Function
Function
ABS function
While driving, the brake control module (BCM) (1) registers the acceleration and deceleration of the wheels. The 4 wheel sensors (3) (one on each wheel) provide the brake control module (BCM) with information about the rotation speed of the wheels. The brake control module (BCM) uses this information to calculate the vehicle speed. The control module transmits the signal on the controller area network (CAN) where it can be accessed by other systems.
During braking, the brake control module (BCM) receives a CAN signal from the engine control module (ECM) and stop lamp switch (2) indicating braking. The brake control module (BCM) then goes into standby mode for ABS control. The signal from the stop lamp switch is not required for ABS control, but does give softer control at the beginning of ABS control.
If the brake control module (BCM) detects that there is a risk of wheel lock-up, the ABS hydraulic modulator (4) is affected and the hydraulic pressure for the relevant wheel is adapted so that wheel lock up is avoided.
The pressure in the circuit is controlled so that the maximum possible braking force is transferred to the road surface, which means that the wheel rotates with 12-20% less slippage on the surface.
ABS is first activated when the speed of the car exceeds approximately 7 km/h. This means that at speeds below approximately 7 km/h the wheels can lock up. This has little practical effect on the function of the system or from the driver's point of view.
ABS control can be divided into three stages:
1. Open position (Pressure increase). This is the normal condition during braking. The inlet valve is open and the outlet valve is closed. This permits the brake pressure to pass from the master cylinder to the wheel
2. Pressure maintenance phase. If the brake control module (BCM) detects that one of the wheels is rotating too slowly, the brake control module (BCM) will close the inlet valve. This stops the brake pressure to the wheel from increasing, even if the driver increases the pressure on the brake pedal. If the rotation speed of the wheel continues to fall the control moves into the pressure reduction phase
3. Pressure release phase. The inlet valve remains closed. The brake control module (BCM) opens the outlet valve and starts the pump. This transfers brake fluid from the wheel, reducing the brake pressure. When the rotation speed has increased sufficiently the control module closes the outlet valve, opens the inlet valve and control returns to the open position.
Steps 1, 2 or 3 are repeated until braking finishes or the ABS system stops.
ABS control
No braking
The brake system is unaffected while the driver is not braking. This means that the master cylinder is in the standby position and the connection between the brake fluid reservoir and the rest of the hydraulic system is open. There is no pressure in the system. In the ABS hydraulic modulator valve block, the hydraulic valves are in their rest position. In other words, the inlet valves are open and the outlet valves closed.
Braking without ABS
When the brake pedal is depressed, the movement of the pedal is transmitted via the power brake booster to the master cylinder pistons and the stop (brake) lamp switch trips. When the primary and secondary pistons move forwards in the master cylinder, the openings for the brake fluid reservoir are closed. The hydraulic pressure in the two brake circuits increases and affects the pistons in the brake calipers which press the brake pads against the brake discs.
When the brake pedal is released, the pistons revert to the rest position in the master cylinder. The connection to the brake fluid reservoir opens and the brake system pressure falls. The brake caliper pistons return to their rest positions with the aid of the sealing rings.
Braking with ABS
When a wheel locks-up under braking, the brake control module (BCM) begins to close the inlet valve for the relevant brake circuit so that the supply of brake fluid from the master cylinder is interrupted. This means that the hydraulic pressure cannot increase irrespective of the pressure in the master cylinder (constant pressure, see the right-hand front wheel - FR). The hydraulic pump starts at the same time.
If the wheel locks anyway, the outlet valve opens and the brake fluid is allowed back to the accumulators resulting in a reduction in the brake circuit pressure (pressure release, see the left front wheel - FL). This reduces the hydraulic pressure and braking effect, allowing the wheel to accelerate. The hydraulic pump returns the brake fluid from the accumulators to the master cylinder.
When the acceleration is sufficient, the brake control module (BCM) closes the outlet valve and opens the inlet valve. This increases the hydraulic pressure in the circuit. During the pressure increase phase the required fluid volume is supplied by the master cylinder and the pump if the accumulators contain brake fluid. As the volume of fluid supplied to the brake circuit is greater than the volume released back into the accumulators from the brake circuits, the accumulators only compensate for the volume peaks on the suction side of the pump. The pump returns the brake fluid from the low pressure accumulators to the master cylinder or brake circuits depending on the position of the inlet valves.
When the pump operates the brake pedal is raised and lowered (pulses). This is normal and is also a warning to the driver that ABS control is active.
The hydraulic pump is operational for approximately 1 second after the ABS control has finished to ensure that the accumulators are empty.
