Antilock Brakes / Traction Control Systems: Description and Operation
Anti-Lock Control - Stability Assist
COMPONENT LOCATION
INTRODUCTION
The ABS (anti-lock brake system) and DSC (dynamic stability control) system features a Bosch modulator, which is an integrated four-channel HCU (hydraulic control unit) and ABS (anti-lock brake system) module. The unit is installed in the brake hydraulic circuit between the brake master cylinder and the four brake calipers.
The ABS (anti-lock brake system) module is connected to the high speed CAN (controller area network) bus, and actively interacts with other vehicle system control modules and associated sensors to receive and transmit current vehicle operating information.
When required, the ABS (anti-lock brake system) module will actively intervene and operate the HCU (hydraulic control unit) during braking or vehicle maneuvers to correct the vehicle attitude, stability, traction or speed. During incidents of vehicle correction, the ABS (anti-lock brake system) module may also request the ECM (engine control module) to control engine power in order to further stabilize and correct the vehicle.
To provide full system functionality, the ABS (anti-lock brake system) and DSC system comprise the following components:
- DSC switch.
- Four wheel speed sensors.
- Steering angle sensor.
- Yaw rate and lateral acceleration sensor.
- Stoplamp switch.
- Instrument cluster indicator lamps.
- Integrated ABS (anti-lock brake system) module and HCU (hydraulic control unit).
Two variants of ABS (anti-lock brake system) module are available, Bosch ESP(R)8.1 and Bosch ESP(R)plus8.1. The Bosch ESP(R)plus8.1 system is fitted to vehicles with ASC (adaptive speed control) and incorporates a feature known as 'electronic brake prefill'.
The ABS (anti-lock brake system) provides the following brake functions that are designed to assist the vehicle or aid the driver:
- ABS (anti-lock brake system).
- DSC, including TracDSC.
- CBC (corner brake control).
- EBD (electronic brake force distribution).
- ETC (electronic traction control).
- EBA (emergency brake assist).
- EDC (engine drag-torque control).
- Understeer control.
- Electronic brake prefill (vehicles with ASC only).
All the brake functions listed are automatically active when the ignition is in power mode and the engine is running. The DSC system can be selected to off using the DSC switch.
WARNING: Although the vehicle is fitted with DSC, it remains the drivers responsibility to drive safely according to the prevailing conditions.
DYNAMIC STABILITY CONTROL SWITCH
The DSC switch is mounted in the floor console adjacent to the JaguarDrive selector.
DSC becomes active whenever the engine is running. A momentary press of the switch allows the driver to toggle between the standard DSC settings and the optimized TracDSC settings. The message TRAC DSC or DSC ON will temporarily be displayed in the instrument cluster message center. The amber DSC warning indicator in the instrument cluster remains illuminated while TracDSC is selected.
The DSC can be switched off by pressing and holding the switch for more than 10 seconds.
In each case the message DSC OFF will be displayed in the instrument cluster message center to confirm DSC has been switched off. The amber DSC warning indicator in the instrument cluster will remain illuminated. The system can be switched back on again by simply pressing and releasing the switch. The message DSC ON will then temporarily appear in the instrument cluster message center to confirm the system is on.
NOTE:
Switch requests may be delayed if the switch is pressed while a DSC operation is taking place. The switch request will be displayed in the instrument cluster but the ABS (anti-lock brake system) module will not initiate any stability changes until it is safe to do so.
DSC returns to ON at the beginning of each ignition cycle, regardless of the previous setting. The exception to this is when the ignition is switched off only briefly and the ABS module remains powered, as indicated by the continued illumination of the P indicator on the JaguarDrive selector.
If a fault is detected with the DSC switch, the ABS (anti-lock brake system) module defaults to the DSC on setting and any switch requests are ignored.
WARNING: It is recommended that when using show chains, DSC is switched on and JaguarDrive control winter mode is selected.
WHEEL SPEED SENSORS
An active wheel speed sensor is installed in each wheel hub to provide the ABS (anti-lock brake system) module with a rotational speed signal from each road wheel. The head of each front wheel speed sensor is positioned close to a magnetic encoder ring incorporated into the inboard seal of the wheel bearing. The head of each rear wheel speed sensor is positioned close to a magnetic encoder ring incorporated into the rear wheel bearing assembly. Each encoder ring contains 46 north and south poles. A fly lead connects each sensor to the vehicle harness.
