ABS/ABD - ABS/ETS brake torque control circuit, function

ABS/ABD control module

with relay

for return flow

pump and

activation of

integrated

solenoid valves

Front left wheel speed sensor

Front right wheel speed sensor

Rear right wheel speed sensor

2-pin stop lamp switch

ABS/ABD hydraulic unit

Return flow pump

Solenoid valves

ABS safety lamp

(ABS MIL)

ABD display

38-pin diagnostic socket

Rear left wheel speed sensor

General description

The ABS/ABD control module controls the various functions of the hydraulic unit. The control module is supplied with power direct via fuse F5.

Input signals to the control module:

 Wheel speed sensors

 Stop lamp switch

The input signals are processed in the control module. By evaluating this information, the output signals to the following components can be controlled:

 Solenoid valves via relays

 Return flow pump via relays

 ABS MIL

 ABD display

The ABS/ABD uses the input signals to decide which functions are to be performed by the hydraulic system

 Normal operation

 ABS/ABD pressure build-up

 ABS/ABD pressure hold

 ABS/ABD pressure reduction

Driving under normal conditions

Under normal operating conditions, i.e. no speed difference between the individual wheels, all the solenoid valves in the ABS/ABD hydraulic system are in their initial positions. The system is operational.

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ABS operation

The ABS is operational after the ignition is switched on and the vehicle reaches a speed of 6 km/h. All braking actions in the lock-up range are performed at control speeds at and above 8 km/h.

ABS control modes

Due to different vehicle designs with different axle loads and varying dynamic behavior under braking, a number of different control modes are required which can be employed and combined as necessary.

The following control modes are employed:

On the front axle  < 50 km/h individual control (IR)

> 50 km/h full GMA (permanently active yaw moment damping)

On the rear axle  < 30 km/h select-low control (SL)

< 30 km/h individual control (IR)

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Individual control (IR)

The two wheels on an axle are regulated completely independently (individually).

The braking effect of each wheel is regulated to suit the existing adhesion conditions so that these can be exploited to the maximum.

This control mode provides the best possible braking effect in all road conditions. This produces a strong yawing moment during ABS braking on μ-split road surfaces, which requires severe countersteering by the driver in the case of ABS control on the front axle.

For this reason a so-called "braking yaw moment damping mode" is employed on the front axle, which slightly delays the braking force of the wheel at the high friction coefficient initially so that the vehicle is more easy to control.

Select-low control (SL)

The two wheels on an axle are regulated so that, in the event of different adhesion conditions on the right and left wheels, both wheels are regulated according to the one with the lowest adhesion. The wheel with the greater adhesion is underbraked and therefore has better lateral stability, which improves the vehicle's directional control.

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Permanently active yaw moment damping (full GMA)

During braking on surfaces with different friction coefficients, the different transmissible braking forces produce a yawing moment (rotation of the vehicle about the vertical axis in the direction of the higher friction coefficient).

To counteract this reaction, the build-up of the maximum transmissible braking force at the wheel with the higher friction coefficient is deliberately slowed in order to slow down the yaw response of the vehicle. The driver is thus given the opportunity to countersteer.

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The control cycle

Terms:

Reference speed

This is one reference variable in calculating wheel slip. It is permanently available in the electronic control module for the purpose of comparing the wheel speeds, and is constantly being recalculated by the electronic control module itself from the generated wheel speed signals.

Slip threshold

The wheel slip is the difference between the reference speed and the actual wheel speed. When certain limits (slip thresholds) are exceeded, there is a risk of the wheels locking up.

An example of the control cycle

The following describes the control cycle on one wheel. Approx. 4 ... 10 control cycles are performed per second, depending on the road surface conditions. The control sequence on the other wheels is identical.

From the alternating voltage generated from the wheel speed, the electronic control module calculates the wheel rotation, deceleration and acceleration signals. By combining the individual wheel speeds, a so-called reference speed is calculated, which approximates to the actual vehicle speed. Slip signals are derived from comparing the wheel speed and the reference speed.

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If a wheel is about to lock up due to excessive pressure in the brake cylinder - a situation which is detected from the wheel speed curve (wheel slip) - then the pressure is kept constant, i.e. no further pressure increase is possible.

If the tendency to lock up continues to exist because the constant pressure is still too high, the pressure is reduced by opening the solenoid valve (AV..). At the same time the brake fluid in the reservoir is pumped back into the brake circuit upstream of the brake master cylinder by the return flow pump. When the pressure is too low for the wheel to start to accelerate again, there is no further pressure reduction and the pressure is again kept constant.

If the re-acceleration of the wheel exceeds a threshold value, the pressure is temporarily increased again by opening the inlet valve in the solenoid valve. The pressure does not exceed the pressure in the brake master cylinder generated by the pedal force.

By means of appropriate signals from the electronic control module, the three control phases

 Pressure build-up

 Pressure hold

 Pressure reduction

can be actuated in the hydraulic unit.

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Control phases during ABS braking

Pressure build-up phase

In the pressure build-up phase the pressure can be increased via the open solenoid valve (EV) up to the pressure built up by the brake master cylinder.

Pressure holding phase

In the pressure holding phase, which precedes every pressure reduction phase, the pressure from the hydraulic unit to the wheel brakes is kept constant because the inlet and outlet in the solenoid valve (EV) and the outlet valve in the solenoid valve (AV) are closed.

Pressure reduction phase

During the pressure reduction phase the solenoid valve (EV) remains closed. The brake fluid is pumped back via the open pressure reduction valve (AV), a reservoir and the check valve (RVR) by the return flow pump (RFP), against the existing pressure in the brake circuit upstream of the brake master cylinder in order not to exhaust its capacity.

The control cycle is continuously repeated during a regulated braking action until the brake pedal is released or until shortly before the vehicle comes to a stop.

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Control phases during ABD braking

Pressure build-up phase

In the pressure build-up phase brake fluid is drawn in by the return flow pump via the brake master cylinder and into the brake circuit of the wheel to be braked. This produces a gradual increase in the brake pressure in the wheel brake cylinder up to max. 170 bar, limited by the pressure relief valve integrated in the switchover valve (USV).

Pressure holding phase

In the pressure holding phase, which precedes every pressure reduction phase, the pressure from the hydraulic unit to the wheel brakes is kept constant by closing the ASV and USV solenoid valve. The ABS wheel valves (EV, AV) are not actuated.

Pressure reduction phase

The pressure is reduced via the USV. It is actuated in pulses to reduce the pressure in the brake circuit from the wheel back to the brake master cylinder.

The control cycle is repeated until the speed difference is < 6 km/h or until the maximum control speed of 50km/h is reached, although any control sequence in progress is completed first.

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