Electronic brake force distribution (EBD)
Electronic brake force distribution (EBD) is integrated in the ABS system and controls the braking forces to the rear wheel brakes. Electronic brake force distribution controls the hydraulic pressure in the brake circuit for the rear wheels the rear wheels slip considerably less than the front wheels under braking. This ensures optimal and stable braking.
The brake control module (BCM) regulates the brake pressure by controlling the inlet and outlet valves in the brake circuits for the rear wheels.
The brake pressure in the rear wheel circuits varies, depending on how much the rear wheels slip in relation to the front wheels. The purpose of the control is to keep the rear wheel slippage to 0-2% lower than the front wheels. This means that the function is load sensitive.
Generally, the difference between the rear wheel and front wheel slippage under braking is dependent on how hard the brakes are applied and on the load of the car. Hard braking /light loads cause more slippage, light braking/heavy loads cause less slippage.
Therefore the requirement for brake fluid when the hydraulic pump is not activated during electronic brake force distribution (EBD) control is less than during stability and traction control. The fluid volume returned from the brake circuits is taken up by the accumulators.
STC (stability and traction control) system
The STC system consists of two components:
- Stability Control
- Traction Control.
Stability Control
Stability control occurs when the brake control module (BCM) transmits a signal to the engine control module (ECM) to lower the engine torque if any of the drive wheels spin and road friction is reduced. Stability control is available between 0 km/h and top speed.
Traction Control
General
Components
1. Separation valve, normally open
2. Electronic shuttle valve, normally closed
3. Hydraulic pump
4. Outlet valve, normally closed
5. Inlet valve, normally open
6. Pressure equalizer
7. By-pass valve
8. Non-return valve.
Traction control is integrated in the brake system.
The system is mainly designed to assist with moving off on slippery surfaces at speeds up to approximately 60 km/h. Traction control has only a very limited effect at speeds over 60 km/h.
If the brake control module (BCM) registers, via the front wheel sensors, that any of the drive wheels are spinning quicker than the others, the separation valve (1) closes. The hydraulic pump (3) is started so that brake fluid is pumped to the caliper of the slipping wheel. Pressure builds up in the brake circuit and the brakes are applied as much as required so that the drive force is distributed between the driven wheels. The wheel which has the best traction receives the most driven force against the surface. This process is independent of engine output.
On vehicles with traction control, the brake control module (BCM) continuously registers the extent of brake usage. This allows the control module to calculate the temperature of the front wheel brake discs. If traction control is engaged and the calculated temperature becomes too high, approximately 450 °C, traction control cuts out. The general warning lamp comes on and a text is displayed in the combined instrument panel. A diagnostic trouble code (DTC) is stored in the brake control module (BCM). Traction control is blocked to prevent the brakes from overheating. The light goes out once the calculated temperature falls below approximately 300 °C and traction control is engaged again. However, ABS is always available. There is information about the calculated temperature of the brake discs in the brake control module (BCM) as long as the 30-supply is connected. If the brake system has been used too hard, this may also cause high brake temperatures.
No traction control
If no drive wheel is spinning, the brake system is unaffected. This means that the master cylinder is in the standby position and the connection between the brake fluid reservoir and the rest of the hydraulic system is open. There is no pressure in the system. In the ABS hydraulic modulator valve block, the hydraulic valves are in their rest position, the inlet valves and the separation valves are open and the outlet valves and electronic shuttle valves are closed.
Traction control when one or more drive wheels are spinning
If the car is started and moves off and one or both driven wheels are threatening to spin, the brake control module (BCM) registers this by comparing the signals from the wheel sensors with a calculated reference speed. The control module closes one or both separation valves and the hydraulic pump starts. The separation valve blocks the connection between the pump pressure side and the primary circuit /secondary circuit in the master cylinder. The inlet valve for the non-spinning wheel closes so that the brake is not applied on that wheel. The hydraulic pump draws brake fluid from the brake fluid reservoir via the electronic shuttle valve and increases the pressure until the by-pass valve opens. The hydraulic pressure reaches the spinning wheel and slows down the wheel so that the driven force is distributed between the driven wheels so that the wheel with the best traction receives the most driven force.
As the pump supplies a greater flow than required by traction control, the excess brake fluid is drained off by the by-pass valve for the master cylinder or is drawn directly up by the pump.
The inlet valve closes as soon as the spinning wheel has been slowed down by the brake to a relatively normal speed. Depending on the acceleration of the wheel, the outlet valve opens (brake fluid flows back to the suction side of the pump) so that the pressure in the brake circuit drops or the outlet valve remains closed to maintain the pressure (constant pressure). The outlet valve closes to increase the pressure in the brake circuit, the inlet valve opens and the pressure rises in the brake circuit.