The wheel speed sensors each have a signal and a return connection with the ABS (anti-lock brake system) module. When the ignition is ON the ABS (anti-lock brake system) module supplies a signal feed to the wheel speed sensors and monitors the return signals. Any rotation of the road wheels induces current fluctuations in the return signals, which are converted into individual wheel speeds and overall vehicle speed by the ABS (anti-lock brake system) module.
The ABS (anti-lock brake system) module broadcasts the individual wheel speeds and the vehicle speed on the high speed CAN (controller area network) bus for use by other systems.
If a wheel speed sensor fault is detected by the ABS (anti-lock brake system) module, 'ABS FAULT' will be displayed in the instrument cluster message center and an amber warning indicator will illuminate.
STEERING ANGLE SENSOR
The steering angle sensor measures the steering wheel angle and the rate of change of the steering wheel angle. These measurements are received by the ABS (anti-lock brake system) module and broadcast on the high speed CAN (controller area network) bus for use by other systems.
The steering angle sensor is mounted on the steering column upper shroud mounting bracket, immediately behind the multifunction switches, and is secured by two screws. A fly lead connects the sensor to the passenger compartment wiring harness via a four pin multiplug.
The sensor is housed in a 'U' shaped plastic casing and contains two offset LED (light emitting diode) facing two detectors.
An encoder ring is mounted on the inner steering column shaft and intersects the LED (light emitting diode)'s and detectors. The encoder ring contains 60 slots which break and restore the light beams between the LED (light emitting diode)'s and the detectors as the steering wheel is rotated. The ABS (anti-lock brake system) module is able to determine the direction of rotation of the steering wheel by monitoring when the light beams change state. The LED (light emitting diode)'s and detectors are mounted in such a way that only one beam will change state, either to broken or restored, at any one time.
The center (straight ahead) position of the steering wheel has to be learned by the ABS (anti-lock brake system) module every time the ignition is switched ON. The steering angle sensor is unable to determine the center position so inputs from the yaw rate and lateral acceleration sensor and wheel speed signals are also used by the ABS (anti-lock brake system) module to help it perform this process. If extreme weather conditions are present, for example ice causing extreme wheel spin or understeer/oversteer, the ABS (anti-lock brake system) module may not be able to determine the center position of the steering wheel. In this situation 'DSC NOT AVAILABLE' will be displayed in the instrument cluster message center and the amber warning indicator will illuminate.
'DSC NOT AVAILABLE' will also be displayed if the ABS (anti-lock brake system) module detects a steering angle sensor fault. The amber warning indicator will illuminate until the fault is rectified.
YAW RATE AND LATERAL ACCELERATION SENSOR
The yaw rate and lateral acceleration sensor is mounted on the transmission tunnel, forward of the gear selector module. The sensor is secured by 2 screws and connects to the vehicle wiring via a 4 pin multiplug.
When the ignition is ON, the sensor receives a power feed from the CJB (central junction box). The ground path for the sensor is located on the instrument panel cross-car beam. The sensor measures the yaw rate and lateral acceleration of the vehicle, providing values to the ABS (anti-lock brake system) module via a dedicated, private high speed CAN (controller area network) bus connection. The ABS (anti-lock brake system) module broadcasts these values on the high speed CAN (controller area network) bus for use by other systems.
If a sensor fault is detected by the ABS (anti-lock brake system) module, 'DSC NOT AVAILABLE' will be displayed in the instrument cluster message center and the amber warning indicator will illuminate.
STOPLAMP SWITCH
The stoplamp switch is mounted on the brake pedal box and is connected to the vehicle harness via a four pin multiplug.
When the brake pedal is pressed, the switch contacts close. This allows a hard wired signal feed to be sent to the ECM (engine control module). A stoplamp switch status message is then sent from the ECM (engine control module) to the ABS (anti-lock brake system) module on the high speed CAN (controller area network) bus. The ABS (anti-lock brake system) module is then able to control braking force accordingly in conjunction with the HCU (hydraulic control unit).
NOTE:
The stoplamp switch also forms part of the speed control system.
BRAKE WARNING INDICATORS
The instrument cluster and message center contains warning indicators and warning messages to display the operating status of the anti-lock control - stability assist functions. The warning indicators and messages provide a visual notification of either a system warning or information indication to the driver. There are three warning indicators on the instrument cluster, which vary dependant on market, and several types of message relating to the anti-lock control - stability assist functions. The DSC OFF message is accompanied by an audible warning.
