive torque modulation,
  • NCR (robotised gearbox node) for gear shift management
  • NBC (body computer node) to transmit vehicle speed values and control warning lights.
Information is exchanged between these components by means of the C-CAN and B-CAN lines.System diagnosis is carried out via a dedicated line (K line).The system is linked to a power unit with a special brake pump; the pipes between brake pump and ABS control unit are also fitted with a Titaflex insert because the pipe diameter is bigger (6 mm) than normal pipes (4 mm). this is to prevent negative effects on VDC operation at low brake fluid temperatures.The VDC comes on automatically when the vehicle is started up and cannot be switched off by the driver. the button on the central console turns off only the ASR function and only in recommended circumstances (see ASR system)

INPUT SIGNALS

  • Wheel speed sensor (from direct line) (3)
  • Brake pedal switch (4)
  • Brake pedal switch wiring diagram (5)
  • ASR button off (from direct line) (6)
  • Engine control unit (2)
  • Throttle angle position (from C-CAN line)
  • Body computer (14)
  • Handbrake lever position (from C-CAN line)
  • Warning light status indicator (from C-CAN line)
  • Yaw sensor (rotation around vehicle vertical axis (from direct line) (10)
  • Lateral acceleration sensor (from direct line) (10)
  • Steering wheel steering angle/rotation sensor ((from C-CAN line) (12)
  • Robotised gearbox control unit (speed engaged status) (from C-CAN line) (13)
  • Hydraulic system pressure sensor(from direct line) (1)

OUTPUT SIGNALS

  • Brake pressure modulation command (15)
  • Ignition advance reduction command (from C-CAN line) (9)
  • Engine power management command (from C-CAN line) (8)
  • Gear change inhibition (from C-CAN line) (13)
  • Wheel speed signal for speedometer and mileometer (from C-CAN line) (14).
  • VSO (vehicle speed) signal (14)
  • VDC warning light control on panel (from C-CAN line) (11)
  • ASR led off (7)
As explained previously, the VDC controls longitudinal vehicle slip and also transverse slip, i.e. vehicle lateral stability.Vehicle lateral stability is provided by tyre reaction to lateral forces and depends on the strength with which the wheel grips the road surface.A wheel's adhesive force depends on vertical load, which depends in turn on wheel situation (supporting or unladen) and friction coefficient, which depends in turn on surface and tyre conditions.When the vehicle is moving in a straight line, the effect of side forces may be disregarded, unless external factors increase their intensity (e.g. wind gusts or changeover to a different surface). Conversely, when the car corners, side forces increase greatly due to the increase in centrifugal force.The action of side forces brings about a change in wheel drift angle and thus a change in axle drift (drift angle = difference between required trajectory and actual trajectory).

The side forces do not act equally on all four wheels because the wheels are not subject to the same load conditions. Wheels are loaded differently in different situations. The following situations may occur:

  • acceleration (lightening of front axle and loading of rear axle)
  • braking (loading of front axle and lightening of rear axle)
  • right/left bend (loading of outside wheels and lightening of inside wheels)
  • cornering while accelerating/decelerating (combination of the above cases)
If the side forces acting on individual wheels vary, it follows that the resultant forces acting on the car axles also vary. This means that if side forces acting on the front axle prevail over the forces acting on the rear axle or viceversa, the vehicle's vertical axis (yaw axis) is subject to a rotatory force (moment)The yawing moment affects car behaviour and generates a state of understeer or oversteer.A vehicle is said to be understeered when the front axle drift angle increase to a greater extent than that of the rear axle when subject to increasing side acceleration. In this case, the car tends to go straight when turning a corner (it tends to take the curve wide).
A car is said to be oversteered when the rear axle drift angle increases to a greater extent than that of the front axis with increasing transverse acceleration. In this case, the vehicle tends to spin around (the rear axle tends to go straight and the vehicle cuts the corner.

To keep the influence of side forces under control and thus limit the yawing moment, the ABS 5.7 control unit must firstly compute the vehicle behaviour set by the driver on the basis of:

  • steering angle sensor/steering wheel rotation speed
  • accelerator pedal position
  • braking system pressure

after which the control unit assesses actual vehicle behaviour by means of:

  • the sensors on the wheels (car speed/wheel speed)
  • side acceleration sensor
  • yaw sensor.

The above shows that the control unit is able to:

  • perceive the driver's actions because steering wheel position shows the number of degrees by which (large radius or short radius bend) and the speed with which the steering wheel is turned (sudden or gradual turns). Throttle position and brake pressure also show whether the car is accelerating or braking. In other words, how the driver takes the corner or deviates from a straight trajectory.
  • perceive actual vehicle behaviour on the basis of environmental variables e.g. slippery surface, car reactions to incorrect manoeuvres by the driver etc. in order to identify yawing moment and axle side slip by means of sensors on all four wheels and the yaw and side acceleration sensor.
These two operations are necessary to compare the mathematical model mapped in the control unit with actual car behaviour in order to identify vehicle status (understeer or oversteer) and decide the action to be taken on the brakes and engine

Understeering on corners

The control unit monitors to detect understeer (prevalence of front axle drift) and corrects car behaviour by braking the wheels on the inside of the bend to create an opposite moment in order to bring the car toward the middle of the bend and reducing drive torque if necessary.
Oversteering on cornersThe car detects the presence of oversteer (prevalence of rear axle drift) and corrects vehicle behaviour by braking the front wheel on the outside of the bend to create an opposite yawing moment, possibly augmented by an increase in the driving torque.
The system intervenes before understeer and oversteer reach excessive levels in order to limit countersteering manoeuvres that could be difficult to handle.

Abrupt changes in straight trajectory (swerving/overtaking)

In the case of abrupt changes in trajectory (e.g. overtaking/swerving, the control unit identifies possible oversteer or understeer conditions and correct vehicle trajectory as described above.

Abrupt change in straight trajectory (driving over different surfaces)

The control unit can preceive deviations from the trajectory and prevalence of axle drift, and corrects the trajectory with appropriate actions on the brakes and engine.

Brusque acceleration/deceleration

The control unit implements the ASR/MSR strategy it controls vehicle side acceleration and thus regulates the action on the front and rear brakes and driving torque more fully than on cars with ASR only.

Cutting out asr

If the ASR/MSR is cut out the following functions remain active:

  • A.B.S./E.B.D.
  • TC up to a speed of 40 km /h
  • partial V.D.C.

Displaying vdc intervention

When the VDC system intervenes, a warning light on the control panel flashes (5 Hz c.c. 50%).
The VDC increases driving safety but cannot control extreme situations. The system should not be seen as a performance-enhancing device but as a device that increases vehicle safety.

Diagnostic functions

The ABS 5.7 control unit is able to carry out automatic diagnosis on the system and if a fault is found is able to:

  • deactivate the entire system
  • deactive the system partially (maintaining the ABS/EBD function)
A list of anomalies detectable by the control unit, warning light statuses and recovery strategies is given below.
ErrorABS warning lightEBD warning lightVDC warning lightRecovery
electrical fault in rh front sensorONOFFONR3
electrical fault in lh front sensorONOFFONR3
electrical error in rh rear sensorONOFFONR3
electrical error in lh rear sensorONOFFONR3
error in rh front sensorONOFFONR3
error in lh front sensor
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