This control brakes the wheel in proportion to optimum slippage until one of the following occurs:
- as friction against the surface becomes greater wheel spin stops
- control is stopped by the brake control module (BCM) to prevent the brakes from overheating
- braking takes place
- the car reaches the speed where traction control ceases.
The separation valve is activated during traction control.
Activation of the brakes, which is registered by the brake control module (BCM) via the signal from the stop lamp switch, enables the traction control to be interrupted and the separation valve to open. The electronic shuttle valve closes, so that the pump is unable to draw brake fluid from the master cylinder. The hydraulic modulator now operates as a normal ABS hydraulic modulator.
A difference in speed between the driven wheels requires traction control. This difference varies with the speed of the car. When the vehicle speed is 0 km/h, a difference in speed of 8 km/h between the drive wheels is required for traction control to engage. At a vehicle speed of 20 km/h, a difference of 18 km/h, and at 40 km/h a difference of 25 km/h is required. At higher speeds the differences required between the driven wheel speeds is so great that in practice traction control does not function above 60 km/h.
Active yaw control system
Brake control modules (BCM) with DSTC use continuous information from the different sensors in the system to calculate the positions of the steering wheel and car. The brake control module (BCM) calculates the driving style by measuring:
- steering angle
- steering angle speed
- engine torque
- the vehicle speed
- brake pressure.
The brake control module (BCM) also calculates the behavior of the car by monitoring:
- yaw rate
- lateral acceleration.
When the differences between the intended direction of the driver and the actual direction exceeds a certain level, active yaw control regulates the engine torque (stability function). This maintains the lateral stability of the car under most driving conditions. Active yaw control also activates the individual brake control to regulate the separation and electronic shuttle valves as well as the inlet and outlet valves. This should correct the vehicle in the event of a skid.
For further information about the steering angle and steering angle speed, see Design and Function, steering wheel module (SWM).
Active yaw control
Components
1. Separation valve, normally open
2. Electronic shuttle valve, normally closed
3. Hydraulic pump
4. Outlet valve, normally closed
5. Inlet valve, normally open
6. Pressure equalizer
7. By-pass valve
8. Non-return valve.
During active yaw control, i.e. when the vehicle looses traction, the hydraulic pump begins to generate brake pressure even if the driver has not applied the brake pedal. The brake control module (BCM) then activates the valves in the ABS hydraulic modulator and checks the braking forces at each wheel. This is done by raising, maintaining or lowering the brake pressure individually to stabilize the vehicle. If the driver presses the brake pedal during stability control, the control module, through the pedal pressure sensor in the power brake booster, will take into account the pressure exerted by the driver during stability control.
Active yaw control oversteer
Oversteer is a very dangerous condition which is difficult for the average driver to control. Oversteer is when the car turns more than the steering wheel has been turned. If not corrected, oversteer can lead to the car beginning to spin uncontrollably.
In the example above, the car turns to the right and the car oversteers. To counter this the DSTC system has closed the separation valve, opened the electronic shuttle valve and started the pump. The inlet valve (C1) has been left open, while the other inlet valves are closed. This causes brakes the left front wheel, which causes the wheel to lose grip and reduces oversteer. If the car oversteers, the brake control module (BCM) transmits a request for torque reduction to the engine control module (ECM) via the controller area network (CAN).
Active yaw control understeer
Understeer is when the car tends to continue straight forward even though the steering wheel has been turned. If the car understeers, the brake control module (BCM) transmits a request for torque reduction to the engine control module (ECM) via the controller area network (CAN).
In the example above, the steering wheel has been turned to the right, but the car continues straight on. The separation valve closes, the electronic shuttle valve opens and the pump starts. The DSTC system applies the brakes to the right rear wheel by leaving inlet valve C2 open and closing and closing the other inlet valves (C1, C3 and C4). In the event that braking the right rear wheel is not sufficient to get the car back on course, the brake control module (BCM) opens inlet valve C4 and brakes the right-hand front wheel.
Dynamic stability control
If the driver presses the brake pedal during stability control, the brake control module (BCM), through the brake pressure sensor in the hydraulic modulator, will take into account the pressure exerted by the driver during stability control.
Included in the calculations by the control module are the driver's desired brake pressure and functions for:
- Active yaw control
- ABS
- EBD
- Stability control.
In the event of a hand brake turn, the active yaw control function is normally activated if the speed exceeds approximately 40 km/h. At lower speeds active yaw control is decreased successively, and at 20-25 km/h will not be noticeable.