The following anti-lock control - stability assist warning indicators are installed in the instrument cluster:
- An amber ABS (anti-lock brake system) warning indicator.
- A red brake warning indicator.
- An amber DSC warning indicator.
ABS MODULE
The ABS (anti-lock brake system) module is secured to a mounting bracket located in the LH (left-hand) front wheel arch. The module is mounted on the rear face of the HCU (hydraulic control unit), which it uses to control all braking and stability functions by modulating hydraulic pressure to the individual wheel brakes.
Two types of ABS (anti-lock brake system) modules are available; one for vehicles with standard speed control, one for vehicles fitted with adaptive speed control.
If an ABS (anti-lock brake system) modulator fault is detected, 'ABS FAULT' will be displayed in the instrument cluster message center and the amber warning indicator will illuminate.
CAUTION: The ABS (anti-lock brake system) module and the HCU (hydraulic control unit) comprise a single unit and must not be separated.
HYDRAULIC CONTROL UNIT
The HCU (hydraulic control unit) is a four channel unit, secured to a mounting bracket located in the LH front wheel arch. The HCU (hydraulic control unit) modulates the supply of hydraulic pressure to the brakes under the control of the ABS (anti-lock brake system) module.
CONTROL DIAGRAM
NOTE:
A = Hardwired; D = High speed CAN (controller area network) bus; V = Private CAN (controller area network) bus.
PRINCIPLES OF OPERATION
Anti-Lock Brake System
ABS (anti-lock brake system) controls the speed of all road wheels to ensure optimum wheel slip when braking at the adhesion limit. The wheels are prevented from locking to retain effective steering control of the vehicle.
The brake pressures are modulated separately for each wheel. Rear brake pressures are controlled to maintain rear stability on split friction surfaces.
Dynamic Stability Control
DSC uses brakes and powertrain torque control to assist in maintaining the yaw stability of the vehicle. While the ignition is energized the DSC function is permanently enabled, unless selected off using the DSC switch.
DSC enhances driving safety in abrupt maneuvers and in under-steer or over-steer situations that may occur in a bend. The ABS (anti-lock brake system) module monitors the yaw rate and lateral acceleration of the vehicle, steering input and individual wheel speeds, then selectively applies individual brakes and signals for powertrain torque adjustments to reduce under-steer or over-steer conditions.
In general:
- In an under-steer situation the inner wheels are braked to counteract the yaw movement towards the outer edge of the bend.
- In an over-steer situation the outer wheels are braked to prevent the rear end of the vehicle from pushing towards the outer edge of the bend.
The ABS (anti-lock brake system) module monitors the tracking stability of the vehicle using inputs from the wheel speed sensors, the steering angle sensor, and the yaw rate and lateral acceleration sensor. The tracking stability is compared with stored target data. Whenever the tracking stability deviates from the target data, the ABS (anti-lock brake system) module intervenes by applying the appropriate control strategy.
The following interactions occur in an intervention situation:
- High speed CAN (controller area network) signal to the ECM (engine control module), to reduce engine torque.
- Application of braking to the appropriate corner of the vehicle.
TracDSC
TracDSC is an alternative setting of DSC with reduced system interventions. With TracDSC engaged, traction may be somewhat increased, although stability may be reduced compared to normal DSC. TracDSC is intended for use only on dry tarmac by suitably experienced drivers and should not be selected for other surfaces or by drivers with insufficient skill and training to operate the vehicle safely with the TracDSC function engaged. The less restrictive TracDSC setting may be preferred, for example, by expert drivers engaged in high performance driving on dry tarmac surfaces such as tracks and circuits.
WARNING: Vehicle safety may be reduced by inappropriate use of TracDSC. TracDSC should only be used in suitable conditions.
Briefly pressing and releasing the DSC switch will switch the vehicle between normal DSC settings and TracDSC settings. To confirm which setting has been selected, either DSC ON or TracDSC will be temporarily displayed in the instrument cluster message center.
When TracDSC is selected, the amber DSC warning indicator located in the instrument cluster will illuminate. The warning indicator will remain illuminated unless TracDSC becomes active. At this point the warning indicator will flash.
NOTE:
If speed control is engaged it will automatically disengage if DSC or TracDSC becomes active.
Corner Brake Control
CBC (corner brake control) influences the brake pressures, below and within DSC and ABS (anti-lock brake system) thresholds, to counteract the yawing moment produced when braking in a corner. CBC produces a correction torque by limiting the brake pressure on one side of the vehicle.
Electronic Brake Force Distribution
EBD (electronic brake force distribution) limits the brake pressure applied to the rear wheels. When the brakes are applied, the weight of the vehicle transfers forwards, reducing the ability of the rear wheels to transfer braking effort to the road surface. This may cause the rear wheels to slip and make the vehicle unstable.
EBD (electronic brake force distribution) uses the ABS (anti-lock brake system) braking hardware to automatically optimize the pressure to the rear brakes, below the point where ABS (anti-lock brake system) is normally invoked.
NOTE:
Only the rear brakes are controlled by the EBD (electronic brake force distribution) function.
Electronic Traction Control
ETC (electronic traction control) attempts to optimize forward traction by reducing engine torque, or by applying the brake of a spinning wheel until traction is regained.
ETC is activated if an individual wheel speed is above that of the vehicle reference speed (positive slip) and the brake pedal is not pressed. The brake is applied to the spinning wheel, allowing the excess torque to be transmitted to the non-spinning wheel through the drive line. If necessary, the ABS (anti-lock brake system) module also sends a high speed CAN (controller area network) bus message to the ECM (engine control module) to request a reduction in engine torque.
When the DSC function is selected off using the DSC switch, the braking and engine torque reduction features are both disabled.
Emergency Brake Assist
EBA (emergency brake assist) assists the driver in emergency braking situations by automatically increasing the applied braking effort. The ABS (anti-lock brake system) module invokes EBA (emergency brake assist) when:
- The brake pedal is rapidly pressed.
- The brake pedal is pressed hard enough to bring the front brakes into ABS (anti-lock brake system) operation.
When the brake pedal is rapidly pressed, the ABS (anti-lock brake system) module increases the hydraulic pressure to all of the brakes until the threshold for ABS (anti-lock brake system) operation is reached. This action applies the maximum braking effort for the available traction. The ABS (anti-lock brake system) module monitors for the sudden application of the brakes, using inputs from the brake pedal switch and from the pressure sensor within the HCU (hydraulic control unit). With the brake pedal pressed, if the rate of increase of hydraulic pressure exceeds the predetermined limit, the ABS (anti-lock brake system) module invokes emergency braking.
When the brake pedal is pressed hard enough to bring the front brakes into ABS (anti-lock brake system) operation, the ABS (anti-lock brake system) module increases the hydraulic pressure to the rear brakes up to the ABS (anti-lock brake system) threshold.
EBA (emergency brake assist) operation continues until the driver releases the brake pedal, sufficiently for the hydraulic pressure in the HCU (hydraulic control unit) to drop below a threshold value stored in the ABS (anti-lock brake system) module.
Engine Drag-Torque Control
EDC (engine drag-torque control) prevents wheel slip caused by any of the following:
- A sudden decrease in engine torque when the accelerator is suddenly released.
- A downshift using the Jaguar sequential shift function.
When the ABS (anti-lock brake system) module detects the onset of wheel slip without the brakes being applied, the ABS (anti-lock brake system) module signals the ECM (engine control module) via the high speed CAN (controller area network) bus to request a momentary increase in engine torque.
Understeer Control
Understeer Logic Control is a proactive system which monitors the vehicle for understeer by comparing signals from the yaw rate and lateral acceleration sensor with signals from the steering angle sensor and wheel speed sensors.
When the ABS (anti-lock brake system) module detects the onset of understeer, the ABS (anti-lock brake system) module signals the ECM (engine control module) via the high speed CAN (controller area network) bus to request a decrease in engine torque. At the same time the ABS (anti-lock brake system) module will control the HCU (hydraulic control unit) to apply brake pressure to the relevant wheels to correct the understeer.
Electronic Brake Prefill (Vehicles With ASC Only)
Electronic brake prefill (Bosch ESP(R)plus8.1) senses any rapid throttle lift off, activating a small brake hydraulic pressure build-up of approximately 3 to 5 bar (43.5 to 72.5 lbf/in2) in anticipation of the brakes being applied.
This application produces a quicker brake pedal response and consequently slightly shorter stopping distances. The system supports vehicles with ASC.
When the ABS (anti-lock brake system) module detects rapid throttle lift off (from the signals received from the ECM (engine control module) over the high speed CAN (controller area network) bus), it controls the HCU (hydraulic control unit) to apply a low brake pressure to assist in a quicker brake application.